Fetch the sigfastblock value in syscalls that wait for signals

We have seen several cases of processes which have become "stuck" in
kern_sigsuspend(). When this occurs, the kernel's td_sigblock_val
is set to 0x10 (one block outstanding) and the userspace copy of the
word is set to 0 (unblocked). Because the kernel's cached value
shows that signals are blocked, kern_sigsuspend() blocks almost all
signals, which means the process hangs indefinitely in sigsuspend().

It is not entirely clear what is causing this condition to occur.
However, it seems to make sense to add some protection against this
case by fetching the latest sigfastblock value from userspace for
syscalls which will sleep waiting for signals. Here, the change is
applied to kern_sigsuspend() and kern_sigtimedwait().

Reviewed by:	kib
Sponsored by:	Netflix
Differential Revision:	https://reviews.freebsd.org/D29225

1 files changed, 6 insertions, 0 deletions

diff --git a/sys/kern/kern_sig.c b/sys/kern/kern_sig.c
index 7884b5be9f91..3d55405d3151 100644
--- a/sys/kern/kern_sig.c
+++ b/sys/kern/kern_sig.c
@@ -1268,6 +1268,9 @@ kern_sigtimedwait(struct thread *td, sigset_t waitset, ksiginfo_t *ksi,
 	ets.tv_nsec = 0;
 	traced = false;
 
+	/* Ensure the sigfastblock value is up to date. */
+	sigfastblock_fetch(td);
+
 	if (timeout != NULL) {
 		if (timeout->tv_nsec >= 0 && timeout->tv_nsec < 1000000000) {
 			timevalid = 1;
@@ -1527,6 +1530,9 @@ kern_sigsuspend(struct thread *td, sigset_t mask)
 	struct proc *p = td->td_proc;
 	int has_sig, sig;
 
+	/* Ensure the sigfastblock value is up to date. */
+	sigfastblock_fetch(td);
+
 	/*
 	 * When returning from sigsuspend, we want
 	 * the old mask to be restored after the