linux-kernel - [PATCH] seccomp: suppress fatal signals that will never be delivered before seccomp forces an exit because of said signals

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Date:	Wed, 10 Aug 2016 12:50:57 -0700
From:	Kyle Huey <me@...ehuey.com>
To:	Robert O'Callahan <robert@...llahan.org>
Cc:	Kees Cook <keescook@...omium.org>,
	Andy Lutomirski <luto@...capital.net>,
	Will Drewry <wad@...omium.org>,
	linux-kernel@...r.kernel.org (open list)
Subject: [PATCH] seccomp: suppress fatal signals that will never be delivered before seccomp forces an exit because of said signals

This fixes rr. It doesn't quite fix the provided testcase, because the testcase fails to wait on the tracee after awakening from the nanosleep. Instead the testcase immediately does a PTHREAD_CONT, discarding the PTHREAD_EVENT_EXIT. The slightly modified testcase at https://gist.github.com/khuey/3c43ac247c72cef8c956c does pass.

I don't see any obvious way to dequeue only the fatal signal, so instead I dequeue them all. Since none of these signals will ever be delivered it shouldn't affect the executing task.

Suggested-by: Robert O'Callahan <robert@...llahan.org>
Signed-off-by: Kyle Huey <khuey@...ehuey.com>
---
 kernel/seccomp.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index ef6c6c3..728074d 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -609,8 +609,20 @@ static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
 		 * Terminating the task now avoids executing a system
 		 * call that may not be intended.
 		 */
-		if (fatal_signal_pending(current))
+		if (fatal_signal_pending(current)) {
+			/*
+			 * Swallow the signals we will never deliver.
+			 * If we do not do this, the PTRACE_EVENT_EXIT will
+			 * be suppressed by those signals.
+			 */
+			siginfo_t info;
+
+			spin_lock_irq(&current->sighand->siglock);
+			while (dequeue_signal(current, &current->blocked, &info));
+			spin_unlock_irq(&current->sighand->siglock);
+
 			do_exit(SIGSYS);
+		}
 		/* Check if the tracer forced the syscall to be skipped. */
 		this_syscall = syscall_get_nr(current, task_pt_regs(current));
 		if (this_syscall < 0)
-- 
2.7.4