[PATCH] fs/select: Fix data races to pwq->triggered

[Marco Elver <elver@google.com>]

Fix data races to pwq->triggered by using READ_ONCE. These accesses are
expected to race per comment and already provided memory barriers
surrounding the accesses.

Original KCSAN report:

write to 0xffffc90001043c30 of 4 bytes by task 17695 on cpu 0:
 __pollwake fs/select.c:197 [inline]
 pollwake+0xe3/0x140 fs/select.c:217
 __wake_up_common+0x7b/0x180 kernel/sched/wait.c:93
 __wake_up_common_lock+0x77/0xb0 kernel/sched/wait.c:123
 __wake_up+0xe/0x10 kernel/sched/wait.c:142
 signalfd_notify include/linux/signalfd.h:22 [inline]
 signalfd_notify include/linux/signalfd.h:19 [inline]
 __send_signal+0x70e/0x870 kernel/signal.c:1158
 send_signal+0x224/0x2b0 kernel/signal.c:1236
 __group_send_sig_info kernel/signal.c:1275 [inline]
 do_notify_parent+0x55b/0x5e0 kernel/signal.c:1992
 exit_notify kernel/exit.c:670 [inline]
 do_exit+0x16ef/0x18c0 kernel/exit.c:818
 do_group_exit+0xb4/0x1c0 kernel/exit.c:895
 __do_sys_exit_group kernel/exit.c:906 [inline]
 __se_sys_exit_group kernel/exit.c:904 [inline]
 __x64_sys_exit_group+0x2e/0x30 kernel/exit.c:904
 do_syscall_64+0xcc/0x3a0 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffffc90001043c30 of 4 bytes by task 15995 on cpu 1:
 poll_schedule_timeout.constprop.0+0x50/0xc0 fs/select.c:242
 do_poll fs/select.c:951 [inline]
 do_sys_poll+0x66d/0x990 fs/select.c:1001
 __do_sys_poll fs/select.c:1059 [inline]
 __se_sys_poll fs/select.c:1047 [inline]
 __x64_sys_poll+0x10f/0x250 fs/select.c:1047
 do_syscall_64+0xcc/0x3a0 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported-by: syzbot+bd0076c6e839503096b5@syzkaller.appspotmail.com
Signed-off-by: Marco Elver <elver@google.com>
---
 fs/select.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/select.c b/fs/select.c
index 11d0285d46b7..152987ddf40e 100644
--- a/fs/select.c
+++ b/fs/select.c
@@ -194,7 +194,7 @@ static int __pollwake(wait_queue_entry_t *wait, unsigned mode, int sync, void *k
 	 * and is paired with smp_store_mb() in poll_schedule_timeout.
 	 */
 	smp_wmb();
-	pwq->triggered = 1;
+	WRITE_ONCE(pwq->triggered, 1);
 
 	/*
 	 * Perform the default wake up operation using a dummy
@@ -239,7 +239,7 @@ static int poll_schedule_timeout(struct poll_wqueues *pwq, int state,
 	int rc = -EINTR;
 
 	set_current_state(state);
-	if (!pwq->triggered)
+	if (!READ_ONCE(pwq->triggered))
 		rc = schedule_hrtimeout_range(expires, slack, HRTIMER_MODE_ABS);
 	__set_current_state(TASK_RUNNING);
 
-- 
2.24.1.735.g03f4e72817-goog