Re: [PATCH V5 1/4] virtio_ring: validate used buffer length

[Halil Pasic <pasic@linux.ibm.com>]

On Tue, 23 Nov 2021 07:17:05 -0500
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Mon, Nov 22, 2021 at 02:50:03PM +0100, Halil Pasic wrote:
> > On Mon, 22 Nov 2021 14:25:26 +0800
> > Jason Wang <jasowang@redhat.com> wrote:
> >   
> > > On Mon, Nov 22, 2021 at 1:49 PM Halil Pasic <pasic@linux.ibm.com> wrote:  
> > > >
> > > > On Mon, 22 Nov 2021 06:35:18 +0100
> > > > Halil Pasic <pasic@linux.ibm.com> wrote:
> > > >    
> > > > > > I think it should be a common issue, looking at
> > > > > > vhost_vsock_handle_tx_kick(), it did:
> > > > > >
> > > > > > len += sizeof(pkt->hdr);
> > > > > > vhost_add_used(vq, head, len);
> > > > > >
> > > > > > which looks like a violation of the spec since it's TX.    
> > > > >
> > > > > I'm not sure the lines above look like a violation of the spec. If you
> > > > > examine vhost_vsock_alloc_pkt() I believe that you will agree that:
> > > > > len == pkt->len == pkt->hdr.len
> > > > > which makes sense since according to the spec both tx and rx messages
> > > > > are hdr+payload. And I believe hdr.len is the size of the payload,
> > > > > although that does not seem to be properly documented by the spec.    
> > > 
> > > Sorry for being unclear, what I meant is that we probably should use
> > > zero here. TX doesn't use in buffer actually.
> > > 
> > > According to the spec, 0 should be the used length:
> > > 
> > > "and len the total of bytes written into the buffer."  
> > 
> > Right, I was wrong. I somehow assumed this is the total length and not
> > just the number of bytes written.
> >   
> > >   
> > > > >
> > > > > On the other hand tx messages are stated to be device read-only (in the
> > > > > spec) so if the device writes stuff, that is certainly wrong.
> > > > >    
> > > 
> > > Yes.
> > >   
> > > > > If that is what happens.
> > > > >
> > > > > Looking at virtqueue_get_buf_ctx_split() I'm not sure that is what
> > > > > happens. My hypothesis is that we just a last descriptor is an 'in'
> > > > > type descriptor (i.e. a device writable one). For tx that assumption
> > > > > would be wrong.
> > > > >
> > > > > I will have another look at this today and send a fix patch if my
> > > > > suspicion is confirmed.  
> > 
> > Yeah, I didn't remember the semantic of
> > vq->split.vring.used->ring[last_used].len
> > correctly, and in fact also how exactly the rings work. So your objection
> > is correct. 
> > 
> > Maybe updating some stuff would make it easier to not make this mistake.
> > 
> > For example the spec and also the linux header says:
> > 
> > /* le32 is used here for ids for padding reasons. */ 
> > struct virtq_used_elem { 
> >         /* Index of start of used descriptor chain. */ 
> >         le32 id; 
> >         /* Total length of the descriptor chain which was used (written to) */ 
> >         le32 len; 
> > };
> > 
> > I think that comment isn't as clear as it could be. I would prefer:
> > /* The number of bytes written into the device writable portion of the
> > buffer described by the descriptor chain. */
> > 
> > I believe "the descriptor chain which was used" includes both the
> > descriptors that map the device read only and the device write
> > only portions of the buffer described by the descriptor chain. And the
> > total length of that descriptor chain may be defined either as a number
> > of the descriptors that form the chain, or the length of the buffer.
> > 
> > One has to use the descriptor chain even if the whole buffer is device
> > read only. So "used" == "written to" does not make any sense to me.  
> 
> The virtio spec actually says
> 
> Total length of the descriptor chain which was written to
> 
> without the "used" part.

Yes, that is in the text after the (pseudo-)code listing which contains
the description I was referring to (in 2.6.8 The Virtqueue Used Ring).
> 
> > Also something like
> > int vhost_add_used(struct vhost_virtqueue *vq, unsigned int head, int bytes_written)
> > instead of
> > int vhost_add_used(struct vhost_virtqueue *vq, unsigned int head, int len)
> > would make it easier to read the code correctly.  
> 
> I think we agree here. Patches?
> 

Will send some :D

Thanks!

[..]
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