[PATCH 5.10 055/161] gue: Fix skb memleak with inner IP protocol 0.

Greg Kroah-Hartman
2026-02-04T14:45:31Z

From: Kuniyuki Iwashima <kuniyu@google.com>

[ Upstream commit 9a56796ad258786d3624eef5aefba394fc9bdded ]

syzbot reported skb memleak below. [0]

The repro generated a GUE packet with its inner protocol 0.

gue_udp_recv() returns -guehdr->proto_ctype for "resubmit"
in ip_protocol_deliver_rcu(), but this only works with
non-zero protocol number.

Let's drop such packets.

Note that 0 is a valid number (IPv6 Hop-by-Hop Option).

I think it is not practical to encap HOPOPT in GUE, so once
someone starts to complain, we could pass down a resubmit
flag pointer to distinguish two zeros from the upper layer:

* no error
* resubmit HOPOPT

[0]
BUG: memory leak
unreferenced object 0xffff888109695a00 (size 240):
comm "syz.0.17", pid 6088, jiffies 4294943096
hex dump (first 32 bytes):
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
00 40 c2 10 81 88 ff ff 00 00 00 00 00 00 00 00 .@..............
backtrace (crc a84b336f):
kmemleak_alloc_recursive include/linux/kmemleak.h:44 [inline]
slab_post_alloc_hook mm/slub.c:4958 [inline]
slab_alloc_node mm/slub.c:5263 [inline]
kmem_cache_alloc_noprof+0x3b4/0x590 mm/slub.c:5270
__build_skb+0x23/0x60 net/core/skbuff.c:474
build_skb+0x20/0x190 net/core/skbuff.c:490
__tun_build_skb drivers/net/tun.c:1541 [inline]
tun_build_skb+0x4a1/0xa40 drivers/net/tun.c:1636
tun_get_user+0xc12/0x2030 drivers/net/tun.c:1770
tun_chr_write_iter+0x71/0x120 drivers/net/tun.c:1999
new_sync_write fs/read_write.c:593 [inline]
vfs_write+0x45d/0x710 fs/read_write.c:686
ksys_write+0xa7/0x170 fs/read_write.c:738
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xa4/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

Fixes: 37dd0247797b1 ("gue: Receive side for Generic UDP Encapsulation")
Reported-by: syzbot+4d8c7d16b0e95c0d0f0d@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/6965534b.050a0220.38aacd.0001.GAE@google.com/
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260115172533.693652-2-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
net/ipv4/fou.c | 3 +++
1 file changed, 3 insertions(+)

diff --git a/net/ipv4/fou.c b/net/ipv4/fou.c
index b1a8e4eec3f6e..e63aa6b52460c 100644
--- a/net/ipv4/fou.c
+++ b/net/ipv4/fou.c
@@ -213,6 +213,9 @@ static int gue_udp_recv(struct sock *sk, struct sk_buff *skb)
return gue_control_message(skb, guehdr);

proto_ctype = guehdr->proto_ctype;
+ if (unlikely(!proto_ctype))
+ goto drop;
+
__skb_pull(skb, sizeof(struct udphdr) + hdrlen);
skb_reset_transport_header(skb);

--
2.51.0