Re: [syzbot] [kvm?] [net?] [virt?] BUG: sleeping function called from invalid context in vhost_get_avail_idx
From: Kohei Enju
Date: Tue Apr 21 2026 - 13:13:26 EST
On 04/20 15:09, syzbot wrote:
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit: 8541d8f725c6 Merge tag 'mtd/for-7.1' of git://git.kernel.o..
> git tree: upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=136454ce580000
> kernel config: https://syzkaller.appspot.com/x/.config?x=7e54da1916e8d11f
> dashboard link: https://syzkaller.appspot.com/bug?extid=6985cb8e543ea90ba8ee
> compiler: gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44
> syz repro: https://syzkaller.appspot.com/x/repro.syz?x=15d264ce580000
> C reproducer: https://syzkaller.appspot.com/x/repro.c?x=143ec1ba580000
>
> Downloadable assets:
> disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/d900f083ada3/non_bootable_disk-8541d8f7.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/22dfea2c37c2/vmlinux-8541d8f7.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/e2f93ad68fe3/bzImage-8541d8f7.xz
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+6985cb8e543ea90ba8ee@xxxxxxxxxxxxxxxxxxxxxxxxx
>
> BUG: sleeping function called from invalid context at drivers/vhost/vhost.c:1527
> in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 6110, name: vhost-6109
> preempt_count: 1, expected: 0
> RCU nest depth: 0, expected: 0
> 2 locks held by vhost-6109/6110:
> #0: ffff888055624cb0 (&vq->mutex/1){+.+.}-{4:4}, at: handle_tx+0x2d/0x160 drivers/vhost/net.c:971
> #1: ffff888055620248 (&vq->mutex){+.+.}-{4:4}, at: vhost_net_busy_poll+0x9c/0x730 drivers/vhost/net.c:554
> Preemption disabled at:
> [<ffffffff88f1a006>] vhost_net_busy_poll+0x1c6/0x730 drivers/vhost/net.c:563
I think the blamed commit may be commit 030881372460 ("vhost_net: basic
polling support"), since it introduced preempt_{disable,enable}() around
the busy-poll loop, which calls a sleepable function inside the loop.
Also, from the changelog of the series,
https://lore.kernel.org/netdev/1448435489-5949-4-git-send-email-jasowang@xxxxxxxxxx/T/#u
Changes from RFC V1:
...
- Disable preemption during busy looping to make sure local_clock() was
correctly used.
So my understanding is that preempt_disable() was introduced to keep
local_clock() based timeout accounting on a single CPU, rather than as a
requirement of busy polling itself.
If my understanding is correct, migrate_disable() is sufficient here
instead of preempt_disable(), avoiding sleepable accesses from a
preempt-disabled context.
#syz test
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);
> CPU: 0 UID: 0 PID: 6110 Comm: vhost-6109 Not tainted syzkaller #0 PREEMPT(full)
> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
> Call Trace:
> <TASK>
> __dump_stack lib/dump_stack.c:94 [inline]
> dump_stack_lvl+0x100/0x190 lib/dump_stack.c:120
> __might_resched.cold+0x1ec/0x232 kernel/sched/core.c:9162
> __might_fault+0x8b/0x140 mm/memory.c:7322
> vhost_get_avail_idx+0x31c/0x4f0 drivers/vhost/vhost.c:1527
> vhost_vq_avail_empty drivers/vhost/vhost.c:3206 [inline]
> vhost_vq_avail_empty+0xa9/0xe0 drivers/vhost/vhost.c:3199
> vhost_net_busy_poll+0x297/0x730 drivers/vhost/net.c:574
> vhost_net_tx_get_vq_desc drivers/vhost/net.c:610 [inline]
> get_tx_bufs.constprop.0+0x338/0x600 drivers/vhost/net.c:650
> handle_tx_copy+0x28c/0x12e0 drivers/vhost/net.c:778
> handle_tx+0x139/0x160 drivers/vhost/net.c:985
> vhost_run_work_list+0x183/0x220 drivers/vhost/vhost.c:454
> vhost_task_fn+0x156/0x430 kernel/vhost_task.c:49
> ret_from_fork+0x72b/0xd50 arch/x86/kernel/process.c:158
> ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
> </TASK>
>
>
> ---
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