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

[Kyle Huey]

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@xxxxxxxxxxxxx>
Signed-off-by: Kyle Huey <khuey@xxxxxxxxxxxx>
---
 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