![Kees Cook](/assets/img/avatar_default.png)
In preparation for adding seccomp locking, move filter creation away from where it is checked and applied. This will allow for locking where no memory allocation is happening. The validation, filter attachment, and seccomp mode setting can all happen under the future locks. For extreme defensiveness, I've added a BUG_ON check for the calculated size of the buffer allocation in case BPF_MAXINSN ever changes, which shouldn't ever happen. The compiler should actually optimize out this check since the test above it makes it impossible. Signed-off-by: Kees Cook <keescook@chromium.org> Reviewed-by: Oleg Nesterov <oleg@redhat.com> Reviewed-by: Andy Lutomirski <luto@amacapital.net>
653 lines
17 KiB
C
653 lines
17 KiB
C
/*
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* linux/kernel/seccomp.c
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*
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* Copyright 2004-2005 Andrea Arcangeli <andrea@cpushare.com>
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*
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* Copyright (C) 2012 Google, Inc.
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* Will Drewry <wad@chromium.org>
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*
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* This defines a simple but solid secure-computing facility.
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*
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* Mode 1 uses a fixed list of allowed system calls.
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* Mode 2 allows user-defined system call filters in the form
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* of Berkeley Packet Filters/Linux Socket Filters.
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*/
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#include <linux/atomic.h>
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#include <linux/audit.h>
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#include <linux/compat.h>
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#include <linux/sched.h>
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#include <linux/seccomp.h>
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#include <linux/slab.h>
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#include <linux/syscalls.h>
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/* #define SECCOMP_DEBUG 1 */
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#ifdef CONFIG_SECCOMP_FILTER
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#include <asm/syscall.h>
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#include <linux/filter.h>
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#include <linux/ptrace.h>
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#include <linux/security.h>
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#include <linux/tracehook.h>
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#include <linux/uaccess.h>
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/**
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* struct seccomp_filter - container for seccomp BPF programs
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*
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* @usage: reference count to manage the object lifetime.
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* get/put helpers should be used when accessing an instance
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* outside of a lifetime-guarded section. In general, this
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* is only needed for handling filters shared across tasks.
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* @prev: points to a previously installed, or inherited, filter
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* @len: the number of instructions in the program
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* @insnsi: the BPF program instructions to evaluate
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*
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* seccomp_filter objects are organized in a tree linked via the @prev
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* pointer. For any task, it appears to be a singly-linked list starting
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* with current->seccomp.filter, the most recently attached or inherited filter.
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* However, multiple filters may share a @prev node, by way of fork(), which
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* results in a unidirectional tree existing in memory. This is similar to
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* how namespaces work.
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*
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* seccomp_filter objects should never be modified after being attached
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* to a task_struct (other than @usage).
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*/
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struct seccomp_filter {
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atomic_t usage;
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struct seccomp_filter *prev;
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struct sk_filter *prog;
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};
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/* Limit any path through the tree to 256KB worth of instructions. */
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#define MAX_INSNS_PER_PATH ((1 << 18) / sizeof(struct sock_filter))
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/*
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* Endianness is explicitly ignored and left for BPF program authors to manage
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* as per the specific architecture.
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*/
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static void populate_seccomp_data(struct seccomp_data *sd)
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{
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struct task_struct *task = current;
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struct pt_regs *regs = task_pt_regs(task);
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unsigned long args[6];
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sd->nr = syscall_get_nr(task, regs);
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sd->arch = syscall_get_arch();
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syscall_get_arguments(task, regs, 0, 6, args);
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sd->args[0] = args[0];
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sd->args[1] = args[1];
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sd->args[2] = args[2];
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sd->args[3] = args[3];
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sd->args[4] = args[4];
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sd->args[5] = args[5];
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sd->instruction_pointer = KSTK_EIP(task);
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}
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/**
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* seccomp_check_filter - verify seccomp filter code
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* @filter: filter to verify
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* @flen: length of filter
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*
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* Takes a previously checked filter (by sk_chk_filter) and
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* redirects all filter code that loads struct sk_buff data
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* and related data through seccomp_bpf_load. It also
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* enforces length and alignment checking of those loads.
