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

[Sabrina Dubroca <sd@queasysnail.net>]

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.


Thanks,

-- 
Sabrina