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@xxxxxxx>
> > > ---
> >
> > 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,
--
Luís
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