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Date:   Sun, 20 Feb 2022 05:08:30 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Hillf Danton <hdanton@...a.com>
Cc:     syzbot <syzbot+3140b17cb44a7b174008@...kaller.appspotmail.com>,
        jasowang@...hat.com, linux-kernel@...r.kernel.org,
        syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] kernel BUG in vhost_get_vq_desc

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);
> --

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