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From:   Jason Wang <jasowang@redhat.com>
Date:   Tue, 22 Feb 2022 15:11:07 +0800
Message-ID: <CACGkMEvd7ETC_ANyrOSAVz_i64xqpYYazmm=+39E51=DMRFXdw@mail.gmail.com>
Subject: Re: [PATCH] vhost: validate range size before adding to iotlb
To:     Anirudh Rayabharam <mail@anirudhrb.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>
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On Tue, Feb 22, 2022 at 12:57 PM Anirudh Rayabharam <mail@anirudhrb.com> wrote:
>
> On Tue, Feb 22, 2022 at 10:50:20AM +0800, Jason Wang wrote:
> > On Tue, Feb 22, 2022 at 3:53 AM Anirudh Rayabharam <mail@anirudhrb.com> wrote:
> > >
> > > In vhost_iotlb_add_range_ctx(), validate the range size is non-zero
> > > before proceeding with adding it to the iotlb.
> > >
> > > Range size can overflow to 0 when start is 0 and last is (2^64 - 1).
> > > One instance where it can happen is when userspace sends an IOTLB
> > > message with iova=size=uaddr=0 (vhost_process_iotlb_msg). So, an
> > > entry with size = 0, start = 0, last = (2^64 - 1) ends up in the
> > > iotlb. Next time a packet is sent, iotlb_access_ok() loops
> > > indefinitely due to that erroneous entry:
> > >
> > >         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>
> > >
> > > Reported by syzbot at:
> > >         https://syzkaller.appspot.com/bug?extid=0abd373e2e50d704db87
> > >
> > > Reported-by: syzbot+0abd373e2e50d704db87@syzkaller.appspotmail.com
> > > Tested-by: syzbot+0abd373e2e50d704db87@syzkaller.appspotmail.com
> > > Signed-off-by: Anirudh Rayabharam <mail@anirudhrb.com>
> > > ---
> > >  drivers/vhost/iotlb.c | 6 ++++--
> > >  1 file changed, 4 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/vhost/iotlb.c b/drivers/vhost/iotlb.c
> > > index 670d56c879e5..b9de74bd2f9c 100644
> > > --- a/drivers/vhost/iotlb.c
> > > +++ b/drivers/vhost/iotlb.c
> > > @@ -53,8 +53,10 @@ int vhost_iotlb_add_range_ctx(struct vhost_iotlb *iotlb,
> > >                               void *opaque)
> > >  {
> > >         struct vhost_iotlb_map *map;
> > > +       u64 size = last - start + 1;
> > >
> > > -       if (last < start)
> > > +       // size can overflow to 0 when start is 0 and last is (2^64 - 1).
> > > +       if (last < start || size == 0)
> > >                 return -EFAULT;
> >
> > I'd move this check to vhost_chr_iter_write(), then for the device who
> > has its own msg handler (e.g vDPA) can benefit from it as well.
>
> Thanks for reviewing!
>
> I kept the check here thinking that all devices would benefit from it
> because they would need to call vhost_iotlb_add_range() to add an entry
> to the iotlb. Isn't that correct?

Correct for now but not for the future, it's not guaranteed that the
per device iotlb message handler will use vhost iotlb.

But I agree that we probably don't need to care about it too much now.

> Do you see any other benefit in moving
> it to vhost_chr_iter_write()?
>
> One concern I have is that if we move it out some future caller to
> vhost_iotlb_add_range() might forget to handle this case.

Yes.

Rethink the whole fix, we're basically rejecting [0, ULONG_MAX] range
which seems a little bit odd. I wonder if it's better to just remove
the map->size. Having a quick glance at the the user, I don't see any
blocker for this.

Thanks

>
> Thanks!
>
>         - Anirudh.
>
> >
> > Thanks
> >
> > >
> > >         if (iotlb->limit &&
> > > @@ -69,7 +71,7 @@ int vhost_iotlb_add_range_ctx(struct vhost_iotlb *iotlb,
> > >                 return -ENOMEM;
> > >
> > >         map->start = start;
> > > -       map->size = last - start + 1;
> > > +       map->size = size;
> > >         map->last = last;
> > >         map->addr = addr;
> > >         map->perm = perm;
> > > --
> > > 2.35.1
> > >
> >
>