Message-ID: <aMu7Hw-lJeFPEEUI@shredder>
Date: Thu, 18 Sep 2025 10:56:15 +0300
From: Ido Schimmel <idosch@...sch.org>
To: syzbot <syzbot+6596516dd2b635ba2350@...kaller.appspotmail.com>
Cc: davem@...emloft.net, dsahern@...nel.org, edumazet@...gle.com,
	horms@...nel.org, kuba@...nel.org, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, pabeni@...hat.com,
	syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [net?] general protection fault in find_match (6)

On Tue, Sep 16, 2025 at 10:56:34AM -0700, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    e2291551827f Merge tag 'probes-fixes-v6.16-rc6' of git://g..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=126aad8c580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=f62a2ef17395702a
> dashboard link: https://syzkaller.appspot.com/bug?extid=6596516dd2b635ba2350
> compiler:       Debian clang version 20.1.7 (++20250616065708+6146a88f6049-1~exp1~20250616065826.132), Debian LLD 20.1.7
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1091958c580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=13c89382580000
> 
> Downloadable assets:
> disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/d900f083ada3/non_bootable_disk-e2291551.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/8873881d7728/vmlinux-e2291551.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/c85b06341ad0/bzImage-e2291551.xz
> mounted in repro: https://storage.googleapis.com/syzbot-assets/6dce6e633409/mount_8.gz
>   fsck result: failed (log: https://syzkaller.appspot.com/x/fsck.log?x=17c3e7d4580000)
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+6596516dd2b635ba2350@...kaller.appspotmail.com
> 
> Oops: general protection fault, probably for non-canonical address 0xdffffc0000000018: 0000 [#1] SMP KASAN NOPTI
> KASAN: null-ptr-deref in range [0x00000000000000c0-0x00000000000000c7]
> CPU: 0 UID: 0 PID: 3043 Comm: kworker/u4:11 Not tainted 6.16.0-rc6-syzkaller-00037-ge2291551827f #0 PREEMPT(full) 
> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
> Workqueue: ipv6_addrconf addrconf_dad_work
> RIP: 0010:__in6_dev_get include/net/addrconf.h:347 [inline]

Problem is that FDB nexthops can end up in non-FDB nexthop groups and
FDB nexthops are not associated with a nexthop device.

The kernel rejects such configurations upon addition:

# ip nexthop add id 1 via 192.0.2.1 fdb
# ip nexthop add id 2 group 1
Error: Non FDB nexthop group cannot have fdb nexthops.

But not upon replacement:

# ip nexthop add id 1 blackhole
# ip nexthop add id 2 group 1
# ip nexthop replace id 1 via 192.0.2.1 fdb
# ip nexthop
id 1 via 192.0.2.1 scope link fdb
id 2 group 1

I will try to send a fix later today.

> RIP: 0010:ip6_ignore_linkdown include/net/addrconf.h:443 [inline]
> RIP: 0010:find_match+0xa3/0xc90 net/ipv6/route.c:781
> Code: 00 00 00 00 00 fc ff df 42 80 7c 25 00 00 74 08 48 89 df e8 cf c4 fc f7 48 89 d8 bb c0 00 00 00 48 03 18 48 89 d8 48 c1 e8 03 <42> 80 3c 20 00 74 08 48 89 df e8 ae c4 fc f7 48 8b 1b e8 f6 60 48
> RSP: 0018:ffffc9000d5ce430 EFLAGS: 00010206
> RAX: 0000000000000018 RBX: 00000000000000c0 RCX: 0000000000000000
> RDX: ffff88803f90c880 RSI: 0000000000000000 RDI: 0000000000000000
> RBP: 1ffff1100b37b324 R08: ffffc9000d5ce7c0 R09: ffffc9000d5ce7d0
> R10: ffffc9000d5ce620 R11: ffffffff8a26ecd0 R12: dffffc0000000000
> R13: 0000000000000002 R14: 1ffff1100b37b326 R15: ffff888059bd9937
> FS:  0000000000000000(0000) GS:ffff88808d21b000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007ffd76fadfd8 CR3: 000000000df38000 CR4: 0000000000352ef0
> Call Trace:
>  <TASK>
>  rt6_nh_find_match+0xd9/0x150 net/ipv6/route.c:821
>  nexthop_for_each_fib6_nh+0x1cd/0x400 net/ipv4/nexthop.c:1516
>  __find_rr_leaf+0x461/0x6d0 net/ipv6/route.c:862
>  find_rr_leaf net/ipv6/route.c:890 [inline]
>  rt6_select net/ipv6/route.c:934 [inline]
>  fib6_table_lookup+0x39f/0xa80 net/ipv6/route.c:2232
>  ip6_pol_route+0x222/0x1180 net/ipv6/route.c:2268
>  pol_lookup_func include/net/ip6_fib.h:617 [inline]
>  fib6_rule_lookup+0x348/0x6f0 net/ipv6/fib6_rules.c:125
>  ip6_route_output_flags_noref net/ipv6/route.c:2683 [inline]
>  ip6_route_output_flags+0x364/0x5d0 net/ipv6/route.c:2695
>  ip6_route_output include/net/ip6_route.h:93 [inline]
>  ip6_dst_lookup_tail+0x1ae/0x1510 net/ipv6/ip6_output.c:1128
>  ip6_dst_lookup_flow+0x47/0xe0 net/ipv6/ip6_output.c:1259
>  udp_tunnel6_dst_lookup+0x231/0x3c0 net/ipv6/ip6_udp_tunnel.c:165
>  geneve6_xmit_skb drivers/net/geneve.c:957 [inline]
>  geneve_xmit+0xd2e/0x2b70 drivers/net/geneve.c:1043
>  __netdev_start_xmit include/linux/netdevice.h:5215 [inline]
>  netdev_start_xmit include/linux/netdevice.h:5224 [inline]
>  xmit_one net/core/dev.c:3830 [inline]
>  dev_hard_start_xmit+0x2d4/0x830 net/core/dev.c:3846
>  __dev_queue_xmit+0x1adf/0x3a70 net/core/dev.c:4713
>  dev_queue_xmit include/linux/netdevice.h:3355 [inline]
>  neigh_hh_output include/net/neighbour.h:523 [inline]
>  neigh_output include/net/neighbour.h:537 [inline]
>  ip6_finish_output2+0x11bc/0x16a0 net/ipv6/ip6_output.c:141
>  __ip6_finish_output net/ipv6/ip6_output.c:-1 [inline]
>  ip6_finish_output+0x234/0x7d0 net/ipv6/ip6_output.c:226
>  NF_HOOK+0x9e/0x380 include/linux/netfilter.h:317
>  mld_sendpack+0x800/0xd80 net/ipv6/mcast.c:1868
>  ipv6_mc_dad_complete+0x88/0x4b0 net/ipv6/mcast.c:2293
>  addrconf_dad_completed+0x6d5/0xd60 net/ipv6/addrconf.c:4339
>  addrconf_dad_work+0xc36/0x14b0 net/ipv6/addrconf.c:-1
>  process_one_work kernel/workqueue.c:3238 [inline]
>  process_scheduled_works+0xae1/0x17b0 kernel/workqueue.c:3321
>  worker_thread+0x8a0/0xda0 kernel/workqueue.c:3402
>  kthread+0x70e/0x8a0 kernel/kthread.c:464
>  ret_from_fork+0x3fc/0x770 arch/x86/kernel/process.c:148
>  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
>  </TASK>

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