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[34.85.134.180]) by smtp.gmail.com with UTF8SMTPSA id 00721157ae682-79b1e45254bsm14981827b3.48.2026.03.26.09.25.16 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 26 Mar 2026 09:25:16 -0700 (PDT) Date: Thu, 26 Mar 2026 12:25:13 -0400 From: Willem de Bruijn To: Guoyu Su , 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 Message-ID: In-Reply-To: <20260326121813.457049-1-yss2813483011xxl@gmail.com> References: <20260326121813.457049-1-yss2813483011xxl@gmail.com> Subject: Re: [PATCH net v6] net: use skb_header_pointer() for TCPv4 GSO frag_off Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Guoyu Su wrote: > Syzbot reported a KMSAN uninit-value warning in gso_features_check() > called from netif_skb_features() [1]. > > gso_features_check() reads iph->frag_off to decide whether to clear > mangleid_features. Accessing the IPv4 header via ip_hdr()/inner_ip_hdr() > can rely on skb header offsets that are not always safe for direct > dereference on packets injected from PF_PACKET paths. > > Use skb_header_pointer() for the TCPv4 frag_off check so the header read > is robust whether data is already linear or needs copying. > > This also removes the SKB_GSO_DODGY special casing: skb_header_pointer() > already fast-paths linear data, so a separate direct-access path is not > needed. Does not belong in the commit message. It's a diff vs the previous version. > [1] https://syzkaller.appspot.com/bug?extid=1543a7d954d9c6d00407 > > Link: https://lore.kernel.org/netdev/willemdebruijn.kernel.1a9f35039caab@gmail.com/ > 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 > --- > v6: > - Use skb_header_pointer() for both DODGY and non-DODGY TCPv4 GSO > packets in gso_features_check(). > - Drop the SKB_GSO_DODGY special-casing for IPv4 header access. > > v5: https://lore.kernel.org/netdev/20260320141459.9691-1-yss2813483011xxl@gmail.com/ > 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 | 12 +++++++++--- > 1 file changed, 9 insertions(+), 3 deletions(-) > > diff --git a/net/core/dev.c b/net/core/dev.c > index 14a83f2035b9..8a15ca67cfed 100644 > --- a/net/core/dev.c > +++ b/net/core/dev.c > @@ -3805,10 +3805,16 @@ 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))) minor: no whitespace in the middle of the variable definition block. > + int nhoff = skb->encapsulation ? > + skb_inner_network_offset(skb) : > + skb_network_offset(skb); > + > + iph = skb_header_pointer(skb, nhoff, sizeof(_iph), &_iph); > + > + if (!iph || !(iph->frag_off & htons(IP_DF))) > features &= ~dev->mangleid_features; > } > > -- > 2.34.1 >