From fb3c434ba2ef9fe6a3ac531ae710e586568a80c7 Mon Sep 17 00:00:00 2001
From: Konstantin Belousov <kib@FreeBSD.org>
Date: Mon, 11 May 2020 22:38:32 +0000
Subject: [PATCH] sigfastblock: fix delivery of the pending signals in
 single-threaded processes.

If single-threaded process receives a signal during critical section
established by sigfastblock(2) word, unblock did not caused signal
delivery because sigfastblock(SIGFASTBLOCK_UNBLOCK) failed to request
ast handling of the pending signals.

Set TDF_ASTPENDING | TDF_NEEDSIGCHK on unblock or when kernel forces
end of sigfastblock critical section, to cause syscall exit to recheck
and deliver any signal pending.

Reported by:	corydoras@ridiculousfish.com
PR:	246385
Sponsored by:	The FreeBSD Foundation
---
 sys/kern/kern_sig.c | 31 +++++++++++++++++++------------
 1 file changed, 19 insertions(+), 12 deletions(-)

diff --git a/sys/kern/kern_sig.c b/sys/kern/kern_sig.c
index d36c382d3e23..7a99de63c655 100644
--- a/sys/kern/kern_sig.c
+++ b/sys/kern/kern_sig.c
@@ -3976,6 +3976,22 @@ sigfastblock_fetch_sig(struct thread *td, bool sendsig, uint32_t *valp)
 	return (true);
 }
 
+static void
+sigfastblock_resched(struct thread *td, bool resched)
+{
+	struct proc *p;
+
+	if (resched) {
+		p = td->td_proc;
+		PROC_LOCK(p);
+		reschedule_signals(p, td->td_sigmask, 0);
+		PROC_UNLOCK(p);
+	}
+	thread_lock(td);
+	td->td_flags |= TDF_ASTPENDING | TDF_NEEDSIGCHK;
+	thread_unlock(td);
+}
+
 int
 sys_sigfastblock(struct thread *td, struct sigfastblock_args *uap)
 {
@@ -4046,11 +4062,8 @@ sys_sigfastblock(struct thread *td, struct sigfastblock_args *uap)
 		 * signals to current thread.  But notify others about
 		 * fake unblock.
 		 */
-		if (error == 0 && p->p_numthreads != 1) {
-			PROC_LOCK(p);
-			reschedule_signals(p, td->td_sigmask, 0);
-			PROC_UNLOCK(p);
-		}
+		sigfastblock_resched(td, error == 0 && p->p_numthreads != 1);
+
 		break;
 
 	case SIGFASTBLOCK_UNSETPTR:
@@ -4079,7 +4092,6 @@ sys_sigfastblock(struct thread *td, struct sigfastblock_args *uap)
 void
 sigfastblock_clear(struct thread *td)
 {
-	struct proc *p;
 	bool resched;
 
 	if ((td->td_pflags & TDP_SIGFASTBLOCK) == 0)
@@ -4088,12 +4100,7 @@ sigfastblock_clear(struct thread *td)
 	resched = (td->td_pflags & TDP_SIGFASTPENDING) != 0 ||
 	    SIGPENDING(td);
 	td->td_pflags &= ~(TDP_SIGFASTBLOCK | TDP_SIGFASTPENDING);
-	if (resched) {
-		p = td->td_proc;
-		PROC_LOCK(p);
-		reschedule_signals(p, td->td_sigmask, 0);
-		PROC_UNLOCK(p);
-	}
+	sigfastblock_resched(td, resched);
 }
 
 void