[tip: sched/urgent] freezer,umh: Fix call_usermode_helper_exec() vs SIGKILL

[tip-bot2 for Peter Zijlstra]

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID:     eedeb787ebb53de5c5dcf7b7b39d01bf1b0f037d
Gitweb:        https://git.kernel.org/tip/eedeb787ebb53de5c5dcf7b7b39d01bf1b0f037d
Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate:    Fri, 03 Feb 2023 15:31:11 +01:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Mon, 13 Feb 2023 16:36:14 +01:00

freezer,umh: Fix call_usermode_helper_exec() vs SIGKILL

Tetsuo-San noted that commit f5d39b020809 ("freezer,sched: Rewrite
core freezer logic") broke call_usermodehelper_exec() for the KILLABLE
case.

Specifically it was missed that the second, unconditional,
wait_for_completion() was not optional and ensures the on-stack
completion is unused before going out-of-scope.

Fixes: f5d39b020809 ("freezer,sched: Rewrite core freezer logic")
Reported-by: syzbot+6cd18e123583550cf469@xxxxxxxxxxxxxxxxxxxxxxxxx
Reported-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Debugged-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/Y90ar35uKQoUrLEK@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
---
 kernel/umh.c | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/kernel/umh.c b/kernel/umh.c
index 8506315..fbf872c 100644
--- a/kernel/umh.c
+++ b/kernel/umh.c
@@ -438,21 +438,27 @@ int call_usermodehelper_exec(struct subprocess_info *sub_info, int wait)
 	if (wait == UMH_NO_WAIT)	/* task has freed sub_info */
 		goto unlock;
 
-	if (wait & UMH_KILLABLE)
-		state |= TASK_KILLABLE;
-
 	if (wait & UMH_FREEZABLE)
 		state |= TASK_FREEZABLE;
 
-	retval = wait_for_completion_state(&done, state);
-	if (!retval)
-		goto wait_done;
-
 	if (wait & UMH_KILLABLE) {
+		retval = wait_for_completion_state(&done, state | TASK_KILLABLE);
+		if (!retval)
+			goto wait_done;
+
 		/* umh_complete() will see NULL and free sub_info */
 		if (xchg(&sub_info->complete, NULL))
 			goto unlock;
+
+		/*
+		 * fallthrough; in case of -ERESTARTSYS now do uninterruptible
+		 * wait_for_completion_state(). Since umh_complete() shall call
+		 * complete() in a moment if xchg() above returned NULL, this
+		 * uninterruptible wait_for_completion_state() will not block
+		 * SIGKILL'ed processes for long.
+		 */
 	}
+	wait_for_completion_state(&done, state);
 
 wait_done:
 	retval = sub_info->retval;