linux-kernel - Re: [PATCH] vhost: handle zero regions in vhost_set

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Message-ID: <YhPS78dGWh3LY+8t@anirudhrb.com>
Date:   Mon, 21 Feb 2022 23:29:11 +0530
From:   Anirudh Rayabharam <mail@...rudhrb.com>
To:     Stefano Garzarella <sgarzare@...hat.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        syzbot+0abd373e2e50d704db87@...kaller.appspotmail.com,
        kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vhost: handle zero regions in vhost_set_memory

On Mon, Feb 21, 2022 at 05:48:17PM +0100, Stefano Garzarella wrote:
> On Mon, Feb 21, 2022 at 12:58:51PM +0530, Anirudh Rayabharam wrote:
> > Return early when userspace sends zero regions in the VHOST_SET_MEM_TABLE
> > ioctl.
> > 
> > Otherwise, this causes an erroneous entry to be added to the iotlb. This
> > entry has a range size of 0 (due to u64 overflow). This then causes
> > iotlb_access_ok() to loop indefinitely resulting in a hung thread.
> > Syzbot has reported this here:
> > 
> > https://syzkaller.appspot.com/bug?extid=0abd373e2e50d704db87
> 
> IIUC vhost_iotlb_add_range() in the for loop is never called if mem.nregions
> is 0, so I'm not sure the problem reported by syzbot is related.

Oh you're right. My bad! The problem is actually elsewhere.

> 
> In any case maybe this patch is fine, but currently I think we're just
> registering an iotlb without any regions, which in theory shouldn't cause
> any problems.

Yeah, there shouldn't be any problems here. The problem actually seems
to be in vhost_process_iotlb_msg() where we end up adding a zero sized
region to the iotlb. I will send a new patch...

Thanks!
	- Anirudh.

> 
> Thanks,
> Stefano
> 
> > 
> > Reported-and-tested-by: syzbot+0abd373e2e50d704db87@...kaller.appspotmail.com
> > Signed-off-by: Anirudh Rayabharam <mail@...rudhrb.com>
> > ---
> > drivers/vhost/vhost.c | 2 ++
> > 1 file changed, 2 insertions(+)
> > 
> > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > index 59edb5a1ffe2..821aba60eac2 100644
> > --- a/drivers/vhost/vhost.c
> > +++ b/drivers/vhost/vhost.c
> > @@ -1428,6 +1428,8 @@ static long vhost_set_memory(struct vhost_dev *d, struct vhost_memory __user *m)
> > 		return -EFAULT;
> > 	if (mem.padding)
> > 		return -EOPNOTSUPP;
> > +	if (mem.nregions == 0)
> > +		return 0;
> > 	if (mem.nregions > max_mem_regions)
> > 		return -E2BIG;
> > 	newmem = kvzalloc(struct_size(newmem, regions, mem.nregions),
> > -- 
> > 2.35.1
> > 
>