![Catherine Zhang](/assets/img/avatar_default.png)
This patch implements an application of the LSM-IPSec networking controls whereby an application can determine the label of the security association its TCP or UDP sockets are currently connected to via getsockopt and the auxiliary data mechanism of recvmsg. Patch purpose: This patch enables a security-aware application to retrieve the security context of an IPSec security association a particular TCP or UDP socket is using. The application can then use this security context to determine the security context for processing on behalf of the peer at the other end of this connection. In the case of UDP, the security context is for each individual packet. An example application is the inetd daemon, which could be modified to start daemons running at security contexts dependent on the remote client. Patch design approach: - Design for TCP The patch enables the SELinux LSM to set the peer security context for a socket based on the security context of the IPSec security association. The application may retrieve this context using getsockopt. When called, the kernel determines if the socket is a connected (TCP_ESTABLISHED) TCP socket and, if so, uses the dst_entry cache on the socket to retrieve the security associations. If a security association has a security context, the context string is returned, as for UNIX domain sockets. - Design for UDP Unlike TCP, UDP is connectionless. This requires a somewhat different API to retrieve the peer security context. With TCP, the peer security context stays the same throughout the connection, thus it can be retrieved at any time between when the connection is established and when it is torn down. With UDP, each read/write can have different peer and thus the security context might change every time. As a result the security context retrieval must be done TOGETHER with the packet retrieval. The solution is to build upon the existing Unix domain socket API for retrieving user credentials. Linux offers the API for obtaining user credentials via ancillary messages (i.e., out of band/control messages that are bundled together with a normal message). Patch implementation details: - Implementation for TCP The security context can be retrieved by applications using getsockopt with the existing SO_PEERSEC flag. As an example (ignoring error checking): getsockopt(sockfd, SOL_SOCKET, SO_PEERSEC, optbuf, &optlen); printf("Socket peer context is: %s\n", optbuf); The SELinux function, selinux_socket_getpeersec, is extended to check for labeled security associations for connected (TCP_ESTABLISHED == sk->sk_state) TCP sockets only. If so, the socket has a dst_cache of struct dst_entry values that may refer to security associations. If these have security associations with security contexts, the security context is returned. getsockopt returns a buffer that contains a security context string or the buffer is unmodified. - Implementation for UDP To retrieve the security context, the application first indicates to the kernel such desire by setting the IP_PASSSEC option via getsockopt. Then the application retrieves the security context using the auxiliary data mechanism. An example server application for UDP should look like this: toggle = 1; toggle_len = sizeof(toggle); setsockopt(sockfd, SOL_IP, IP_PASSSEC, &toggle, &toggle_len); recvmsg(sockfd, &msg_hdr, 0); if (msg_hdr.msg_controllen > sizeof(struct cmsghdr)) { cmsg_hdr = CMSG_FIRSTHDR(&msg_hdr); if (cmsg_hdr->cmsg_len <= CMSG_LEN(sizeof(scontext)) && cmsg_hdr->cmsg_level == SOL_IP && cmsg_hdr->cmsg_type == SCM_SECURITY) { memcpy(&scontext, CMSG_DATA(cmsg_hdr), sizeof(scontext)); } } ip_setsockopt is enhanced with a new socket option IP_PASSSEC to allow a server socket to receive security context of the peer. A new ancillary message type SCM_SECURITY. When the packet is received we get the security context from the sec_path pointer which is contained in the sk_buff, and copy it to the ancillary message space. An additional LSM hook, selinux_socket_getpeersec_udp, is defined to retrieve the security context from the SELinux space. The existing function, selinux_socket_getpeersec does not suit our purpose, because the security context is copied directly to user space, rather than to kernel space. Testing: We have tested the patch by setting up TCP and UDP connections between applications on two machines using the IPSec policies that result in labeled security associations being built. For TCP, we can then extract the peer security context using getsockopt on either end. For UDP, the receiving end can retrieve the security context using the auxiliary data mechanism of recvmsg. Signed-off-by: Catherine Zhang <cxzhang@watson.ibm.com> Acked-by: James Morris <jmorris@namei.org> Acked-by: Herbert Xu <herbert@gondor.apana.org.au> Signed-off-by: David S. Miller <davem@davemloft.net>
374 lines
8.3 KiB
C
374 lines
8.3 KiB
C
/*
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* NSA Security-Enhanced Linux (SELinux) security module
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*
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* This file contains the SELinux XFRM hook function implementations.
