[PATCH net] net: clear mangleid_features for SKB_GSO_DODGY TCPv4

[Guoyu Su <yss2813483011xxl@gmail.com>]

Syzbot reported a KMSAN uninit-value warning in netif_skb_features() [1], which originates from gso_features_check():

  BUG: KMSAN: uninit-value in gso_features_check net/core/dev.c:3804 [inline]
  BUG: KMSAN: uninit-value in netif_skb_features+0x6fb/0x1870 net/core/dev.c:3828
   gso_features_check net/core/dev.c:3804 [inline]
   netif_skb_features+0x6fb/0x1870 net/core/dev.c:3828
   validate_xmit_skb+0xb6/0x2400 net/core/dev.c:4003
   ...
   __dev_queue_xmit+0x3016/0x5a50 net/core/dev.c:4795
   packet_snd net/packet/af_packet.c:3077 [inline]
   packet_sendmsg+0x91d9/0xa320 net/packet/af_packet.c:3109

SKB_GSO_DODGY marks packets whose GSO-related header metadata should be treated as untrusted (for example, packets injected via AF_PACKET).

gso_features_check() checks IP_DF through iph->frag_off for SKB_GSO_TCPV4 packets. For DODGY packets with untrusted metadata, this may result in a KMSAN uninit-value report.

Handle such packets conservatively by clearing mangleid_features when SKB_GSO_DODGY is set, without relying on iph->frag_off.

[1] https://syzkaller.appspot.com/bug?extid=1543a7d954d9c6d00407

Reported-by: syzbot+1543a7d954d9c6d00407@syzkaller.appspotmail.com
Tested-by: syzbot+1543a7d954d9c6d00407@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/68e4b5d8.050a0220.256323.0018.GAE@google.com/T/
Signed-off-by: Guoyu Su <yss2813483011xxl@gmail.com>
---
 net/core/dev.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 14a83f2035b9..9b19154b6f7f 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -3808,7 +3808,12 @@ static netdev_features_t gso_features_check(const struct sk_buff *skb,
 		struct iphdr *iph = skb->encapsulation ?
 				    inner_ip_hdr(skb) : ip_hdr(skb);
 
-		if (!(iph->frag_off & htons(IP_DF)))
+		/* SKB_GSO_DODGY packets carry untrusted L3/L4 header metadata.
+		 * Avoid dereferencing IPv4 header fields and conservatively
+		 * clear mangleid support in that case.
+		 */
+		if ((skb_shinfo(skb)->gso_type & SKB_GSO_DODGY) ||
+		    !(iph->frag_off & htons(IP_DF)))
 			features &= ~dev->mangleid_features;
 	}
 
-- 
2.34.1