[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 <andreyk...@google.com>
Signed-off-by: Paolo Abeni <pab...@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