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

From: Jason Wang <jasowang@redhat.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
	kvm@vger.kernel.org, virtualization@lists.linux-foundation.org
Subject: Re: [PATCH net] vhost: validate log when IOTLB is enabled
Date: Fri, 30 Mar 2018 10:27:07 +0800	[thread overview]
Message-ID: <a6d20da3-cebb-b672-ad4a-8e17083b7234@redhat.com> (raw)
In-Reply-To: <20180329173438-mutt-send-email-mst@kernel.org>

On 2018年03月29日 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@syzkaller.appspotmail.com
>> Signed-off-by: Jason Wang<jasowang@redhat.com>
> 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:

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

So this is expected I think.

Thanks
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