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>

1 files changed, 11 insertions, 11 deletions

diff --git a/fs/proc/base.c b/fs/proc/base.c
index f3c2e4085fed..175db258942f 100644
--- 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;
 }