Patch "ptrace: fix PTRACE_LISTEN race corrupting task->state" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

    ptrace: fix PTRACE_LISTEN race corrupting task->state

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
ptrace-fix-ptrace_listen-race-corrupting-task-state.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: "[email protected]" <[email protected]>
Date: Fri, 7 Apr 2017 16:04:51 -0700
Subject: ptrace: fix PTRACE_LISTEN race corrupting task->state

From: [email protected] <[email protected]>

commit 5402e97af667e35e54177af8f6575518bf251d51 upstream.

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.

Oleg said:
"The kernel can crash or this can lead to other hard-to-debug problems.
In short, "task->state = TASK_TRACED" in ptrace_unfreeze_traced() assumes that nobody else can wake it up, but PTRACE_LISTEN breaks the contract. Obviusly it is very wrong to manipulate task->state if this task is already running, or WAKING, or it sleeps again"

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

@@ -151,11 +151,17 @@ static void ptrace_unfreeze_traced(struc
 
 	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.
+	 */
 	spin_lock_irq(&task->sighand->siglock);
-	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;
+	}
 	spin_unlock_irq(&task->sighand->siglock);
 }
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/ptrace-fix-ptrace_listen-race-corrupting-task-state.patch