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[2001:b030:8055:ffc7:2d87:7642:b84f:7a21]) by smtp.gmail.com with ESMTPSA id 41be03b00d2f7-c743a7ff529sm2027246a12.2.2026.03.20.07.15.27 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 20 Mar 2026 07:15:32 -0700 (PDT) From: Guoyu Su To: edumazet@google.com, davem@davemloft.net, kuba@kernel.org, pabeni@redhat.com Cc: willemdebruijn.kernel@gmail.com, netdev@vger.kernel.org, horms@kernel.org, linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com, syzbot+1543a7d954d9c6d00407@syzkaller.appspotmail.com, Guoyu Su Subject: [PATCH net v5] net: use skb_header_pointer() only for DODGY TCPv4 GSO skbs Date: Fri, 20 Mar 2026 22:14:59 +0800 Message-Id: <20260320141459.9691-1-yss2813483011xxl@gmail.com> X-Mailer: git-send-email 2.34.1 In-Reply-To: References: Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Syzbot reported a KMSAN uninit-value warning in gso_features_check() called from netif_skb_features() [1]. The current direct skb->len check is not sufficient for SKB_GSO_DODGY packets. In the AF_PACKET/PACKET_VNET_HDR path, packet_snd() can build a DODGY GSO skb whose total length is large enough, while the IPv4 header is not fully available as initialized linear data for a direct iph->frag_off access. Use skb_header_pointer() to fetch the IPv4 header only for DODGY packets and clear mangleid_features conservatively if the header cannot be accessed or the DF bit is not set. Keep the existing direct ip_hdr()/inner_ip_hdr() access for non-DODGY packets so that buggy internal callers are still exposed. [1] https://syzkaller.appspot.com/bug?extid=1543a7d954d9c6d00407 Fixes: cbc53e08a793 ("GSO: Add GSO type for fixed IPv4 ID") Reported-by: syzbot+1543a7d954d9c6d00407@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=1543a7d954d9c6d00407 Tested-by: syzbot+1543a7d954d9c6d00407@syzkaller.appspotmail.com Signed-off-by: Guoyu Su --- v5: - Move nhoff into the SKB_GSO_DODGY branch. - Drop the explicit nhoff < 0 check. v4: https://lore.kernel.org/netdev/20260319005421.14908-1-yss2813483011xxl@gmail.com/ v3: https://lore.kernel.org/netdev/20260312104351.185370-1-yss2813483011xxl@gmail.com/ v2: https://lore.kernel.org/netdev/20260308083319.1255118-1-yss2813483011xxl@gmail.com/ v1: https://lore.kernel.org/netdev/20260307162905.3697050-1-yss2813483011xxl@gmail.com/ net/core/dev.c | 16 +++++++++++++--- 1 file changed, 13 insertions(+), 3 deletions(-) diff --git a/net/core/dev.c b/net/core/dev.c index 14a83f2035b9..be0cc69fa534 100644 --- a/net/core/dev.c +++ b/net/core/dev.c @@ -3805,10 +3805,20 @@ static netdev_features_t gso_features_check(const struct sk_buff *skb, * segmentation-offloads.rst). */ if (skb_shinfo(skb)->gso_type & SKB_GSO_TCPV4) { - struct iphdr *iph = skb->encapsulation ? - inner_ip_hdr(skb) : ip_hdr(skb); + const struct iphdr *iph; + struct iphdr _iph; - if (!(iph->frag_off & htons(IP_DF))) + if (unlikely(skb_shinfo(skb)->gso_type & SKB_GSO_DODGY)) { + int nhoff = skb->encapsulation ? + skb_inner_network_offset(skb) : + skb_network_offset(skb); + + iph = skb_header_pointer(skb, nhoff, sizeof(_iph), &_iph); + } else { + iph = skb->encapsulation ? inner_ip_hdr(skb) : ip_hdr(skb); + } + + if (!iph || !(iph->frag_off & htons(IP_DF))) features &= ~dev->mangleid_features; } -- 2.34.1