Re: [PATCH stable<3.19] net: handle null iovec pointer in skb_copy_and_csum_datagram_iovec()

[Luis Henriques <luis.henriques@canonical.com>]

On Fri, Oct 23, 2015 at 11:39:46AM +0200, Michal Kubecek wrote:
> On Fri, Oct 23, 2015 at 11:22:19AM +0200, Sabrina Dubroca wrote:
> > Hello Michal,
> > 
> > 2015-10-23, 10:46:09 +0200, Michal Kubecek wrote:
> > > Mainline commit 89c22d8c3b27 ("net: Fix skb csum races when peeking")
> > > backport into pre-3.19 stable kernels introduces a regression causing
> > > null pointer dererefence in skb_copy_and_csum_datagram_iovec().
> > > 
> > > This commit only sets CHECKSUM_UNNECESSARY for non-shared skb, allowing
> > > udp_recvmsg() to take the "else" branch of if (skb_csum_unnecessary(skb))
> > > when called with null iovec (and len=0, e.g. when peeking for datagram
> > > size first). The problem is that unlike skb_copy_and_csum_datagram_msg()
> > > called in this path since 3.19, skb_copy_and_csum_datagram_iovec() does
> > > not handle null iov parameter and always dereferences iov->iov_len. This
> > > is especially harmful when udp_recvmsg() is called in kernel context,
> > > e.g. from kernel nfsd.
> > > 
> > > Band-aid skb_copy_and_csum_datagram_iovec() by testing iov for null and
> > > only checking the checksum in this case.
> > > 
> > > Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
> > > ---
> > 
> > I ran into this problem too and that was my initial solution to this
> > problem as well, but actually, we need a more complete fix, like the
> > one I submitted a few days ago:
> > 
> > http://patchwork.ozlabs.org/patch/530642/
> > 
> > With your solution, userspace can still receive bogus EFAULT, or the
> > kernel ends up writing data to an unwanted memory location.
> 
> I must admit I wondered why skb_copy_and_csum_datagram_iovec() doesn't
> get (and check) read length and why it cannot overfill the buffer. But
> then I saw the comment "Caller _must_ check that skb will fit to this
> iovec", stopped thinking and assumed it's OK. I guess I should be less
> trusting... :-(
> 
> Thank you for the warning.
> 
>                                                         Michal Kubecek

I can confirm that we had this issue reported in our kernels too
(https://bugs.launchpad.net/bugs/1508510).  I'll queue Sabrina's patch for
the next 3.16 stable kernel release.  Thanks a lot!


Cheers,
--
Lu�s