[PATCH net 1/2] net: gro: do not undo remote checksum offload on merged packets

skb_gro_flush_final_remcsum() calls skb_gro_remcsum_cleanup() to undo
the checksum adjustment done by skb_gro_remcsum_process() when the
packet was not consumed by GRO.

When the inner handler merged the packet (NAPI_GRO_CB(skb)->same_flow
is set), skb->data might have been moved past the headers:

- skb_gro_receive() uses __skb_pull(skb, offset) before adding the skb
  to the frag_list of the held packet.

- skb_gro_receive_list() uses skb_pull(skb, skb_gro_offset(skb)).

grc->offset is relative to the original skb->data, so the 2-byte
read-modify-write done by skb_gro_remcsum_cleanup() can land in
the payload of a packet which now belongs to the GRO packet.
pskb_may_pull() can also be called on this frag_list member.

With fraglist GRO, this can also be a use-after-free:
pskb_may_pull(skb, skb_gro_offset(skb)) in skb_gro_receive_list()
or udp_gro_receive_segment() releases the first page fragment if it
only contains headers. NAPI_GRO_CB(skb)->frag0 and frag0_len are not
updated, so skb_gro_remcsum_cleanup() takes the fast path and writes
through frag0 into a page the skb no longer holds a reference on.
The non merged case, where frag0 is used after this pull, is handled
in the following patch.

Headers of a merged skb are not used anymore: the skb is either freed,
or its headers are rebuilt from the head skb in skb_segment() or
skb_segment_list(). Skip the cleanup in this case.

This affects VXLAN and GUE with remote checksum offload enabled.

Fixes: 26c4f7da3e41 ("net: Fix remcsum in GRO path to not change packet")
Tested-by: Cen Zhang <redacted>
Signed-off-by: Eric Dumazet <edumazet@kernel.org>
---
 include/net/gro.h | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/include/net/gro.h b/include/net/gro.h
index 2300b6da05b2728ec40f42228f8fa9c195d8479c..b560e866683b03314b3e80a887ac4467d1a5acb9 100644
--- a/include/net/gro.h
+++ b/include/net/gro.h

@@ -351,7 +351,10 @@ static inline void skb_gro_remcsum_cleanup(struct sk_buff *skb,
 	void *ptr;
 	size_t plen = grc->offset + sizeof(u16);
 
-	if (!grc->delta)
+	/* Once merged, skb->data was moved past the headers, and
+	 * the headers of this skb will not be used anymore.
+	 */
+	if (!grc->delta || NAPI_GRO_CB(skb)->same_flow)
 		return;
 
 	ptr = skb_gro_header(skb, plen, grc->offset);

-- 
2.53.0