Re: [PATCH] vhost: validate range size before adding to iotlb

[Anirudh Rayabharam <mail@anirudhrb.com>]

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? 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.

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
> >
>