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mm_for_maps: take ->cred_guard_mutex to fix the race with exec

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The problem is minor, but without ->cred_guard_mutex held we can race
with exec() and get the new ->mm but check old creds.

Now we do not need to re-check task->mm after ptrace_may_access(), it
can't be changed to the new mm under us.

Strictly speaking, this also fixes another very minor problem. Unless
security check fails or the task exits mm_for_maps() should never
return NULL, the caller should get either old or new ->mm.

Signed-off-by: Oleg Nesterov <[email protected]>
Acked-by: Serge Hallyn <[email protected]>
Signed-off-by: James Morris <[email protected]>
This commit is contained in:
Oleg Nesterov 2009-07-10 03:27:40 +02:00 • committed by James Morris
commit 704b836cbf
1 changed files with 11 additions and 11 deletions
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22
fs/proc/base.c
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@ -234,19 +234,19 @@ static int check_mem_permission(struct task_struct *task)
struct mm_struct *mm_for_maps(struct task_struct *task)
{
struct mm_struct *mm = get_task_mm(task);
struct mm_struct *mm;
if (mm && mm != current->mm) {
/*
* task->mm can be changed before security check,
* in that case we must notice the change after.
*/
if (!ptrace_may_access(task, PTRACE_MODE_READ) ||
mm != task->mm) {
mmput(mm);
mm = NULL;
}
if (mutex_lock_killable(&task->cred_guard_mutex))
return NULL;
mm = get_task_mm(task);
if (mm && mm != current->mm &&
!ptrace_may_access(task, PTRACE_MODE_READ)) {
mmput(mm);
mm = NULL;
}
mutex_unlock(&task->cred_guard_mutex);
return mm;
}