Dear Willem

frag_list assignment trigger:
frag_list is assigned to the next skb without to enable device feature
NETIF_F_GRO_FRAGLIST if head_frag is zero.

After packet in fraglist is processed by BPF pull tial, performing GSO
segmentation directly will cause problems.


udp_gro_receive_segment
  skb_gro_receive
   if (skb->head_frag==0)
     goto merge;


merge:
        /* sk ownership - if any - completely transferred to the
aggregated packet */
        skb->destructor = NULL;
        skb->sk = NULL;
        delta_truesize = skb->truesize;
        if (offset > headlen) {
                unsigned int eat = offset - headlen;

                skb_frag_off_add(&skbinfo->frags[0], eat);
                skb_frag_size_sub(&skbinfo->frags[0], eat);
                skb->data_len -= eat;
                skb->len -= eat;
                offset = headlen;
        }

        __skb_pull(skb, offset);

        if (NAPI_GRO_CB(p)->last == p)
                skb_shinfo(p)->frag_list = skb;   
        <<< here frag_list is assigned to the next skb without to
enable device feature NETIF_F_GRO_FRAGLIST if head_frag is zero
        else
                NAPI_GRO_CB(p)->last->next = skb;
        NAPI_GRO_CB(p)->last = skb;
        __skb_header_release(skb);
        lp = p;




On Tue, 2026-01-06 at 14:15 -0500, Willem de Bruijn wrote:

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Shiming Cheng wrote:

quoted

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.

quoted

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

  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)-

quoted

saddr,

                                                ip_hdr(gso_skb)-

quoted

daddr, 0);

+     } else if (skb_shinfo(gso_skb)->frag_list && gso_skb-

quoted

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