![Andy Lutomirski](/assets/img/avatar_default.png)
It turns out that there's a lurking ABI issue. GCC, when compiling this in a 32-bit program: struct user_desc desc = { .entry_number = idx, .base_addr = base, .limit = 0xfffff, .seg_32bit = 1, .contents = 0, /* Data, grow-up */ .read_exec_only = 0, .limit_in_pages = 1, .seg_not_present = 0, .useable = 0, }; will leave .lm uninitialized. This means that anything in the kernel that reads user_desc.lm for 32-bit tasks is unreliable. Revert the .lm check in set_thread_area(). The value never did anything in the first place. Fixes:0e58af4e1d
("x86/tls: Disallow unusual TLS segments") Signed-off-by: Andy Lutomirski <luto@amacapital.net> Acked-by: Thomas Gleixner <tglx@linutronix.de> Cc: stable@vger.kernel.org # Only if0e58af4e1d
is backported Cc: Linus Torvalds <torvalds@linux-foundation.org> Link: http://lkml.kernel.org/r/d7875b60e28c512f6a6fc0baf5714d58e7eaadbb.1418856405.git.luto@amacapital.net Signed-off-by: Ingo Molnar <mingo@kernel.org>
253 lines
5.6 KiB
C
253 lines
5.6 KiB
C
#include <linux/kernel.h>
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#include <linux/errno.h>
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#include <linux/sched.h>
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#include <linux/user.h>
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#include <linux/regset.h>
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#include <linux/syscalls.h>
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#include <asm/uaccess.h>
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#include <asm/desc.h>
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#include <asm/ldt.h>
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#include <asm/processor.h>
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#include <asm/proto.h>
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#include "tls.h"
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/*
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* sys_alloc_thread_area: get a yet unused TLS descriptor index.
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*/
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static int get_free_idx(void)
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{
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struct thread_struct *t = ¤t->thread;
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int idx;
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for (idx = 0; idx < GDT_ENTRY_TLS_ENTRIES; idx++)
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if (desc_empty(&t->tls_array[idx]))
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return idx + GDT_ENTRY_TLS_MIN;
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return -ESRCH;
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}
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static bool tls_desc_okay(const struct user_desc *info)
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{
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if (LDT_empty(info))
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return true;
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/*
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* espfix is required for 16-bit data segments, but espfix
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* only works for LDT segments.
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*/
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if (!info->seg_32bit)
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return false;
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/* Only allow data segments in the TLS array. */
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if (info->contents > 1)
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return false;
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/*
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* Non-present segments with DPL 3 present an interesting attack
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* surface. The kernel should handle such segments correctly,
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* but TLS is very difficult to protect in a sandbox, so prevent
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* such segments from being created.
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*
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* If userspace needs to remove a TLS entry, it can still delete
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* it outright.
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*/
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if (info->seg_not_present)
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return false;
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return true;
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}
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static void set_tls_desc(struct task_struct *p, int idx,
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const struct user_desc *info, int n)
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{
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struct thread_struct *t = &p->thread;
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struct desc_struct *desc = &t->tls_array[idx - GDT_ENTRY_TLS_MIN];
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int cpu;
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/*
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* We must not get preempted while modifying the TLS.
