From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pf1-f182.google.com (mail-pf1-f182.google.com [209.85.210.182]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id AFB743264E9 for ; Tue, 4 Aug 2026 05:41:00 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.210.182 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785822061; cv=none; b=V/U+SR+4RdxL7dO+MhuAseEQgPl6baf16raahXX+wiz14GcwdBvXnVqtxwmeg0bEmFg3YzmbXwtmTYAmNLd6lQ/8Rss2wTfsvpxE+R5PZgTQHSmMSRwfvDaCnOKOlCJRmlugXF6GyHR0DAvaIFUN5N7B/AIJqokcYvhGpQI11MM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785822061; c=relaxed/simple; bh=+HpGf3LoSzduFxk132QfTs1NVy4vbdD0zOb0d/ZqjPA=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=snI3jfz7yTXpZFlVEyBEgCsi3g7RH6BhhGcdj47q2wLmiGZ0jzKzcVfj197tvrYAjQe60TGbYxAOnqUg+z1ETG9W0WMbTV5MAw3BsQ67yRFb2LWNUKcNF25Q5/EZfNOxsD4lOYgEu2wkEGgF/J5xeHbl8VQ2lgRHLJ+JN6tiCsI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com; spf=pass smtp.mailfrom=gmail.com; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b=E3GPIwQC; arc=none smtp.client-ip=209.85.210.182 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=gmail.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="E3GPIwQC" Received: by mail-pf1-f182.google.com with SMTP id d2e1a72fcca58-8484f229529so3386837b3a.2 for ; Mon, 03 Aug 2026 22:41:00 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20251104; t=1785822060; x=1786426860; darn=vger.kernel.org; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:from:to:cc:subject:date :message-id:reply-to:content-type; bh=hVYoynVbiX5DkPohCYrGWG74ca2waR9to+ufYwdKFiY=; b=E3GPIwQCNVm+YqxS+ydy4QDGcerW3x0UUcjMNs6fRL/W/kbBFU+ztm8F1LhuMqe3YV PG+Eprk2lTety8jsAlecig4HIkCFTKQxKHFaCbLdYidwCRDg8OFORm3Fyj1daS8D22Gt e+K/xA9KRV8rGAeUZvx595OGS53fONdIcNzZgPDUKPDHqFNw2YKNw7kvlKYngyHeWoSq 0R2e0Guamp0hpWkkcyAbCWBzy8hAcjS4JVFwhMGhl2sAdVN9TSLL4ctyMG5WOFrfkL91 mpz5S0BOP1ZpcIYgrwVhChtgF3lGAglujnuWCBsA2pLn1hqpFzvddyXf/s3fokEeU6cO xB2g== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20251104; t=1785822060; x=1786426860; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:x-gm-gg:x-gm-message-state:from :to:cc:subject:date:message-id:reply-to:content-type; bh=hVYoynVbiX5DkPohCYrGWG74ca2waR9to+ufYwdKFiY=; b=CxgsDInoop6cS5X5y5Mtksw48O6mVAkvt8HgavtVvPSLjjEJPFQhMNTEmIQb1FVht+ TKQgUIdOdxpBS0CXdcZoPWUXbGE6rdM/i0D35iXZJk/peCMpJN8UXgVV+daAQC3tolvM On9sckUvoWjcNjz2KY+QnqRns/CR5ITHeN8jrqDyrax4jQ5qqRy0o7kDI0s2rvcvPFpv 3gK2+StQB6/fn97lbmv3JWNTC4uvskSVmXgUMwUJwREmEzAkMuly9hFeTGH+hlVbMa7M DtmkZqjwhxZELxj2pJbDas+TsARANwyaC6Rb5FfkTrwWMAnXLlJXvxfPFsxXKo5g7NiE 8iOQ== X-Forwarded-Encrypted: i=1; AHgh+Rrk5OzA/0ZgEhh6MV+LWvW/VZYYK8psNu8uUqfEuldqqERlIxAuUvtB+mFc2dhOEQKdl8+iNlT6Fc5vcRA=@vger.kernel.org X-Gm-Message-State: AOJu0Ywrk8udf1LVvFU7o/2c6JiuzBJWFMdBSKYq2KH2X5hSBwEj7IfK sGggpB/sQDQ3sPrBMwo4Cdpeuc3ylx04VnCOd6P9ElK8/stD+ueSWy4N X-Gm-Gg: AR+sD10wCjb/yOcEoDmFIcmGoAOeATm65UUezTC6Xjp/dcfJ8T3eSGv7F1Yc9fSNTXH 1T0OwlBAhvIgtsu6Z5CB24aLagXL1TfmwfZf18W1H0mjJ4jlGrk52u8rzzskWhc/u3x7occqTo2 7Lwjzh/iCPK+KT7iPw1N2D4w4hHB2C+22pzeci09xXWp3grtuVtaOoHy4/13hsas1p5YVC2pliP mfxQzp9Y1Wxd5c5D7LLWc+7mn4uMm78Ajb+MzamOaI7C67YonK1HewgPuJe2M5m67g0yo5qKoHc 42DGJfh27fN9ZzptkeZbDK28qO/ESk28rdvqGE5yaa2UQTK+2NQnrdtMCBZmql68lq4dpvFt+ch F4L73gL4OXdMaMkaHU8KAIJtWCRDPtvAqotlRG4bR5evJh0ItlF8yBA/7d69rJCDO7aMp2YO2p4 1T15lxCz+EBtbMPnVpZgJXWvs/pzKpVeMEbMsWY+JSVxrHW5fkSlUljxcyQDs9IXaprS4iLH2XG ZmYE0VUK/ghQxV9jsVi/NLUx29mkdlcTxcxeEKCuwzc40b6JqcJ3nq9i+IbBpZjT+qjn4H9Od8+ 04I0do3+zPVz9jPB3gzrw5E= X-Received: by 2002:a05:6a00:3d44:b0:848:7731:d95a with SMTP id d2e1a72fcca58-84ee48bc67emr11526034b3a.37.1785822059843; Mon, 03 Aug 2026 22:40:59 -0700 (PDT) Received: from u2404-VMware-Virtual-Platform.localdomain ([60.168.97.11]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-84edbe5702bsm4678737b3a.16.2026.08.03.22.40.53 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 03 Aug 2026 22:40:59 -0700 (PDT) From: Sun Jian To: netdev@vger.kernel.org Cc: Sun Jian , stable@vger.kernel.org, Mohsin Bashir , Lorenzo Bianconi , "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 , =?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 1/2] net: fix skb length accounting after generic XDP frag adjustment Date: Mon, 3 Aug 2026 22:40:38 -0700 Message-ID: <20260804054040.613675-2-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 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/ Reviewed-by: Mohsin Bashir Acked-by: Lorenzo Bianconi 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; -- 2.43.0