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*
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* Authors: Serge Hallyn <sergeh@us.ibm.com>
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* Trent Jaeger <jaegert@us.ibm.com>
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*
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* Copyright (C) 2005 International Business Machines Corporation
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License version 2,
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* as published by the Free Software Foundation.
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*/
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/*
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* USAGE:
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* NOTES:
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* 1. Make sure to enable the following options in your kernel config:
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* CONFIG_SECURITY=y
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* CONFIG_SECURITY_NETWORK=y
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* CONFIG_SECURITY_NETWORK_XFRM=y
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* CONFIG_SECURITY_SELINUX=m/y
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* ISSUES:
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* 1. Caching packets, so they are not dropped during negotiation
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* 2. Emulating a reasonable SO_PEERSEC across machines
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* 3. Testing addition of sk_policy's with security context via setsockopt
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*/
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#include <linux/config.h>
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#include <linux/module.h>
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#include <linux/kernel.h>
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#include <linux/init.h>
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#include <linux/security.h>
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#include <linux/types.h>
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#include <linux/netfilter.h>
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#include <linux/netfilter_ipv4.h>
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#include <linux/netfilter_ipv6.h>
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#include <linux/ip.h>
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#include <linux/tcp.h>
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#include <linux/skbuff.h>
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#include <linux/xfrm.h>
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#include <net/xfrm.h>
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#include <net/checksum.h>
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#include <net/udp.h>
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#include <asm/semaphore.h>
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#include "avc.h"
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#include "objsec.h"
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#include "xfrm.h"
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/*
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* Returns true if an LSM/SELinux context
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*/
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static inline int selinux_authorizable_ctx(struct xfrm_sec_ctx *ctx)
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{
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return (ctx &&
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(ctx->ctx_doi == XFRM_SC_DOI_LSM) &&
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(ctx->ctx_alg == XFRM_SC_ALG_SELINUX));
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}
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/*
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* Returns true if the xfrm contains a security blob for SELinux
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*/
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static inline int selinux_authorizable_xfrm(struct xfrm_state *x)
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{
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return selinux_authorizable_ctx(x->security);
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}
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/*
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* LSM hook implementation that authorizes that a socket can be used
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* with the corresponding xfrm_sec_ctx and direction.
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*/
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int selinux_xfrm_policy_lookup(struct xfrm_policy *xp, u32 sk_sid, u8 dir)
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{
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int rc = 0;
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u32 sel_sid = SECINITSID_UNLABELED;
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struct xfrm_sec_ctx *ctx;
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/* Context sid is either set to label or ANY_ASSOC */
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if ((ctx = xp->security)) {
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if (!selinux_authorizable_ctx(ctx))
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return -EINVAL;
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sel_sid = ctx->ctx_sid;
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}
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rc = avc_has_perm(sk_sid, sel_sid, SECCLASS_ASSOCIATION,
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((dir == FLOW_DIR_IN) ? ASSOCIATION__RECVFROM :
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((dir == FLOW_DIR_OUT) ? ASSOCIATION__SENDTO :
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(ASSOCIATION__SENDTO | ASSOCIATION__RECVFROM))),
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NULL);
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return rc;
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}
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/*
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* Security blob allocation for xfrm_policy and xfrm_state
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* CTX does not have a meaningful value on input
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*/
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static int selinux_xfrm_sec_ctx_alloc(struct xfrm_sec_ctx **ctxp, struct xfrm_user_sec_ctx *uctx)
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{
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int rc = 0;
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struct task_security_struct *tsec = current->security;
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struct xfrm_sec_ctx *ctx;
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BUG_ON(!uctx);
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BUG_ON(uctx->ctx_doi != XFRM_SC_ALG_SELINUX);
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if (uctx->ctx_len >= PAGE_SIZE)
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return -ENOMEM;
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*ctxp = ctx = kmalloc(sizeof(*ctx) +
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uctx->ctx_len,
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GFP_KERNEL);
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if (!ctx)
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return -ENOMEM;
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ctx->ctx_doi = uctx->ctx_doi;
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ctx->ctx_len = uctx->ctx_len;
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ctx->ctx_alg = uctx->ctx_alg;
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memcpy(ctx->ctx_str,
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uctx+1,
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ctx->ctx_len);
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rc = security_context_to_sid(ctx->ctx_str,
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ctx->ctx_len,
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&ctx->ctx_sid);
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if (rc)
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goto out;
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/*
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* Does the subject have permission to set security or permission to
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* do the relabel?
