linux-kernel - [PATCH 3.16 155/202] signal: Restore the stop PTRACE_EVENT

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Message-ID: <lsq.1556377989.384011412@decadent.org.uk>
Date:   Sat, 27 Apr 2019 16:13:09 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, Denis Kirjanov <kda@...ux-powerpc.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "Oleg Nesterov" <oleg@...hat.com>,
        "Ivan Delalande" <colona@...sta.com>
Subject: [PATCH 3.16 155/202] signal: Restore the stop PTRACE_EVENT_EXIT

3.16.66-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: "Eric W. Biederman" <ebiederm@...ssion.com>

commit cf43a757fd49442bc38f76088b70c2299eed2c2f upstream.

In the middle of do_exit() there is there is a call
"ptrace_event(PTRACE_EVENT_EXIT, code);" That call places the process
in TACKED_TRACED aka "(TASK_WAKEKILL | __TASK_TRACED)" and waits for
for the debugger to release the task or SIGKILL to be delivered.

Skipping past dequeue_signal when we know a fatal signal has already
been delivered resulted in SIGKILL remaining pending and
TIF_SIGPENDING remaining set.  This in turn caused the
scheduler to not sleep in PTACE_EVENT_EXIT as it figured
a fatal signal was pending.  This also caused ptrace_freeze_traced
in ptrace_check_attach to fail because it left a per thread
SIGKILL pending which is what fatal_signal_pending tests for.

This difference in signal state caused strace to report
strace: Exit of unknown pid NNNNN ignored

Therefore update the signal handling state like dequeue_signal
would when removing a per thread SIGKILL, by removing SIGKILL
from the per thread signal mask and clearing TIF_SIGPENDING.

Acked-by: Oleg Nesterov <oleg@...hat.com>
Reported-by: Oleg Nesterov <oleg@...hat.com>
Reported-by: Ivan Delalande <colona@...sta.com>
Fixes: 35634ffa1751 ("signal: Always notice exiting tasks")
Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 kernel/signal.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2274,9 +2274,12 @@ relock:
 	}

 	/* Has this task already been marked for death? */
-	ksig->info.si_signo = signr = SIGKILL;
-	if (signal_group_exit(signal))
+	if (signal_group_exit(signal)) {
+		ksig->info.si_signo = signr = SIGKILL;
+		sigdelset(&current->pending.signal, SIGKILL);
+		recalc_sigpending();
 		goto fatal;
+	}

 	for (;;) {
 		struct k_sigaction *ka;