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Sun, 26 Jul 2026 20:25:54 -0700 (PDT) From: Sun Jian To: netdev@vger.kernel.org Cc: bpf@vger.kernel.org, stable@vger.kernel.org, sun.jian.kdev@gmail.com, davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, andrew+netdev@lunn.ch, horms@kernel.org, ast@kernel.org, daniel@iogearbox.net, hawk@kernel.org, john.fastabend@gmail.com, sdf@fomichev.me, lorenzo@kernel.org, toke@redhat.com, maciej.fijalkowski@intel.com, matt@readmodwrite.com Subject: [PATCH net 1/2] net: fix skb length accounting after generic XDP frag adjustment Date: Sun, 26 Jul 2026 20:25:34 -0700 Message-ID: <20260727032535.13469-2-sun.jian.kdev@gmail.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20260727032535.13469-1-sun.jian.kdev@gmail.com> References: <20260727032535.13469-1-sun.jian.kdev@gmail.com> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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/r/20260720141859.19FF41F000E9@smtp.kernel.org Link: https://lore.kernel.org/bpf/al9T9Eto%2FhRIzP5W@boxer/ Signed-off-by: Sun Jian --- net/core/dev.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) diff --git a/net/core/dev.c b/net/core/dev.c index 5933c5dab09e..94204702cb02 100644 --- a/net/core/dev.c +++ b/net/core/dev.c @@ -5517,12 +5517,14 @@ 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), update skb length fields here. */ + skb->len -= skb->data_len; if (xdp_buff_has_frags(xdp)) skb->data_len = skb_shinfo(skb)->xdp_frags_size; else skb->data_len = 0; + skb->len += skb->data_len; /* check if XDP changed eth hdr such SKB needs update */ eth = (struct ethhdr *)xdp->data; -- 2.43.0