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Subject[patch 69/74] mm_for_maps: take ->cred_guard_mutex to fix the race with exec
2.6.30-stable review patch.  If anyone has any objections, please let us know.

------------------

From: Oleg Nesterov <oleg@redhat.com>

commit 704b836cbf19e885f8366bccb2e4b0474346c02d upstream.

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 <oleg@redhat.com>
Acked-by: Serge Hallyn <serue@us.ibm.com>
Signed-off-by: James Morris <jmorris@namei.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
fs/proc/base.c | 22 +++++++++++-----------
1 file changed, 11 insertions(+), 11 deletions(-)

--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -234,19 +234,19 @@ static int check_mem_permission(struct t

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_exec_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_exec_mutex);
+
return mm;
}




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