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* Must be permitted to relabel from default socket type (process type)
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* to specified context
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*/
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rc = avc_has_perm(tsec->sid, ctx->ctx_sid,
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SECCLASS_ASSOCIATION,
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ASSOCIATION__SETCONTEXT, NULL);
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if (rc)
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goto out;
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return rc;
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out:
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*ctxp = NULL;
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kfree(ctx);
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return rc;
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}
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/*
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* LSM hook implementation that allocs and transfers uctx spec to
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* xfrm_policy.
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*/
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int selinux_xfrm_policy_alloc(struct xfrm_policy *xp, struct xfrm_user_sec_ctx *uctx)
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{
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int err;
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BUG_ON(!xp);
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err = selinux_xfrm_sec_ctx_alloc(&xp->security, uctx);
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return err;
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}
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/*
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* LSM hook implementation that copies security data structure from old to
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* new for policy cloning.
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*/
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int selinux_xfrm_policy_clone(struct xfrm_policy *old, struct xfrm_policy *new)
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{
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struct xfrm_sec_ctx *old_ctx, *new_ctx;
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old_ctx = old->security;
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if (old_ctx) {
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new_ctx = new->security = kmalloc(sizeof(*new_ctx) +
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old_ctx->ctx_len,
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GFP_KERNEL);
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if (!new_ctx)
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return -ENOMEM;
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memcpy(new_ctx, old_ctx, sizeof(*new_ctx));
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memcpy(new_ctx->ctx_str, old_ctx->ctx_str, new_ctx->ctx_len);
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}
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return 0;
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}
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/*
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* LSM hook implementation that frees xfrm_policy security information.
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*/
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void selinux_xfrm_policy_free(struct xfrm_policy *xp)
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{
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struct xfrm_sec_ctx *ctx = xp->security;
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if (ctx)
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kfree(ctx);
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}
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/*
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* LSM hook implementation that allocs and transfers sec_ctx spec to
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* xfrm_state.
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*/
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int selinux_xfrm_state_alloc(struct xfrm_state *x, struct xfrm_user_sec_ctx *uctx)
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{
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int err;
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BUG_ON(!x);
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err = selinux_xfrm_sec_ctx_alloc(&x->security, uctx);
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return err;
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}
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/*
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* LSM hook implementation that frees xfrm_state security information.
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*/
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void selinux_xfrm_state_free(struct xfrm_state *x)
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{
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struct xfrm_sec_ctx *ctx = x->security;
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if (ctx)
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kfree(ctx);
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}
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/*
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* SELinux internal function to retrieve the context of a connected
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* (sk->sk_state == TCP_ESTABLISHED) TCP socket based on its security
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* association used to connect to the remote socket.
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*
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* Retrieve via getsockopt SO_PEERSEC.
