linux-hardening - [PATCH 1/3] signal: HANDLER_EXIT should clear SIGNAL

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Message-Id: <20220210025321.787113-2-keescook@chromium.org>
Date:   Wed,  9 Feb 2022 18:53:19 -0800
From:   Kees Cook <keescook@...omium.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Robert Święcki <robert@...ecki.net>,
        stable@...r.kernel.org, Andy Lutomirski <luto@...capital.net>,
        Will Drewry <wad@...omium.org>, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: [PATCH 1/3] signal: HANDLER_EXIT should clear SIGNAL_UNKILLABLE

Fatal SIGSYS signals were not being delivered to pid namespace init
processes. Make sure the SIGNAL_UNKILLABLE doesn't get set for these
cases.

Reported-by: Robert Święcki <robert@...ecki.net>
Suggested-by: "Eric W. Biederman" <ebiederm@...ssion.com>
Fixes: 00b06da29cf9 ("signal: Add SA_IMMUTABLE to ensure forced siganls do not get changed")
Cc: stable@...r.kernel.org
Signed-off-by: Kees Cook <keescook@...omium.org>
---
 kernel/signal.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index 38602738866e..33e3ee4f3383 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1342,9 +1342,10 @@ force_sig_info_to_task(struct kernel_siginfo *info, struct task_struct *t,
 	}
 	/*
 	 * Don't clear SIGNAL_UNKILLABLE for traced tasks, users won't expect
-	 * debugging to leave init killable.
+	 * debugging to leave init killable, unless it is intended to exit.
 	 */
-	if (action->sa.sa_handler == SIG_DFL && !t->ptrace)
+	if (action->sa.sa_handler == SIG_DFL &&
+	    (!t->ptrace || (handler == HANDLER_EXIT)))
 		t->signal->flags &= ~SIGNAL_UNKILLABLE;
 	ret = send_signal(sig, info, t, PIDTYPE_PID);
 	spin_unlock_irqrestore(&t->sighand->siglock, flags);
-- 
2.30.2