[syzbot] [net?] KMSAN: uninit-value in IP6_ECN_decapsulate (3)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 9bb956508c9d Merge tag 'riscv-for-linus-6.18-rc3' of git:/..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=157ccbe2580000
kernel config: https://syzkaller.appspot.com/x/.config?x=dce7eac4016da338
dashboard link: https://syzkaller.appspot.com/bug?extid=d4dda070f833dc5dc89a
compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=12022258580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=140eaf34580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/e72b2dd5c2f0/disk-9bb95650.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/a1b145971798/vmlinux-9bb95650.xz
kernel image: https://storage.googleapis.com/syzbot-assets/bb711e135af2/bzImage-9bb95650.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+d4dda0...@syzkaller.appspotmail.com

syz.0.17 uses obsolete (PF_INET,SOCK_PACKET)
=====================================================
BUG: KMSAN: uninit-value in __INET_ECN_decapsulate include/net/inet_ecn.h:253 [inline]
BUG: KMSAN: uninit-value in INET_ECN_decapsulate include/net/inet_ecn.h:275 [inline]
BUG: KMSAN: uninit-value in IP6_ECN_decapsulate+0x7a8/0x1fa0 include/net/inet_ecn.h:321
__INET_ECN_decapsulate include/net/inet_ecn.h:253 [inline]
INET_ECN_decapsulate include/net/inet_ecn.h:275 [inline]
IP6_ECN_decapsulate+0x7a8/0x1fa0 include/net/inet_ecn.h:321
ip6ip6_dscp_ecn_decapsulate+0x16f/0x1b0 net/ipv6/ip6_tunnel.c:729
__ip6_tnl_rcv+0xed9/0x1b50 net/ipv6/ip6_tunnel.c:860
ip6_tnl_rcv+0xc3/0x100 net/ipv6/ip6_tunnel.c:903
gre_rcv+0x14fd/0x1b60 net/ipv6/ip6_gre.c:-1
ip6_protocol_deliver_rcu+0x1c89/0x2c60 net/ipv6/ip6_input.c:438
ip6_input_finish+0x1f4/0x4a0 net/ipv6/ip6_input.c:489
NF_HOOK include/linux/netfilter.h:318 [inline]
ip6_input+0x9c/0x330 net/ipv6/ip6_input.c:500
ip6_mc_input+0x7ca/0xc10 net/ipv6/ip6_input.c:590
dst_input include/net/dst.h:474 [inline]
ip6_rcv_finish+0x958/0x990 net/ipv6/ip6_input.c:79
NF_HOOK include/linux/netfilter.h:318 [inline]
ipv6_rcv+0xf1/0x3c0 net/ipv6/ip6_input.c:311
__netif_receive_skb_one_core net/core/dev.c:6079 [inline]
__netif_receive_skb+0x1df/0xac0 net/core/dev.c:6192
netif_receive_skb_internal net/core/dev.c:6278 [inline]
netif_receive_skb+0x57/0x630 net/core/dev.c:6337
tun_rx_batched+0x1df/0x980 drivers/net/tun.c:1485
tun_get_user+0x5d60/0x6d70 drivers/net/tun.c:1953
tun_chr_write_iter+0x3e9/0x5c0 drivers/net/tun.c:1999
new_sync_write fs/read_write.c:593 [inline]
vfs_write+0xbe2/0x15d0 fs/read_write.c:686
ksys_write fs/read_write.c:738 [inline]
__do_sys_write fs/read_write.c:749 [inline]
__se_sys_write fs/read_write.c:746 [inline]
__x64_sys_write+0x1fb/0x4d0 fs/read_write.c:746
x64_sys_call+0x3014/0x3e30 arch/x86/include/generated/asm/syscalls_64.h:2
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xd9/0xfa0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

Uninit was created at:
slab_post_alloc_hook mm/slub.c:4977 [inline]
slab_alloc_node mm/slub.c:5280 [inline]
kmem_cache_alloc_node_noprof+0x989/0x16b0 mm/slub.c:5332
kmalloc_reserve+0x13c/0x4b0 net/core/skbuff.c:579
__alloc_skb+0x347/0x7d0 net/core/skbuff.c:670
alloc_skb include/linux/skbuff.h:1383 [inline]
alloc_skb_with_frags+0xc5/0xa60 net/core/skbuff.c:6671
sock_alloc_send_pskb+0xacc/0xc60 net/core/sock.c:2965
tun_alloc_skb drivers/net/tun.c:1461 [inline]
tun_get_user+0x1142/0x6d70 drivers/net/tun.c:1794
tun_chr_write_iter+0x3e9/0x5c0 drivers/net/tun.c:1999
new_sync_write fs/read_write.c:593 [inline]
vfs_write+0xbe2/0x15d0 fs/read_write.c:686
ksys_write fs/read_write.c:738 [inline]
__do_sys_write fs/read_write.c:749 [inline]
__se_sys_write fs/read_write.c:746 [inline]
__x64_sys_write+0x1fb/0x4d0 fs/read_write.c:746
x64_sys_call+0x3014/0x3e30 arch/x86/include/generated/asm/syscalls_64.h:2
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xd9/0xfa0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

CPU: 1 UID: 0 PID: 6032 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(none)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/02/2025
=====================================================


---
This report is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzk...@googlegroups.com.

syzbot will keep track of this issue. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.

If the report is already addressed, let syzbot know by replying with:
#syz fix: exact-commit-title

If you want syzbot to run the reproducer, reply with:
#syz test: git://repo/address.git branch-or-commit-hash
If you attach or paste a git patch, syzbot will apply it before testing.

If you want to overwrite report's subsystems, reply with:
#syz set subsystems: new-subsystem
(See the list of subsystem names on the web dashboard)

If the report is a duplicate of another one, reply with:
#syz dup: exact-subject-of-another-report

If you want to undo deduplication, reply with:
#syz undup