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Date:   Mon, 21 Feb 2022 13:41:34 +0530
From:   Anirudh Rayabharam <mail@anirudhrb.com>
To:     Jason Wang <jasowang@redhat.com>
Cc:     "Michael S. Tsirkin" <mst@redhat.com>,
        syzbot+0abd373e2e50d704db87@syzkaller.appspotmail.com,
        kvm <kvm@vger.kernel.org>,
        virtualization <virtualization@lists.linux-foundation.org>,
        netdev <netdev@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>, mail@anirudhrb.com
Subject: Re: [PATCH] vhost: handle zero regions in vhost_set_memory
Message-ID: <YhNJNh8N1GW6UZGo@anirudhrb.com>
References: <20220221072852.31820-1-mail@anirudhrb.com>
 <CACGkMEs6HLM3ok29rm4u=Tq2preno_60Z6cvKw2T7=nak2yzkQ@mail.gmail.com>
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On Mon, Feb 21, 2022 at 03:56:29PM +0800, Jason Wang wrote:
> On Mon, Feb 21, 2022 at 3:45 PM Anirudh Rayabharam <mail@anirudhrb.com> 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:
> 
> Interesting, I think iotlb_access_ok() won't be called for memory
> table entries, or anything I missed?

Yeah, vhost_set_memory() doesn't call iotlb_access_ok(). The issue
appears when it is called later in a different code path. Here's the
trace for iotlb_access_ok():

NMI backtrace for cpu 1
CPU: 1 PID: 3633 Comm: vhost-3632 Not tainted 5.17.0-rc3-syzkaller-00029-ge6251ab4551f #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:check_kcov_mode kernel/kcov.c:166 [inline]
RIP: 0010:__sanitizer_cov_trace_pc+0xd/0x60 kernel/kcov.c:200
Code: 00 00 e9 c6 41 66 02 66 0f 1f 44 00 00 48 8b be b0 01 00 00 e8 b4 ff ff ff 31 c0 c3 90 65 8b 05 29 f7 89 7e 89 c1 48 8b 34 24 <81> e1 00 01 00 00 65 48 8b 14 25 00 70 02 00 a9 00 01 ff 00 74 0e
RSP: 0018:ffffc90000cd7c78 EFLAGS: 00000246
RAX: 0000000080000000 RBX: ffff888079ca8a80 RCX: 0000000080000000
RDX: 0000000000000000 RSI: ffffffff86d3f8fb RDI: 0000000000000003
RBP: 0000000000000000 R08: 0000000000000000 R09: ffffc90000cd7c77
R10: ffffffff86d3f8ed R11: 0000000000000001 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000000000001 R15: 0000000000000000
FS:  0000000000000000(0000) GS:ffff8880b9d00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007ffdf716a3b8 CR3: 00000000235b6000 CR4: 00000000003506e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 <TASK>
 iotlb_access_ok+0x21b/0x3e0 drivers/vhost/vhost.c:1340
 vq_meta_prefetch+0xbc/0x280 drivers/vhost/vhost.c:1366
 vhost_transport_do_send_pkt+0xe0/0xfd0 drivers/vhost/vsock.c:104
 vhost_worker+0x23d/0x3d0 drivers/vhost/vhost.c:372
 kthread+0x2e9/0x3a0 kernel/kthread.c:377
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295
 </TASK>
----------------

Thanks,

	- Anirudh
> 
> (If this is not true, we need a kernel patch as well).
> 
> Thanks
> 
> >
> > https://syzkaller.appspot.com/bug?extid=0abd373e2e50d704db87
> >
> > Reported-and-tested-by: syzbot+0abd373e2e50d704db87@syzkaller.appspotmail.com
> > Signed-off-by: Anirudh Rayabharam <mail@anirudhrb.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
> >
>