Re: [PATCH] net: fix udp gso skb_segment after pull from frag_list
From: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Date: 2026-01-06 19:15:22
Also in:
lkml
Shiming Cheng wrote:
Commit 3382a1ed7f77 ("net: fix udp gso skb_segment after pull from
frag_list")
if gso_type is not SKB_GSO_FRAGLIST but skb->head_frag is zero,What codepath triggers this scenario? We should make sure that the fix covers all such instances. Likely instances of where some module in the datapath, like a BPF program, modifies a valid skb into one that is not safe to pass to skb_segment. I don't fully understand yet that skb->head_frag == 0 is the only such condition in scope.
then detected invalid geometry in frag_list skbs and call skb_segment. But some packets with modified geometry can also hit bugs in that code. Instead, linearize all these packets that fail the basic invariants on gso fraglist skbs. That is more robust. call stack information, see below. Valid SKB_GSO_FRAGLIST skbs - consist of two or more segments - the head_skb holds the protocol headers plus first gso_size - one or more frag_list skbs hold exactly one segment - all but the last must be gso_size Optional datapath hooks such as NAT and BPF (bpf_skb_pull_data) can modify fraglist skbs, breaking these invariants. In extreme cases they pull one part of data into skb linear. For UDP, this causes three payloads with lengths of (11,11,10) bytes were pulled tail to become (12,10,10) bytes. The skbs no longer meets the above SKB_GSO_FRAGLIST conditions because payload was pulled into head_skb, it needs to be linearized before pass to regular skb_segment.
Most of this commit message duplicates the text in commit 3382a1ed7f77
("net: fix udp gso skb_segment after pull from frag_list"). And
somewhat garbles it, as in the first sentence.
But this is a different datapath, not related to SKB_GSO_FRAGLIST.
So the fixes tag is also incorrect. The blamed commit fixes an issue
with fraglist GRO. This new issue is with skbs that have a fraglist,
but not one created with that feature. (the naming is confusing, but
fraglist-gro is only one use of the skb frag_list).
quoted hunk ↗ jump to hunk
skb_segment+0xcd0/0xd14 __udp_gso_segment+0x334/0x5f4 udp4_ufo_fragment+0x118/0x15c inet_gso_segment+0x164/0x338 skb_mac_gso_segment+0xc4/0x13c __skb_gso_segment+0xc4/0x124 validate_xmit_skb+0x9c/0x2c0 validate_xmit_skb_list+0x4c/0x80 sch_direct_xmit+0x70/0x404 __dev_queue_xmit+0x64c/0xe5c neigh_resolve_output+0x178/0x1c4 ip_finish_output2+0x37c/0x47c __ip_finish_output+0x194/0x240 ip_finish_output+0x20/0xf4 ip_output+0x100/0x1a0 NF_HOOK+0xc4/0x16c ip_forward+0x314/0x32c ip_rcv+0x90/0x118 __netif_receive_skb+0x74/0x124 process_backlog+0xe8/0x1a4 __napi_poll+0x5c/0x1f8 net_rx_action+0x154/0x314 handle_softirqs+0x154/0x4b8 [118.376811] [C201134] rxq0_pus: [name:bug&]kernel BUG at net/core/skbuff.c:4278! [118.376829] [C201134] rxq0_pus: [name:traps&]Internal error: Oops - BUG: 00000000f2000800 [#1] [118.470774] [C201134] rxq0_pus: [name:mrdump&]Kernel Offset: 0x178cc00000 from 0xffffffc008000000 [118.470810] [C201134] rxq0_pus: [name:mrdump&]PHYS_OFFSET: 0x40000000 [118.470827] [C201134] rxq0_pus: [name:mrdump&]pstate: 60400005 (nZCv daif +PAN -UAO) [118.470848] [C201134] rxq0_pus: [name:mrdump&]pc : [0xffffffd79598aefc] skb_segment+0xcd0/0xd14 [118.470900] [C201134] rxq0_pus: [name:mrdump&]lr : [0xffffffd79598a5e8] skb_segment+0x3bc/0xd14 [118.470928] [C201134] rxq0_pus: [name:mrdump&]sp : ffffffc008013770 Fixes: 3382a1ed7f77 ("net: fix udp gso skb_segment after pull from frag_list") Signed-off-by: Shiming Cheng <redacted> --- net/ipv4/udp_offload.c | 6 ++++++ 1 file changed, 6 insertions(+)diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c index 19d0b5b09ffa..606d9ce8c98e 100644 --- a/net/ipv4/udp_offload.c +++ b/net/ipv4/udp_offload.c@@ -535,6 +535,12 @@ struct sk_buff *__udp_gso_segment(struct sk_buff *gso_skb, uh->check = ~udp_v4_check(gso_skb->len, ip_hdr(gso_skb)->saddr, ip_hdr(gso_skb)->daddr, 0); + } else if (skb_shinfo(gso_skb)->frag_list && gso_skb->head_frag == 0) { + if (skb_pagelen(gso_skb) - sizeof(*uh) != skb_shinfo(gso_skb)->gso_size) { + ret = __skb_linearize(gso_skb); + if (ret) + return ERR_PTR(ret); + } } skb_pull(gso_skb, sizeof(*uh));-- 2.45.2