Re: [syzbot] kernel BUG in vhost_get_vq_desc

Michael S. Tsirkin
2022-02-20T10:08:44Z

On Sun, Feb 20, 2022 at 09:47:15AM +0800, Hillf Danton wrote:
> On Sat, 19 Feb 2022 05:01:10 -0800
> > Hello,
> >
> > syzbot has tested the proposed patch but the reproducer is still triggering an issue:
> > kernel BUG in vhost_get_vq_desc
>
> The WARNING: CPU: 1 PID: 4052 at drivers/vhost/vhost.c:715 got quiesced.
> >
> > ------------[ cut here ]------------
> > kernel BUG at drivers/vhost/vhost.c:2338!
>
> Given the mutex_lock(&vq->mutex) in vhost_vsock_handle_tx_kick(), this
> report proves that the bug is bogus.
>
> > invalid opcode: 0000 [#1] PREEMPT SMP KASAN
> > CPU: 0 PID: 4071 Comm: vhost-4070 Not tainted 5.17.0-rc4-syzkaller-00054-gf71077a4d84b-dirty #0
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> > RIP: 0010:vhost_get_vq_desc+0x1dc5/0x2350 drivers/vhost/vhost.c:2338
> > Code: 00 00 00 48 c7 c6 20 2c 9d 8a 48 c7 c7 98 a6 8e 8d 48 89 ca 48 c1 e1 04 48 01 d9 e8 25 59 28 fd e9 74 ff ff ff e8 cb c7 a1 fa <0f> 0b e8 c4 c7 a1 fa 48 8b 54 24 18 48 b8 00 00 00 00 00 fc ff df
> > RSP: 0018:ffffc900028bfb78 EFLAGS: 00010293
> > RAX: 0000000000000000 RBX: 0000000000000001 RCX: 0000000000000000
> > RDX: ffff88801cbd1d00 RSI: ffffffff86d71655 RDI: 0000000000000003
> > RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000001
> > R10: ffffffff86d7072d R11: 0000000000000000 R12: 0000000000000000
> > R13: 0000000000000000 R14: ffff88806ffc4bb0 R15: dffffc0000000000
> > FS: 0000000000000000(0000) GS:ffff8880b9c00000(0000) knlGS:0000000000000000
> > CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > CR2: 0000000000000002 CR3: 000000001d077000 CR4: 00000000003506f0
> > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > Call Trace:
> > <TASK>
> > vhost_vsock_handle_tx_kick+0x277/0xa20 drivers/vhost/vsock.c:522
> > vhost_worker+0x2e9/0x3e0 drivers/vhost/vhost.c:374
> > kthread+0x2e9/0x3a0 kernel/kthread.c:377
> > ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295
> > </TASK>
> > Modules linked in:
> > ---[ end trace 0000000000000000 ]---
> > RIP: 0010:vhost_get_vq_desc+0x1dc5/0x2350 drivers/vhost/vhost.c:2338
> > Code: 00 00 00 48 c7 c6 20 2c 9d 8a 48 c7 c7 98 a6 8e 8d 48 89 ca 48 c1 e1 04 48 01 d9 e8 25 59 28 fd e9 74 ff ff ff e8 cb c7 a1 fa <0f> 0b e8 c4 c7 a1 fa 48 8b 54 24 18 48 b8 00 00 00 00 00 fc ff df
> > RSP: 0018:ffffc900028bfb78 EFLAGS: 00010293
> > RAX: 0000000000000000 RBX: 0000000000000001 RCX: 0000000000000000
> > RDX: ffff88801cbd1d00 RSI: ffffffff86d71655 RDI: 0000000000000003
> > RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000001
> > R10: ffffffff86d7072d R11: 0000000000000000 R12: 0000000000000000
> > R13: 0000000000000000 R14: ffff88806ffc4bb0 R15: dffffc0000000000
> > FS: 0000000000000000(0000) GS:ffff8880b9d00000(0000) knlGS:0000000000000000
> > CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > CR2: 00007fc7293991d0 CR3: 000000001d077000 CR4: 00000000003506e0
> > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> >
> >
> > Tested on:
> >
> > commit: f71077a4 Merge tag 'mmc-v5.17-rc1-2' of git://git.kern..
> > git tree: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/
> > console output: https://syzkaller.appspot.com/x/log.txt?x=11e82d7a700000
> > kernel config: https://syzkaller.appspot.com/x/.config?x=a78b064590b9f912
> > dashboard link: https://syzkaller.appspot.com/bug?extid=3140b17cb44a7b174008
> > compiler: gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> > patch: https://syzkaller.appspot.com/x/patch.diff?x=11857326700000
>
> Attempted fix is bail out if anything eerie is detected in terms of the
> notify flag.

I mean this will fix the warning for sure, but do we understand how
it might have triggered?

> Hillf
>
> #syz test: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/ f71077a4d84b
>
> --- x/drivers/vhost/vhost.c
> +++ y/drivers/vhost/vhost.c
> @@ -353,14 +353,16 @@ static int vhost_worker(void *data)
> /* mb paired w/ kthread_stop */
> set_current_state(TASK_INTERRUPTIBLE);
>
> - if (kthread_should_stop()) {
> - __set_current_state(TASK_RUNNING);
> - break;
> - }
> -
> node = llist_del_all(&dev->work_list);
> - if (!node)
> + if (!node) {
> + if (kthread_should_stop()) {
> + __set_current_state(TASK_RUNNING);
> + break;
> + }
> +
> schedule();
> + continue;
> + }
>
> node = llist_reverse_order(node);
> /* make sure flag is seen after deletion */
> @@ -712,12 +714,12 @@ void vhost_dev_cleanup(struct vhost_dev
> dev->iotlb = NULL;
> vhost_clear_msg(dev);
> wake_up_interruptible_poll(&dev->wait, EPOLLIN | EPOLLRDNORM);
> - WARN_ON(!llist_empty(&dev->work_list));
> if (dev->worker) {
> kthread_stop(dev->worker);
> dev->worker = NULL;
> dev->kcov_handle = 0;
> }
> + WARN_ON(!llist_empty(&dev->work_list));
> vhost_detach_mm(dev);
> }
> EXPORT_SYMBOL_GPL(vhost_dev_cleanup);
> @@ -2207,7 +2209,10 @@ int vhost_get_vq_desc(struct vhost_virtq
> __virtio16 avail_idx;
> __virtio16 ring_head;
> int ret, access;
> + bool was_set = !!(vq->used_flags & VRING_USED_F_NO_NOTIFY);
>
> + if (!was_set)
> + return -EINVAL;
> /* Check it isn't doing very strange things with descriptor numbers. */
> last_avail_idx = vq->last_avail_idx;
>
> @@ -2327,12 +2332,14 @@ int vhost_get_vq_desc(struct vhost_virtq
> }
> } while ((i = next_desc(vq, &desc)) != -1);
>
> + /* Assume notifications from guest are disabled at this point,
> + * if they aren't we would need to update avail_event index. */
> + if (!!(vq->used_flags & VRING_USED_F_NO_NOTIFY) != was_set)
> + return -EINVAL;
> +
> /* On success, increment avail index. */
> vq->last_avail_idx++;
>
> - /* Assume notifications from guest are disabled at this point,
> - * if they aren't we would need to update avail_event index. */
> - BUG_ON(!(vq->used_flags & VRING_USED_F_NO_NOTIFY));
> return head;
> }
> EXPORT_SYMBOL_GPL(vhost_get_vq_desc);
> --