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*
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* Returns 0 if the rule set is legal or -EINVAL if not.
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*/
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static int seccomp_check_filter(struct sock_filter *filter, unsigned int flen)
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{
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int pc;
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for (pc = 0; pc < flen; pc++) {
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struct sock_filter *ftest = &filter[pc];
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u16 code = ftest->code;
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u32 k = ftest->k;
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switch (code) {
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case BPF_LD | BPF_W | BPF_ABS:
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ftest->code = BPF_LDX | BPF_W | BPF_ABS;
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/* 32-bit aligned and not out of bounds. */
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if (k >= sizeof(struct seccomp_data) || k & 3)
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return -EINVAL;
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continue;
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case BPF_LD | BPF_W | BPF_LEN:
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ftest->code = BPF_LD | BPF_IMM;
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ftest->k = sizeof(struct seccomp_data);
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continue;
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case BPF_LDX | BPF_W | BPF_LEN:
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ftest->code = BPF_LDX | BPF_IMM;
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ftest->k = sizeof(struct seccomp_data);
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continue;
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/* Explicitly include allowed calls. */
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case BPF_RET | BPF_K:
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case BPF_RET | BPF_A:
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case BPF_ALU | BPF_ADD | BPF_K:
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case BPF_ALU | BPF_ADD | BPF_X:
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case BPF_ALU | BPF_SUB | BPF_K:
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case BPF_ALU | BPF_SUB | BPF_X:
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case BPF_ALU | BPF_MUL | BPF_K:
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case BPF_ALU | BPF_MUL | BPF_X:
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case BPF_ALU | BPF_DIV | BPF_K:
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case BPF_ALU | BPF_DIV | BPF_X:
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case BPF_ALU | BPF_AND | BPF_K:
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case BPF_ALU | BPF_AND | BPF_X:
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case BPF_ALU | BPF_OR | BPF_K:
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case BPF_ALU | BPF_OR | BPF_X:
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case BPF_ALU | BPF_XOR | BPF_K:
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case BPF_ALU | BPF_XOR | BPF_X:
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case BPF_ALU | BPF_LSH | BPF_K:
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case BPF_ALU | BPF_LSH | BPF_X:
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case BPF_ALU | BPF_RSH | BPF_K:
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case BPF_ALU | BPF_RSH | BPF_X:
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case BPF_ALU | BPF_NEG:
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case BPF_LD | BPF_IMM:
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case BPF_LDX | BPF_IMM:
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case BPF_MISC | BPF_TAX:
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case BPF_MISC | BPF_TXA:
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case BPF_LD | BPF_MEM:
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case BPF_LDX | BPF_MEM:
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case BPF_ST:
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case BPF_STX:
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case BPF_JMP | BPF_JA:
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case BPF_JMP | BPF_JEQ | BPF_K:
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case BPF_JMP | BPF_JEQ | BPF_X:
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case BPF_JMP | BPF_JGE | BPF_K:
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case BPF_JMP | BPF_JGE | BPF_X:
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case BPF_JMP | BPF_JGT | BPF_K:
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case BPF_JMP | BPF_JGT | BPF_X:
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case BPF_JMP | BPF_JSET | BPF_K:
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case BPF_JMP | BPF_JSET | BPF_X:
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continue;
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default:
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return -EINVAL;
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}
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}
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return 0;
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}
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/**
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* seccomp_run_filters - evaluates all seccomp filters against @syscall
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* @syscall: number of the current system call
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*
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* Returns valid seccomp BPF response codes.
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*/
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static u32 seccomp_run_filters(int syscall)
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{
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struct seccomp_filter *f;
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struct seccomp_data sd;
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u32 ret = SECCOMP_RET_ALLOW;
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/* Ensure unexpected behavior doesn't result in failing open. */
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if (WARN_ON(current->seccomp.filter == NULL))
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return SECCOMP_RET_KILL;
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populate_seccomp_data(&sd);
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/*
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* All filters in the list are evaluated and the lowest BPF return
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* value always takes priority (ignoring the DATA).
