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 (file)
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -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;
 }