[PATCH net v2] ip: fix IP_CHECKSUM handling

[PATCH net v2] ip: fix IP_CHECKSUM handling

[Paolo Abeni]

The skbs processed by ip_cmsg_recv() are not guaranteed to
be linear e.g. when sending UDP packets over loopback with
MSGMORE.
Using csum_partial() on [potentially] the whole skb len
is dangerous; instead be on the safe side and use skb_checksum().

Thanks to syzkaller team to detect the issue and provide the
reproducer.

v1 -> v2:
 - move the variable declaration in a tighter scope

Fixes: ad6f939ab193 ("ip: Add offset parameter to ip_cmsg_recv")
Reported-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
---
 net/ipv4/ip_sockglue.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/net/ipv4/ip_sockglue.c b/net/ipv4/ip_sockglue.c
index 9000117..fc4bf4d 100644
--- a/net/ipv4/ip_sockglue.c
+++ b/net/ipv4/ip_sockglue.c
@@ -116,10 +116,10 @@ static void ip_cmsg_recv_checksum(struct msghdr *msg, struct sk_buff *skb,
 	if (skb->ip_summed != CHECKSUM_COMPLETE)
 		return;
 
-	if (offset != 0)
-		csum = csum_sub(csum,
-				csum_partial(skb_transport_header(skb) + tlen,
-					     offset, 0));
+	if (offset != 0) {
+		int tend_off = skb_transport_offset(skb) + tlen;
+		csum = csum_sub(csum, skb_checksum(skb, tend_off, offset, 0));
+	}
 
 	put_cmsg(msg, SOL_IP, IP_CHECKSUM, sizeof(__wsum), &csum);
 }
-- 
2.9.3

Re: [PATCH net v2] ip: fix IP_CHECKSUM handling

[Eric Dumazet]

On Tue, 2017-02-21 at 09:33 +0100, Paolo Abeni wrote:
> The skbs processed by ip_cmsg_recv() are not guaranteed to
> be linear e.g. when sending UDP packets over loopback with
> MSGMORE.
> Using csum_partial() on [potentially] the whole skb len
> is dangerous; instead be on the safe side and use skb_checksum().
> 
> Thanks to syzkaller team to detect the issue and provide the
> reproducer.

Acked-by: Eric Dumazet <edumazet@google.com>

Thanks Paolo !

Re: [PATCH net v2] ip: fix IP_CHECKSUM handling

[David Miller]

From: Paolo Abeni <pabeni@redhat.com>
Date: Tue, 21 Feb 2017 09:33:18 +0100

> The skbs processed by ip_cmsg_recv() are not guaranteed to
> be linear e.g. when sending UDP packets over loopback with
> MSGMORE.
> Using csum_partial() on [potentially] the whole skb len
> is dangerous; instead be on the safe side and use skb_checksum().
> 
> Thanks to syzkaller team to detect the issue and provide the
> reproducer.
> 
> v1 -> v2:
>  - move the variable declaration in a tighter scope
> 
> Fixes: ad6f939ab193 ("ip: Add offset parameter to ip_cmsg_recv")
> Reported-by: Andrey Konovalov <andreyknvl@google.com>
> Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Applied and queued up for -stable, thanks.