mm_for_maps: take ->cred_guard_mutex to fix the race with exec
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>
diff --git a/fs/proc/base.c b/fs/proc/base.c
index f3c2e40..175db25 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -234,19 +234,19 @@
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;
}