CVE-2026-23095: gue: Fix skb memleak with inner IP protocol 0.

In the Linux kernel, the following vulnerability has been resolved:

gue: Fix skb memleak with inner IP protocol 0.

syzbot reported skb memleak below. [0]

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

gueudprecv() returns -guehdr->protoctype for "resubmit" in ipprotocoldeliverrcu(), 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): kmemleakallocrecursive include/linux/kmemleak.h:44 [inline] slabpostallochook mm/slub.c:4958 [inline] slaballocnode mm/slub.c:5263 [inline] kmemcacheallocnoprof+0x3b4/0x590 mm/slub.c:5270 buildskb+0x23/0x60 net/core/skbuff.c:474 buildskb+0x20/0x190 net/core/skbuff.c:490 tunbuildskb drivers/net/tun.c:1541 [inline] tunbuildskb+0x4a1/0xa40 drivers/net/tun.c:1636 tungetuser+0xc12/0x2030 drivers/net/tun.c:1770 tunchrwriteiter+0x71/0x120 drivers/net/tun.c:1999 newsyncwrite fs/readwrite.c:593 [inline] vfswrite+0x45d/0x710 fs/readwrite.c:686 ksyswrite+0xa7/0x170 fs/readwrite.c:738 dosyscallx64 arch/x86/entry/syscall64.c:63 [inline] dosyscall64+0xa4/0xf80 arch/x86/entry/syscall64.c:94 entrySYSCALL64afterhwframe+0x77/0x7f