[PATCH net v1] vhost_net: fix sleeping with preempt-disabled in vhost_net_busy_poll()

[Kohei Enju <kohei@enjuk.jp>]

syzbot reported "sleeping function called from invalid context" in
vhost_net_busy_poll().

Commit 030881372460 ("vhost_net: basic polling support") introduced a
busy-poll loop and preempt_{disable,enable}() around it, where each
iteration calls a sleepable function inside the loop.

The purpose of disabling preemption was to keep local_clock()-based
timeout accounting on a single CPU, rather than as a requirement of
busy-poll itself:

https://lore.kernel.org/netdev/1448435489-5949-4-git-send-email-jasowang@redhat.com/T/#u

  Changes from RFC V1:
  ...
  - Disable preemption during busy looping to make sure local_clock()
    was correctly used.

From this perspective, migrate_disable() is sufficient here, so replace
preempt_disable() with migrate_disable(), avoiding sleepable accesses
from a preempt-disabled context.

Fixes: 030881372460 ("vhost_net: basic polling support")
Tested-by: syzbot+6985cb8e543ea90ba8ee@syzkaller.appspotmail.com
Reported-by: syzbot+6985cb8e543ea90ba8ee@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/69e6a414.050a0220.24bfd3.002d.GAE@google.com/T/
Signed-off-by: Kohei Enju <kohei@enjuk.jp>
---
 drivers/vhost/net.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index 80965181920c..c6536cad9c4f 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c
@@ -560,7 +560,7 @@ static void vhost_net_busy_poll(struct vhost_net *net,
 	busyloop_timeout = poll_rx ? rvq->busyloop_timeout:
 				     tvq->busyloop_timeout;
 
-	preempt_disable();
+	migrate_disable();
 	endtime = busy_clock() + busyloop_timeout;
 
 	while (vhost_can_busy_poll(endtime)) {
@@ -577,7 +577,7 @@ static void vhost_net_busy_poll(struct vhost_net *net,
 		cpu_relax();
 	}
 
-	preempt_enable();
+	migrate_enable();
 
 	if (poll_rx || sock_has_rx_data(sock))
 		vhost_net_busy_poll_try_queue(net, vq);
-- 
2.51.0