Re: [PATCH net] vhost: validate log when IOTLB is enabled

[Jason Wang]

On 2018å03æ29æ 22:44, Michael S. Tsirkin wrote:
> On Thu, Mar 29, 2018 at 04:00:04PM +0800, Jason Wang wrote:
> > Vq log_base is the userspace address of bitmap which has nothing to do
> > with IOTLB. So it needs to be validated unconditionally otherwise we
> > may try use 0 as log_base which may lead to pin pages that will lead
> > unexpected result (e.g trigger BUG_ON() in set_bit_to_user()).
> >
> > Fixes: 6b1e6cc7855b0 ("vhost: new device IOTLB API")
> > Reported-by:syzbot+6304bf97ef436580fede@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Signed-off-by: Jason Wang<jasowang@xxxxxxxxxx>
> One follow-up question:
>
> We still observe that get user pages returns 0 sometimes. While I agree
> we should not pass in unvalidated addresses, isn't this worth
> documenting?
> 	

Looking at get_user_pages_fast(), it has:

ÂÂÂ if (unlikely(!access_ok(write ? VERIFY_WRITE : VERIFY_READ,
ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ (void __user *)start, len)))
ÂÂÂ ÂÂÂ return 0;

So this is expected I think.

Thanks