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*/
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for (f = current->seccomp.filter; f; f = f->prev) {
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u32 cur_ret = SK_RUN_FILTER(f->prog, (void *)&sd);
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if ((cur_ret & SECCOMP_RET_ACTION) < (ret & SECCOMP_RET_ACTION))
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ret = cur_ret;
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}
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return ret;
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}
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#endif /* CONFIG_SECCOMP_FILTER */
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static inline bool seccomp_may_assign_mode(unsigned long seccomp_mode)
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{
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if (current->seccomp.mode && current->seccomp.mode != seccomp_mode)
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return false;
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return true;
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}
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static inline void seccomp_assign_mode(unsigned long seccomp_mode)
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{
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current->seccomp.mode = seccomp_mode;
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set_tsk_thread_flag(current, TIF_SECCOMP);
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}
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#ifdef CONFIG_SECCOMP_FILTER
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/**
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* seccomp_prepare_filter: Prepares a seccomp filter for use.
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* @fprog: BPF program to install
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*
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* Returns filter on success or an ERR_PTR on failure.
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*/
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static struct seccomp_filter *seccomp_prepare_filter(struct sock_fprog *fprog)
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{
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struct seccomp_filter *filter;
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unsigned long fp_size;
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struct sock_filter *fp;
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int new_len;
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long ret;
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if (fprog->len == 0 || fprog->len > BPF_MAXINSNS)
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return ERR_PTR(-EINVAL);
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BUG_ON(INT_MAX / fprog->len < sizeof(struct sock_filter));
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fp_size = fprog->len * sizeof(struct sock_filter);
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/*
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* Installing a seccomp filter requires that the task has
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* CAP_SYS_ADMIN in its namespace or be running with no_new_privs.
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* This avoids scenarios where unprivileged tasks can affect the
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* behavior of privileged children.
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*/
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if (!task_no_new_privs(current) &&
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security_capable_noaudit(current_cred(), current_user_ns(),
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CAP_SYS_ADMIN) != 0)
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return ERR_PTR(-EACCES);
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fp = kzalloc(fp_size, GFP_KERNEL|__GFP_NOWARN);
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if (!fp)
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return ERR_PTR(-ENOMEM);
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/* Copy the instructions from fprog. */
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ret = -EFAULT;
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if (copy_from_user(fp, fprog->filter, fp_size))
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goto free_prog;
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/* Check and rewrite the fprog via the skb checker */
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ret = sk_chk_filter(fp, fprog->len);
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if (ret)
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goto free_prog;
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/* Check and rewrite the fprog for seccomp use */
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ret = seccomp_check_filter(fp, fprog->len);
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if (ret)
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goto free_prog;
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/* Convert 'sock_filter' insns to 'sock_filter_int' insns */
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ret = sk_convert_filter(fp, fprog->len, NULL, &new_len);
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if (ret)
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goto free_prog;
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/* Allocate a new seccomp_filter */
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ret = -ENOMEM;
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filter = kzalloc(sizeof(struct seccomp_filter),
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GFP_KERNEL|__GFP_NOWARN);
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if (!filter)
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goto free_prog;
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filter->prog = kzalloc(sk_filter_size(new_len),
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GFP_KERNEL|__GFP_NOWARN);
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if (!filter->prog)
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goto free_filter;
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ret = sk_convert_filter(fp, fprog->len, filter->prog->insnsi, &new_len);
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if (ret)
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goto free_filter_prog;
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kfree(fp);
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atomic_set(&filter->usage, 1);
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filter->prog->len = new_len;
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sk_filter_select_runtime(filter->prog);
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return filter;
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free_filter_prog:
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kfree(filter->prog);
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free_filter:
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kfree(filter);
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free_prog:
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kfree(fp);
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return ERR_PTR(ret);
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}
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/**
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* seccomp_prepare_user_filter - prepares a user-supplied sock_fprog
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* @user_filter: pointer to the user data containing a sock_fprog.
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*
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* Returns 0 on success and non-zero otherwise.