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*/
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cpu = get_cpu();
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while (n-- > 0) {
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if (LDT_empty(info))
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desc->a = desc->b = 0;
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else
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fill_ldt(desc, info);
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++info;
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++desc;
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}
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if (t == ¤t->thread)
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load_TLS(t, cpu);
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put_cpu();
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}
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/*
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* Set a given TLS descriptor:
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*/
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int do_set_thread_area(struct task_struct *p, int idx,
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struct user_desc __user *u_info,
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int can_allocate)
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{
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struct user_desc info;
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if (copy_from_user(&info, u_info, sizeof(info)))
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return -EFAULT;
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if (!tls_desc_okay(&info))
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return -EINVAL;
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if (idx == -1)
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idx = info.entry_number;
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/*
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* index -1 means the kernel should try to find and
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* allocate an empty descriptor:
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*/
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if (idx == -1 && can_allocate) {
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idx = get_free_idx();
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if (idx < 0)
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return idx;
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if (put_user(idx, &u_info->entry_number))
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return -EFAULT;
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}
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if (idx < GDT_ENTRY_TLS_MIN || idx > GDT_ENTRY_TLS_MAX)
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return -EINVAL;
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set_tls_desc(p, idx, &info, 1);
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return 0;
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}
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SYSCALL_DEFINE1(set_thread_area, struct user_desc __user *, u_info)
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{
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return do_set_thread_area(current, -1, u_info, 1);
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}
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/*
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* Get the current Thread-Local Storage area:
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*/
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static void fill_user_desc(struct user_desc *info, int idx,
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const struct desc_struct *desc)
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{
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memset(info, 0, sizeof(*info));
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info->entry_number = idx;
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info->base_addr = get_desc_base(desc);
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info->limit = get_desc_limit(desc);
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info->seg_32bit = desc->d;
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info->contents = desc->type >> 2;
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info->read_exec_only = !(desc->type & 2);
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info->limit_in_pages = desc->g;
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info->seg_not_present = !desc->p;
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info->useable = desc->avl;
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#ifdef CONFIG_X86_64
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info->lm = desc->l;
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#endif
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}
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int do_get_thread_area(struct task_struct *p, int idx,
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struct user_desc __user *u_info)
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{
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struct user_desc info;
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if (idx == -1 && get_user(idx, &u_info->entry_number))
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return -EFAULT;
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if (idx < GDT_ENTRY_TLS_MIN || idx > GDT_ENTRY_TLS_MAX)
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return -EINVAL;
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fill_user_desc(&info, idx,
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&p->thread.tls_array[idx - GDT_ENTRY_TLS_MIN]);
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if (copy_to_user(u_info, &info, sizeof(info)))
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return -EFAULT;
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return 0;
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}
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SYSCALL_DEFINE1(get_thread_area, struct user_desc __user *, u_info)
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{
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return do_get_thread_area(current, -1, u_info);
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}
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int regset_tls_active(struct task_struct *target,
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const struct user_regset *regset)
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{
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struct thread_struct *t = &target->thread;
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int n = GDT_ENTRY_TLS_ENTRIES;
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while (n > 0 && desc_empty(&t->tls_array[n - 1]))
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--n;
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return n;
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}
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int regset_tls_get(struct task_struct *target, const struct user_regset *regset,
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unsigned int pos, unsigned int count,
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void *kbuf, void __user *ubuf)
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{
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const struct desc_struct *tls;
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if (pos >= GDT_ENTRY_TLS_ENTRIES * sizeof(struct user_desc) ||
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(pos % sizeof(struct user_desc)) != 0 ||
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(count % sizeof(struct user_desc)) != 0)
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return -EINVAL;
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pos /= sizeof(struct user_desc);
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count /= sizeof(struct user_desc);
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tls = &target->thread.tls_array[pos];
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if (kbuf) {
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struct user_desc *info = kbuf;
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while (count-- > 0)
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fill_user_desc(info++, GDT_ENTRY_TLS_MIN + pos++,
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tls++);
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} else {
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struct user_desc __user *u_info = ubuf;
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while (count-- > 0) {
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struct user_desc info;
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fill_user_desc(&info, GDT_ENTRY_TLS_MIN + pos++, tls++);
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if (__copy_to_user(u_info++, &info, sizeof(info)))
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return -EFAULT;
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}
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}
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return 0;
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}
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int regset_tls_set(struct task_struct *target, const struct user_regset *regset,
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unsigned int pos, unsigned int count,
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const void *kbuf, const void __user *ubuf)
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{
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struct user_desc infobuf[GDT_ENTRY_TLS_ENTRIES];
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const struct user_desc *info;
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int i;
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if (pos >= GDT_ENTRY_TLS_ENTRIES * sizeof(struct user_desc) ||
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(pos % sizeof(struct user_desc)) != 0 ||
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(count % sizeof(struct user_desc)) != 0)
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return -EINVAL;
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if (kbuf)
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info = kbuf;
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else if (__copy_from_user(infobuf, ubuf, count))
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return -EFAULT;
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else
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info = infobuf;
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for (i = 0; i < count / sizeof(struct user_desc); i++)
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if (!tls_desc_okay(info + i))
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return -EINVAL;
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set_tls_desc(target,
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GDT_ENTRY_TLS_MIN + (pos / sizeof(struct user_desc)),
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info, count / sizeof(struct user_desc));
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return 0;
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}
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