linux-kernel - [PATCH 1/3] signal: In get_signal test for signal_group

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Date:   Mon, 15 Nov 2021 23:32:50 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     <linux-kernel@...r.kernel.org>
Cc:     Kyle Huey <me@...ehuey.com>, Jens Axboe <axboe@...nel.dk>,
        Peter Zijlstra <peterz@...radead.org>,
        Marco Elver <elver@...gle.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Collingbourne <pcc@...gle.com>,
        Alexey Gladkov <legion@...nel.org>,
        "Robert O'Callahan" <rocallahan@...il.com>,
        Marko Mäkelä 
        <marko.makela@...iadb.com>, <linux-api@...r.kernel.org>,
        Al Viro <viro@...IV.linux.org.uk>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>
Subject: [PATCH 1/3] signal: In get_signal test for signal_group_exit every time through the loop


Recently while investigating a problem with rr and signals I noticed
that siglock is dropped in ptrace_signal and get_signal does not jump
to relock.

Looking farther to see if the problem is anywhere else I see that
do_signal_stop also returns if signal_group_exit is true.  I believe
that test can now never be true, but it is a bit hard to trace
through and be certain.

Testing signal_group_exit is not expensive, so move the test for
signal_group_exit into the for loop inside of get_signal to ensure
the test is never skipped improperly.

This has been a potential since I added the test for signal_group_exit
was added.

Fixes: 35634ffa1751 ("signal: Always notice exiting tasks")
Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
---
 kernel/signal.c | 20 ++++++++++----------
 1 file changed, 10 insertions(+), 10 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index 7c4b7ae714d4..986fa69c15c5 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2662,19 +2662,19 @@ bool get_signal(struct ksignal *ksig)
 		goto relock;
 	}
 
-	/* Has this task already been marked for death? */
-	if (signal_group_exit(signal)) {
-		ksig->info.si_signo = signr = SIGKILL;
-		sigdelset(&current->pending.signal, SIGKILL);
-		trace_signal_deliver(SIGKILL, SEND_SIG_NOINFO,
-				&sighand->action[SIGKILL - 1]);
-		recalc_sigpending();
-		goto fatal;
-	}
-
 	for (;;) {
 		struct k_sigaction *ka;
 
+		/* Has this task already been marked for death? */
+		if (signal_group_exit(signal)) {
+			ksig->info.si_signo = signr = SIGKILL;
+			sigdelset(&current->pending.signal, SIGKILL);
+			trace_signal_deliver(SIGKILL, SEND_SIG_NOINFO,
+				&sighand->action[SIGKILL - 1]);
+			recalc_sigpending();
+			goto fatal;
+		}
+
 		if (unlikely(current->jobctl & JOBCTL_STOP_PENDING) &&
 		    do_signal_stop(0))
 			goto relock;
-- 
2.20.1