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*/
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static struct seccomp_filter *
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seccomp_prepare_user_filter(const char __user *user_filter)
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{
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struct sock_fprog fprog;
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struct seccomp_filter *filter = ERR_PTR(-EFAULT);
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#ifdef CONFIG_COMPAT
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if (is_compat_task()) {
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struct compat_sock_fprog fprog32;
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if (copy_from_user(&fprog32, user_filter, sizeof(fprog32)))
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goto out;
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fprog.len = fprog32.len;
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fprog.filter = compat_ptr(fprog32.filter);
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} else /* falls through to the if below. */
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#endif
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if (copy_from_user(&fprog, user_filter, sizeof(fprog)))
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goto out;
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filter = seccomp_prepare_filter(&fprog);
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out:
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return filter;
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}
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/**
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* seccomp_attach_filter: validate and attach filter
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* @flags: flags to change filter behavior
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* @filter: seccomp filter to add to the current process
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*
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* Returns 0 on success, -ve on error.
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*/
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static long seccomp_attach_filter(unsigned int flags,
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struct seccomp_filter *filter)
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{
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unsigned long total_insns;
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struct seccomp_filter *walker;
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/* Validate resulting filter length. */
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total_insns = filter->prog->len;
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for (walker = current->seccomp.filter; walker; walker = walker->prev)
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total_insns += walker->prog->len + 4; /* 4 instr penalty */
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if (total_insns > MAX_INSNS_PER_PATH)
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return -ENOMEM;
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/*
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* If there is an existing filter, make it the prev and don't drop its
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* task reference.
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*/
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filter->prev = current->seccomp.filter;
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current->seccomp.filter = filter;
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return 0;
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}
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/* get_seccomp_filter - increments the reference count of the filter on @tsk */
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void get_seccomp_filter(struct task_struct *tsk)
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{
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struct seccomp_filter *orig = tsk->seccomp.filter;
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if (!orig)
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return;
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/* Reference count is bounded by the number of total processes. */
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atomic_inc(&orig->usage);
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}
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static inline void seccomp_filter_free(struct seccomp_filter *filter)
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{
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if (filter) {
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sk_filter_free(filter->prog);
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kfree(filter);
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}
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}
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/* put_seccomp_filter - decrements the ref count of tsk->seccomp.filter */
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void put_seccomp_filter(struct task_struct *tsk)
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{
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struct seccomp_filter *orig = tsk->seccomp.filter;
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/* Clean up single-reference branches iteratively. */
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while (orig && atomic_dec_and_test(&orig->usage)) {
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struct seccomp_filter *freeme = orig;
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orig = orig->prev;
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seccomp_filter_free(freeme);
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}
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}
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/**
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* seccomp_send_sigsys - signals the task to allow in-process syscall emulation
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* @syscall: syscall number to send to userland
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* @reason: filter-supplied reason code to send to userland (via si_errno)
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*
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* Forces a SIGSYS with a code of SYS_SECCOMP and related sigsys info.
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*/
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static void seccomp_send_sigsys(int syscall, int reason)
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{
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struct siginfo info;
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memset(&info, 0, sizeof(info));
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info.si_signo = SIGSYS;
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info.si_code = SYS_SECCOMP;
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info.si_call_addr = (void __user *)KSTK_EIP(current);
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info.si_errno = reason;
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info.si_arch = syscall_get_arch();
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info.si_syscall = syscall;
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force_sig_info(SIGSYS, &info, current);
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}
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#endif /* CONFIG_SECCOMP_FILTER */
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/*
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* Secure computing mode 1 allows only read/write/exit/sigreturn.
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* To be fully secure this must be combined with rlimit
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* to limit the stack allocations too.