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*/
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u32 selinux_socket_getpeer_stream(struct sock *sk)
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{
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struct dst_entry *dst, *dst_test;
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u32 peer_sid = SECSID_NULL;
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if (sk->sk_state != TCP_ESTABLISHED)
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goto out;
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dst = sk_dst_get(sk);
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if (!dst)
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goto out;
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for (dst_test = dst; dst_test != 0;
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dst_test = dst_test->child) {
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struct xfrm_state *x = dst_test->xfrm;
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if (x && selinux_authorizable_xfrm(x)) {
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struct xfrm_sec_ctx *ctx = x->security;
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peer_sid = ctx->ctx_sid;
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break;
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}
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}
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dst_release(dst);
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out:
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return peer_sid;
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}
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/*
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* SELinux internal function to retrieve the context of a UDP packet
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* based on its security association used to connect to the remote socket.
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*
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* Retrieve via setsockopt IP_PASSSEC and recvmsg with control message
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* type SCM_SECURITY.
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*/
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u32 selinux_socket_getpeer_dgram(struct sk_buff *skb)
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{
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struct sec_path *sp;
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if (skb == NULL)
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return SECSID_NULL;
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if (skb->sk->sk_protocol != IPPROTO_UDP)
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return SECSID_NULL;
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sp = skb->sp;
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if (sp) {
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int i;
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for (i = sp->len-1; i >= 0; i--) {
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struct xfrm_state *x = sp->x[i].xvec;
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if (selinux_authorizable_xfrm(x)) {
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struct xfrm_sec_ctx *ctx = x->security;
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return ctx->ctx_sid;
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}
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}
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}
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return SECSID_NULL;
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}
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/*
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* LSM hook that controls access to unlabelled packets. If
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* a xfrm_state is authorizable (defined by macro) then it was
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* already authorized by the IPSec process. If not, then
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* we need to check for unlabelled access since this may not have
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* gone thru the IPSec process.
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*/
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int selinux_xfrm_sock_rcv_skb(u32 isec_sid, struct sk_buff *skb)
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{
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int i, rc = 0;
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struct sec_path *sp;
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sp = skb->sp;
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if (sp) {
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/*
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* __xfrm_policy_check does not approve unless xfrm_policy_ok
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* says that spi's match for policy and the socket.
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*
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* Only need to verify the existence of an authorizable sp.
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*/
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for (i = 0; i < sp->len; i++) {
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struct xfrm_state *x = sp->x[i].xvec;
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if (x && selinux_authorizable_xfrm(x))
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goto accept;
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}
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}
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/* check SELinux sock for unlabelled access */
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rc = avc_has_perm(isec_sid, SECINITSID_UNLABELED, SECCLASS_ASSOCIATION,
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ASSOCIATION__RECVFROM, NULL);
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if (rc)
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goto drop;
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accept:
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return 0;
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drop:
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return rc;
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}
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/*
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* POSTROUTE_LAST hook's XFRM processing:
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* If we have no security association, then we need to determine
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* whether the socket is allowed to send to an unlabelled destination.
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* If we do have a authorizable security association, then it has already been
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* checked in xfrm_policy_lookup hook.
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*/
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int selinux_xfrm_postroute_last(u32 isec_sid, struct sk_buff *skb)
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{
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struct dst_entry *dst;
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int rc = 0;
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dst = skb->dst;
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if (dst) {
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struct dst_entry *dst_test;
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for (dst_test = dst; dst_test != 0;
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dst_test = dst_test->child) {
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struct xfrm_state *x = dst_test->xfrm;
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if (x && selinux_authorizable_xfrm(x))
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goto accept;
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}
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}
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rc = avc_has_perm(isec_sid, SECINITSID_UNLABELED, SECCLASS_ASSOCIATION,
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ASSOCIATION__SENDTO, NULL);
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if (rc)
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goto drop;
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accept:
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return NF_ACCEPT;
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drop:
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return NF_DROP;
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}
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