ptrace_stop: fix the race with ptrace detach+attach

If the tracer went away (may_ptrace_stop() failed), ptrace_stop() drops
tasklist and then changes the ->state from TASK_TRACED to TASK_RUNNING.

This can fool another tracer which attaches to us in between.  Change the
->state under tasklist_lock to ensure that ptrace_check_attach() can't wrongly
succeed.  Also, remove the unnecessary mb().

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Acked-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 3 insertions, 3 deletions

diff --git a/kernel/signal.c b/kernel/signal.c
index 6d6d1ab39e7e..678bffa437c1 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1638,11 +1638,11 @@ static void ptrace_stop(int exit_code, int nostop_code, siginfo_t *info)
         } else {
                 /*
                  * By the time we got the lock, our tracer went away.
-                 * Don't stop here.
+                 * Don't drop the lock yet, another tracer may come.
                  */
-                read_unlock(&tasklist_lock);
-                set_current_state(TASK_RUNNING);
+                __set_current_state(TASK_RUNNING);
                 current->exit_code = nostop_code;
+                read_unlock(&tasklist_lock);
         }
 
         /*