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Message-ID: <6837c3df6b1a7_39fc9c29492@willemb.c.googlers.com.notmuch>
Date: Wed, 28 May 2025 22:18:07 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Shiming Cheng <shiming.cheng@...iatek.com>,
willemdebruijn.kernel@...il.com,
willemb@...gle.com,
edumazet@...gle.com,
davem@...emloft.net,
kuba@...nel.org,
pabeni@...hat.com,
matthias.bgg@...il.com
Cc: linux-kernel@...r.kernel.org,
netdev@...r.kernel.org,
shiming.cheng@...iatek.com,
lena.wang@...iatek.com
Subject: Re: [PATCH net v5] net: fix udp gso skb_segment after pull from
frag_list
Shiming Cheng wrote:
> Commit a1e40ac5b5e9 ("net: gso: fix udp gso fraglist segmentation after
> pull from frag_list") detected invalid geometry in frag_list skbs and
> redirects them from skb_segment_list to more robust skb_segment. But some
> packets with modified geometry can also hit bugs in that code. We don't
> know how many such cases exist. Addressing each one by one also requires
> touching the complex skb_segment code, which risks introducing bugs for
> other types of skbs. Instead, linearize all these packets that fail the
> basic invariants on gso fraglist skbs. That is more robust.
>
> If only part of the fraglist payload is pulled into head_skb, it will
> always cause exception when splitting skbs by skb_segment. For detailed
> 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.
>
> 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] PREEMPT SMP
> [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: a1e40ac5b5e9 ("net: gso: fix udp gso fraglist segmentation after pull from frag_list")
> Signed-off-by: Shiming Cheng <shiming.cheng@...iatek.com>
Reviewed-by: Willem de Bruijn <willemb@...gle.com>
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