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([2601:646:8f02:5350:ce4:701d:f6e3:3a6b]) by smtp.gmail.com with ESMTPSA id 5a478bee46e88-3153e29a126sm7821146eec.31.2026.07.31.08.45.10 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 31 Jul 2026 08:45:11 -0700 (PDT) Message-ID: <6fe86a50-34ec-45aa-b2b4-52429a72369c@gmail.com> Date: Fri, 31 Jul 2026 08:45:09 -0700 Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH net v2 1/2] net: fix skb length accounting after generic XDP frag adjustment To: Sun Jian , netdev@vger.kernel.org Cc: Andrew Lunn , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , Alexei Starovoitov , Daniel Borkmann , Jesper Dangaard Brouer , John Fastabend , Stanislav Fomichev , Kuniyuki Iwashima , Hangbin Liu , Krishna Kumar , Samiullah Khawaja , Martin Karsten , Lorenzo Bianconi , =?UTF-8?Q?Toke_H=C3=B8iland-J=C3=B8rgensen?= , linux-kernel@vger.kernel.org, bpf@vger.kernel.org, maciej.fijalkowski@intel.com, stable@vger.kernel.org References: <20260731032357.6114-1-sun.jian.kdev@gmail.com> <20260731032357.6114-2-sun.jian.kdev@gmail.com> Content-Language: en-US From: Mohsin Bashir In-Reply-To: <20260731032357.6114-2-sun.jian.kdev@gmail.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 7/30/26 8:23 PM, Sun Jian wrote: > Generic XDP exposes non-linear skb fragments through an xdp_buff. If an > XDP program adjusts the fragment area, bpf_prog_run_generic_xdp() copies > xdp_frags_size back to skb->data_len but leaves skb->len containing the > old fragment contribution. > > After a fragment shrink, this makes skb_headlen() larger than the actual > linear area. In the reproduced UDP receive path, __skb_datagram_iter() > copied 1024 bytes past the actual linear tail to userspace, starting at > struct skb_shared_info. The copied bytes included the affected skb's > nr_frags, xdp_frags_size and a kernel pointer from > skb_shinfo(skb)->frags[0]. Real packet data was displaced by the same > amount and truncated at the end. > > Subtract the old data_len before replacing it and add the new data_len > afterwards, keeping skb->len and skb->data_len synchronized. > > A 60000-byte UDP datagram on a veth pair with MTU 64000 was shortened by > 1024 bytes from its fragment area. Before the fix, all 10 runs produced > corrupted payloads. After the fix, all 10 runs matched the expected > payload exactly. > > Fixes: e6d5dbdd20aa ("xdp: add multi-buff support for xdp running in generic mode") > Cc: stable@vger.kernel.org > Link: https://lore.kernel.org/bpf/al9T9Eto%2FhRIzP5W@boxer/ > Signed-off-by: Sun Jian > --- > net/core/dev.c | 10 +++++++--- > 1 file changed, 7 insertions(+), 3 deletions(-) > > diff --git a/net/core/dev.c b/net/core/dev.c > index 5933c5dab09e..5c37cf6c4aa1 100644 > --- a/net/core/dev.c > +++ b/net/core/dev.c > @@ -5517,12 +5517,16 @@ u32 bpf_prog_run_generic_xdp(struct sk_buff *skb, struct xdp_buff *xdp, > } > > /* XDP frag metadata (e.g. nr_frags) are updated in eBPF helpers > - * (e.g. bpf_xdp_adjust_tail), we need to update data_len here. > + * (e.g. bpf_xdp_adjust_tail). Remove the old fragment contribution > + * from skb->len before updating data_len, then add the new one back. > */ > - if (xdp_buff_has_frags(xdp)) > + skb->len -= skb->data_len; > + if (xdp_buff_has_frags(xdp)) { > skb->data_len = skb_shinfo(skb)->xdp_frags_size; > - else > + skb->len += skb->data_len; > + } else { > skb->data_len = 0; > + } > > /* check if XDP changed eth hdr such SKB needs update */ > eth = (struct ethhdr *)xdp->data; Reviewed-by: Mohsin Bashir