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*/
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static int mode1_syscalls[] = {
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__NR_seccomp_read, __NR_seccomp_write, __NR_seccomp_exit, __NR_seccomp_sigreturn,
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0, /* null terminated */
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};
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#ifdef CONFIG_COMPAT
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static int mode1_syscalls_32[] = {
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__NR_seccomp_read_32, __NR_seccomp_write_32, __NR_seccomp_exit_32, __NR_seccomp_sigreturn_32,
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0, /* null terminated */
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};
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#endif
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int __secure_computing(int this_syscall)
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{
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int mode = current->seccomp.mode;
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int exit_sig = 0;
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int *syscall;
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u32 ret;
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switch (mode) {
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case SECCOMP_MODE_STRICT:
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syscall = mode1_syscalls;
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#ifdef CONFIG_COMPAT
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if (is_compat_task())
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syscall = mode1_syscalls_32;
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#endif
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do {
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if (*syscall == this_syscall)
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return 0;
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} while (*++syscall);
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exit_sig = SIGKILL;
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ret = SECCOMP_RET_KILL;
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break;
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#ifdef CONFIG_SECCOMP_FILTER
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case SECCOMP_MODE_FILTER: {
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int data;
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struct pt_regs *regs = task_pt_regs(current);
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ret = seccomp_run_filters(this_syscall);
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data = ret & SECCOMP_RET_DATA;
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ret &= SECCOMP_RET_ACTION;
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switch (ret) {
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case SECCOMP_RET_ERRNO:
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/* Set the low-order 16-bits as a errno. */
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syscall_set_return_value(current, regs,
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-data, 0);
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goto skip;
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case SECCOMP_RET_TRAP:
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/* Show the handler the original registers. */
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syscall_rollback(current, regs);
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/* Let the filter pass back 16 bits of data. */
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seccomp_send_sigsys(this_syscall, data);
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goto skip;
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case SECCOMP_RET_TRACE:
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/* Skip these calls if there is no tracer. */
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if (!ptrace_event_enabled(current, PTRACE_EVENT_SECCOMP)) {
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syscall_set_return_value(current, regs,
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-ENOSYS, 0);
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goto skip;
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}
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/* Allow the BPF to provide the event message */
|
|
ptrace_event(PTRACE_EVENT_SECCOMP, data);
|
|
/*
|
|
* The delivery of a fatal signal during event
|
|
* notification may silently skip tracer notification.
|
|
* Terminating the task now avoids executing a system
|
|
* call that may not be intended.
|
|
*/
|
|
if (fatal_signal_pending(current))
|
|
break;
|
|
if (syscall_get_nr(current, regs) < 0)
|
|
goto skip; /* Explicit request to skip. */
|
|
|
|
return 0;
|
|
case SECCOMP_RET_ALLOW:
|
|
return 0;
|
|
case SECCOMP_RET_KILL:
|
|
default:
|
|
break;
|
|
}
|
|
exit_sig = SIGSYS;
|
|
break;
|
|
}
|
|
#endif
|
|
default:
|
|
BUG();
|
|
}
|
|
|
|
#ifdef SECCOMP_DEBUG
|
|
dump_stack();
|
|
#endif
|
|
audit_seccomp(this_syscall, exit_sig, ret);
|
|
do_exit(exit_sig);
|
|
#ifdef CONFIG_SECCOMP_FILTER
|
|
skip:
|
|
audit_seccomp(this_syscall, exit_sig, ret);
|
|
#endif
|
|
return -1;
|
|
}
|
|
|
|
long prctl_get_seccomp(void)
|
|
{
|
|
return current->seccomp.mode;
|
|
}
|
|
|
|
/**
|
|
* seccomp_set_mode_strict: internal function for setting strict seccomp
|
|
*
|
|
* Once current->seccomp.mode is non-zero, it may not be changed.
|
|
*
|
|
* Returns 0 on success or -EINVAL on failure.
|
|
*/
|
|
static long seccomp_set_mode_strict(void)
|
|
{
|
|
const unsigned long seccomp_mode = SECCOMP_MODE_STRICT;
|
|
long ret = -EINVAL;
|
|
|
|
if (!seccomp_may_assign_mode(seccomp_mode))
|
|
goto out;
|
|
|
|
#ifdef TIF_NOTSC
|
|
disable_TSC();
|
|
#endif
|
|
seccomp_assign_mode(seccomp_mode);
|
|
ret = 0;
|
|
|
|
out:
|
|
|
|
return ret;
|
|
}
|
|
|
|
#ifdef CONFIG_SECCOMP_FILTER
|
|
/**
|
|
* seccomp_set_mode_filter: internal function for setting seccomp filter
|
|
* @flags: flags to change filter behavior
|
|
* @filter: struct sock_fprog containing filter
|
|
*
|
|
* This function may be called repeatedly to install additional filters.
