netdev - Re: [syzbot] [net?] [virt?] [kvm?] KASAN: slab-use-after-free Read in __vhost_vq_attach

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Date: Mon, 26 Jun 2023 11:36:23 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Mike Christie <michael.christie@...cle.com>
Cc: syzbot <syzbot+8540db210d403f1aa214@...kaller.appspotmail.com>,
	jasowang@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	syzkaller-bugs@...glegroups.com,
	virtualization@...ts.linux-foundation.org
Subject: Re: [syzbot] [net?] [virt?] [kvm?] KASAN: slab-use-after-free Read
 in __vhost_vq_attach_worker

On Mon, Jun 26, 2023 at 10:03:25AM -0500, Mike Christie wrote:
> On 6/26/23 2:15 AM, Michael S. Tsirkin wrote:
> > On Mon, Jun 26, 2023 at 12:06:54AM -0700, syzbot wrote:
> >> Hello,
> >>
> >> syzbot found the following issue on:
> >>
> >> HEAD commit:    8d2be868b42c Add linux-next specific files for 20230623
> >> git tree:       linux-next
> >> console+strace: https://syzkaller.appspot.com/x/log.txt?x=12872950a80000
> >> kernel config:  https://syzkaller.appspot.com/x/.config?x=d8ac8dd33677e8e0
> >> dashboard link: https://syzkaller.appspot.com/bug?extid=8540db210d403f1aa214
> >> compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> >> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=15c1b70f280000
> >> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=122ee4cb280000
> >>
> >> Downloadable assets:
> >> disk image: https://storage.googleapis.com/syzbot-assets/2a004483aca3/disk-8d2be868.raw.xz
> >> vmlinux: https://storage.googleapis.com/syzbot-assets/5688cb13b277/vmlinux-8d2be868.xz
> >> kernel image: https://storage.googleapis.com/syzbot-assets/76de0b63bc53/bzImage-8d2be868.xz
> >>
> >> The issue was bisected to:
> >>
> >> commit 21a18f4a51896fde11002165f0e7340f4131d6a0
> >> Author: Mike Christie <michael.christie@...cle.com>
> >> Date:   Tue Jun 13 01:32:46 2023 +0000
> >>
> >>     vhost: allow userspace to create workers
> >>
> >> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=130850bf280000
> >> final oops:     https://syzkaller.appspot.com/x/report.txt?x=108850bf280000
> >> console output: https://syzkaller.appspot.com/x/log.txt?x=170850bf280000
> >>
> >> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> >> Reported-by: syzbot+8540db210d403f1aa214@...kaller.appspotmail.com
> >> Fixes: 21a18f4a5189 ("vhost: allow userspace to create workers")
> > 
> > Mike, would appreciate prompt attention to this as I am preparing
> > a pull request for the merge window and need to make a
> > decision on whether to include your userspace-controlled
> > threading patchset.
> > 
> 
> Do you want me to resubmit the patchset or submit a patch against your vhost
> branch?

Resubmit pls.

> The bug is that vhost-net can call vhost_dev_reset_owner and that will
> free the workers. However, I leave the virtqueue->worker pointer set so
> we end up referencing the freed workers later on. When I handled a
> review comment between v5 and v6, I deleted that code thinking it was
> also not needed.
> 
> So the fix is:
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index ab79b064aade..5a07e220e46d 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -607,6 +607,10 @@ static void vhost_workers_free(struct vhost_dev *dev)
>  
>  	if (!dev->use_worker)
>  		return;
> +
> +	for (i = 0; i < dev->nvqs; i++)
> +		rcu_assign_pointer(dev->vqs[i]->worker, NULL);
> +
>  	/*
>  	 * Free the default worker we created and cleanup workers userspace
>  	 * created but couldn't clean up (it forgot or crashed).
> 
>