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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Alexei Starovoitov , Daniel Borkmann , Jesper Dangaard Brouer , John Fastabend , Stanislav Fomichev , =?UTF-8?q?Toke=20H=C3=B8iland-J=C3=B8rgensen?= , linux-kernel@vger.kernel.org (open list), bpf@vger.kernel.org (open list:XDP (eXpress Data Path):Keyword:(?:\b|_)xdp(?:\b|_)) Subject: [PATCH net v3 2/2] veth: fix skb length accounting after XDP frag adjustment Date: Mon, 3 Aug 2026 22:40:39 -0700 Message-ID: <20260804054040.613675-3-sun.jian.kdev@gmail.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20260804054040.613675-1-sun.jian.kdev@gmail.com> References: <20260804054040.613675-1-sun.jian.kdev@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit veth exposes non-linear skb fragments through an xdp_buff. If an XDP program adjusts the fragment area, veth_xdp_rcv_skb() 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. Additionally, bpf_xdp_pull_data() can advance data_end while leaving frags present. The skb is then still non-linear, so the old __skb_put(skb, off) triggers SKB_LINEAR_ASSERT(). Use skb_set_tail_pointer() and update skb->len explicitly instead, following bpf_prog_run_generic_xdp(). Unlike __skb_put(), skb_set_tail_pointer() does not require a linear skb. 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. A forced-tailroom reproducer also exercises bpf_xdp_pull_data() with frags still present; the old code triggers SKB_LINEAR_ASSERT(), while this fix passes 10/10 runs. Fixes: 718a18a0c8a6 ("veth: Rework veth_xdp_rcv_skb in order to accept non-linear skb") Cc: stable@vger.kernel.org Reported-by: Mohsin Bashir Link: https://lore.kernel.org/bpf/80687d9c-9c27-494c-b3f2-efd0230b1895@gmail.com/ Suggested-by: Lorenzo Bianconi Acked-by: Lorenzo Bianconi Signed-off-by: Sun Jian --- drivers/net/veth.c | 22 ++++++++++++++-------- 1 file changed, 14 insertions(+), 8 deletions(-) diff --git a/drivers/net/veth.c b/drivers/net/veth.c index 00e34afd858e..f80f6584a6ff 100644 --- a/drivers/net/veth.c +++ b/drivers/net/veth.c @@ -865,18 +865,24 @@ static struct sk_buff *veth_xdp_rcv_skb(struct veth_rq *rq, skb_reset_mac_header(skb); - /* check if bpf_xdp_adjust_tail was used */ - off = xdp->data_end - orig_data_end; - if (off != 0) - __skb_put(skb, off); /* positive on grow, negative on shrink */ - /* 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; + } + + /* Synchronize the skb tail with XDP's updated linear area. */ + off = xdp->data_end - orig_data_end; + if (off != 0) { + skb_set_tail_pointer(skb, xdp->data_end - xdp->data); + skb->len += off; /* positive on grow, negative on shrink */ + } skb->protocol = eth_type_trans(skb, rq->dev); -- 2.43.0