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), grc->offset can no longer be trusted: - skb_gro_receive() either gives the frags or the head of the skb to the held packet, or 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. Skipping the cleanup for a merged skb is correct: - With skb_gro_receive(), the skb is freed, or its headers are rebuilt from the head skb in skb_segment(). - skb_segment_list() only copies the link-layer and tunnel headers from the head skb, so fraglist members keep their own inner headers. grc->delta is only set with VXLAN_F_REMCSUM_NOPARTIAL, where skb_remcsum_process() writes the same adjusted checksum in the non GRO path. Keeping it is the intended result. This affects VXLAN and GUE with remote checksum offload enabled. Fixes: 26c4f7da3e41 ("net: Fix remcsum in GRO path to not change packet") Cc: stable@vger.kernel.org Tested-by: Cen Zhang Signed-off-by: Eric Dumazet --- include/net/gro.h | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) diff --git a/include/net/gro.h b/include/net/gro.h index 2300b6da05b2..00b49b6b9e74 100644 --- a/include/net/gro.h +++ b/include/net/gro.h @@ -351,7 +351,13 @@ 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, grc->offset can not be trusted: the frags or head + * of this skb now belong to the GRO packet, or skb->data was pulled + * past the headers before adding it to a frag_list. + * Fraglist members keep their adjusted inner checksum, which is the + * value skb_remcsum_process() would have written. + */ + if (!grc->delta || NAPI_GRO_CB(skb)->same_flow) return; ptr = skb_gro_header(skb, plen, grc->offset); -- 2.53.0