From 78effd896eee11ac9db9bcbb53e7bbcad96073d7 Mon Sep 17 00:00:00 2001 From: Gal Pressman Date: Mon, 18 May 2026 09:22:50 +0300 Subject: [PATCH] udp: Fix UDP length on last GSO_PARTIAL segment Following the cited commit, __udp_gso_segment() writes single MSS length in the UDP header. The cited patch doesn't account for the fact that the last segment could be a GSO skb by itself. This could happen when the size of the packet is a multiple of MSS, hence the first segment is also the last one (there is no need for a remainder skb). When the post-loop segment is a GSO skb, assign the single MSS length in the UDP header. Fixes: b10b446ce7ad ("udp: gso: Use single MSS length in UDP header for GSO_PARTIAL") Reported-by: Matthew Schwartz Closes: https://lore.kernel.org/all/6c3fb15e-711d-4b8d-b152-e03d9b05293f@linux.dev/ Tested-by: Matthew Schwartz Reviewed-by: Dragos Tatulea Signed-off-by: Gal Pressman Link: https://patch.msgid.link/20260518062250.3019914-3-gal@nvidia.com Signed-off-by: Jakub Kicinski --- net/ipv4/udp_offload.c | 9 ++++++--- 1 file changed, 6 insertions(+), 3 deletions(-) diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c index 2578aa7f9ff9..29651b1a0bc7 100644 --- a/net/ipv4/udp_offload.c +++ b/net/ipv4/udp_offload.c @@ -590,9 +590,12 @@ struct sk_buff *__udp_gso_segment(struct sk_buff *gso_skb, uh = udp_hdr(seg); } - /* last packet can be partial gso_size, account for that in checksum */ - newlen = htons(skb_tail_pointer(seg) - skb_transport_header(seg) + - seg->data_len); + /* Unless skb fits perfectly as GSO_PARTIAL, the trailing + * segment may not be full MSS, account for that in the checksum + */ + if (!skb_is_gso(seg)) + newlen = htons(skb_tail_pointer(seg) - + skb_transport_header(seg) + seg->data_len); check = csum16_add(csum16_sub(uh->check, uh->len), newlen); uh->len = newlen;