[PATCH v2] signal: respect the blocked mask in dequeue_synchronous_signal()

[Avi Weiss <thnkslprpt@gmail.com>]

dequeue_synchronous_signal() uses the signal number and a positive
si_code to identify synchronous signals. An ordinarily queued
signal can also match this heuristic.

The initial pending-mask check only establishes that an unblocked
signal in SYNCHRONOUS_MASK is pending. The queue scan can select a
different blocked signal matching the heuristic and deliver it
despite its mask.

For example, a thread can block SIGSEGV and queue it to itself with
a positive si_code using rt_tgsigqueueinfo(). An unblocked SIGTRAP
then allows the scan to select the blocked SIGSEGV.

Check each candidate against the blocked mask. Genuine forced faults
are unblocked by force_sig_info_to_task() when necessary.

Fixes: 7146db3317c6 ("signal: Better detection of synchronous signals")
Signed-off-by: Avi Weiss <thnkslprpt@gmail.com>
---

Reproduced by blocking SIGSEGV, queuing a positive-code SIGSEGV
with rt_tgsigqueueinfo(), then triggering an unblocked SIGTRAP.
Without the fix, SIGSEGV was delivered while still blocked.

Tested unpatched and patched kernels under QEMU on UP x86-64, with
test processes running as UID/GID 1000. The unpatched kernel passed
21/38 checks and the patched kernel passed 38/38.

These results are from the original patch base. The rebased v2 has
not been runtime-tested separately.

Changes in v2:
- Restore the original queue-scan comment following Eric's feedback.
- Clarify that ordinarily queued signals can match the
  synchronous-signal heuristic.
- Rebase onto current mainline.

 kernel/signal.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index cb6ec7054eee..7e690f4fb5bc 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -689,7 +689,8 @@ static int dequeue_synchronous_signal(kernel_siginfo_t *info)
 	list_for_each_entry(q, &pending->list, list) {
 		/* Synchronous signals have a positive si_code */
 		if ((q->info.si_code > SI_USER) &&
-		    (sigmask(q->info.si_signo) & SYNCHRONOUS_MASK)) {
+		    (sigmask(q->info.si_signo) & SYNCHRONOUS_MASK) &&
+		    !sigismember(&tsk->blocked, q->info.si_signo)) {
 			sync = q;
 			goto next;
 		}

base-commit: 4403278e4ea2f654f8130a5c03eb8b4516a0f201
-- 
2.43.0