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Date:   Tue, 04 Apr 2017 14:47:34 -0700
To:     Oleg Nesterov <>
        Roland McGrath <>,
        Andrew Morton <>,
        Tejun Heo <>
Subject: [PATCHv2] ptrace: fix PTRACE_LISTEN race corrupting task->state

In PT_SEIZED + LISTEN mode STOP/CONT signals cause a wakeup against
__TASK_TRACED. If this races with the ptrace_unfreeze_traced at the end
of a PTRACE_LISTEN, this can wake the task /after/ the check against
__TASK_TRACED, but before the reset of state to TASK_TRACED. This causes
it to instead clobber TASK_WAKING, allowing a subsequent wakeup against
TRACED while the task is still on the rq wake_list, corrupting it.

Signed-off-by: Ben Segall <>

v2: slight clarification in comments, put the conditional around the
whole wakeup area

Oleg mentioned a preference for making LISTEN unfreeze instead; I have
no preference there, just wanted to make sure that this doesn't get
forgotten entirely.

 kernel/ptrace.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 0af928712174..7cc49c3e73af 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -184,11 +184,17 @@ static void ptrace_unfreeze_traced(struct task_struct *task)
        WARN_ON(!task->ptrace || task->parent != current);
+       /*
+        * PTRACE_LISTEN can allow ptrace_trap_notify to wake us up
+        * remotely. Recheck state under the lock to close this race.
+        */
-       if (__fatal_signal_pending(task))
-               wake_up_state(task, __TASK_TRACED);
-       else
-               task->state = TASK_TRACED;
+       if (task->state == __TASK_TRACED) {
+               if (__fatal_signal_pending(task))
+                       wake_up_state(task, __TASK_TRACED);
+               else
+                       task->state = TASK_TRACED;
+       }

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