Re: [PATCH net-next] ptr_ring: fix integer overflow

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Jan 26, 2018 at 11:44:22AM +0800, Jason Wang wrote:
> 
> 
> On 2018年01月26日 01:31, Michael S. Tsirkin wrote:
> > On Thu, Jan 25, 2018 at 10:17:38PM +0800, Jason Wang wrote:
> > > 
> > > On 2018年01月25日 21:45, Michael S. Tsirkin wrote:
> > > > On Thu, Jan 25, 2018 at 03:31:42PM +0800, Jason Wang wrote:
> > > > > We try to allocate one more entry for lockless peeking. The adding
> > > > > operation may overflow which causes zero to be passed to kmalloc().
> > > > > In this case, it returns ZERO_SIZE_PTR without any notice by ptr
> > > > > ring. Try to do producing or consuming on such ring will lead NULL
> > > > > dereference. Fix this detect and fail early.
> > > > > 
> > > > > Fixes: bcecb4bbf88a ("net: ptr_ring: otherwise safe empty checks can overrun array bounds")
> > > > > Reported-by:syzbot+87678bcf753b44c39b67@syzkaller.appspotmail.com
> > > > > Cc: John Fastabend<john.fastabend@gmail.com>
> > > > > Signed-off-by: Jason Wang<jasowang@redhat.com>
> > > > Ugh that's just way too ugly.
> > > > I'll work on dropping the extra + 1 - but calling this
> > > > function with -1 size is the real source of the bug.
> > > > Do you know how come we do that?
> > > > 
> > > It looks e.g try to change tx_queue_len to UINT_MAX. And we probably can't
> > > prevent user form trying to do this?
> > > 
> > > Thanks
> > Right. BTW why net-next? Isn't the crash exploitable in net?
> > 
> 
> Commit bcecb4bbf88a exists only in net-next.

Right you are.

> And in net we check r->size
> before trying to dereference the queue.
> 
> Thanks

I was wondering what it's about btw. Does anyone really create 0 size rings?

-- 
MST