[PATCH 5.16 189/203] signal: HANDLER_EXIT should clear SIGNAL_UNKILLABLE

[Greg Kroah-Hartman]

From: Kees Cook <keescook@xxxxxxxxxxxx>

commit 5c72263ef2fbe99596848f03758ae2dc593adf2c upstream.

Fatal SIGSYS signals (i.e. seccomp RET_KILL_* syscall filter actions)
were not being delivered to ptraced pid namespace init processes. Make
sure the SIGNAL_UNKILLABLE doesn't get set for these cases.

Reported-by: Robert Święcki <robert@xxxxxxxxxxx>
Suggested-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
Fixes: 00b06da29cf9 ("signal: Add SA_IMMUTABLE to ensure forced siganls do not get changed")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
Reviewed-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
Link: https://lore.kernel.org/lkml/878rui8u4a.fsf@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 kernel/signal.c |    5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1339,9 +1339,10 @@ force_sig_info_to_task(struct kernel_sig
 	}
 	/*
 	 * Don't clear SIGNAL_UNKILLABLE for traced tasks, users won't expect
-	 * debugging to leave init killable.
+	 * debugging to leave init killable. But HANDLER_EXIT is always fatal.
 	 */
-	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);