vhost_net: fix sleeping with preempt-disabled in vhost_net_busy_poll()

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

Commit 0308813724 ("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/[email protected]

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

Fixes: 0308813724 ("vhost_net: basic polling support")
Tested-by: [email protected]
Reported-by: [email protected]
Closes: https://lore.kernel.org/all/[email protected]/T/
Signed-off-by: Kohei Enju <[email protected]>
Acked-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Jakub Kicinski <[email protected]>

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);