|
|
* Every filter successfully installed will be evaluated (in reverse order)
|
|
* for each system call the task makes.
|
|
*
|
|
* Once current->seccomp.mode is non-zero, it may not be changed.
|
|
*
|
|
* Returns 0 on success or -EINVAL on failure.
|
|
*/
|
|
static long seccomp_set_mode_filter(unsigned int flags,
|
|
const char __user *filter)
|
|
{
|
|
const unsigned long seccomp_mode = SECCOMP_MODE_FILTER;
|
|
struct seccomp_filter *prepared = NULL;
|
|
long ret = -EINVAL;
|
|
|
|
/* Validate flags. */
|
|
if (flags != 0)
|
|
goto out;
|
|
|
|
/* Prepare the new filter before holding any locks. */
|
|
prepared = seccomp_prepare_user_filter(filter);
|
|
if (IS_ERR(prepared))
|
|
return PTR_ERR(prepared);
|
|
|
|
if (!seccomp_may_assign_mode(seccomp_mode))
|
|
goto out;
|
|
|
|
ret = seccomp_attach_filter(flags, prepared);
|
|
if (ret)
|
|
goto out;
|
|
/* Do not free the successfully attached filter. */
|
|
prepared = NULL;
|
|
|
|
seccomp_assign_mode(seccomp_mode);
|
|
out:
|
|
seccomp_filter_free(prepared);
|
|
return ret;
|
|
}
|
|
#else
|
|
static inline long seccomp_set_mode_filter(unsigned int flags,
|
|
const char __user *filter)
|
|
{
|
|
return -EINVAL;
|
|
}
|
|
#endif
|
|
|
|
/* Common entry point for both prctl and syscall. */
|
|
static long do_seccomp(unsigned int op, unsigned int flags,
|
|
const char __user *uargs)
|
|
{
|
|
switch (op) {
|
|
case SECCOMP_SET_MODE_STRICT:
|
|
if (flags != 0 || uargs != NULL)
|
|
return -EINVAL;
|
|
return seccomp_set_mode_strict();
|
|
case SECCOMP_SET_MODE_FILTER:
|
|
return seccomp_set_mode_filter(flags, uargs);
|
|
default:
|
|
return -EINVAL;
|
|
}
|
|
}
|
|
|
|
SYSCALL_DEFINE3(seccomp, unsigned int, op, unsigned int, flags,
|
|
const char __user *, uargs)
|
|
{
|
|
return do_seccomp(op, flags, uargs);
|
|
}
|
|
|
|
/**
|
|
* prctl_set_seccomp: configures current->seccomp.mode
|
|
* @seccomp_mode: requested mode to use
|
|
* @filter: optional struct sock_fprog for use with SECCOMP_MODE_FILTER
|
|
*
|
|
* Returns 0 on success or -EINVAL on failure.
|
|
*/
|
|
long prctl_set_seccomp(unsigned long seccomp_mode, char __user *filter)
|
|
{
|
|
unsigned int op;
|
|
char __user *uargs;
|
|
|
|
switch (seccomp_mode) {
|
|
case SECCOMP_MODE_STRICT:
|
|
op = SECCOMP_SET_MODE_STRICT;
|
|
/*
|
|
* Setting strict mode through prctl always ignored filter,
|
|
* so make sure it is always NULL here to pass the internal
|
|
* check in do_seccomp().
|
|
*/
|
|
uargs = NULL;
|
|
break;
|
|
case SECCOMP_MODE_FILTER:
|
|
op = SECCOMP_SET_MODE_FILTER;
|
|
uargs = filter;
|
|
break;
|
|
default:
|
|
return -EINVAL;
|
|
}
|
|
|
|
/* prctl interface doesn't have flags, so they are always zero. */
|
|
return do_seccomp(op, 0, uargs);
|
|
}
|