[syzbot] [net?] kernel BUG in fib6_add_rt2node (2)

[syzbot] [net?] kernel BUG in fib6_add_rt2node (2)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    1d528e794f3d Merge branch 'bpf-fix-bpf_d_path-helper-proto..
git tree:       bpf
console output: https://syzkaller.appspot.com/x/log.txt?x=159c51c2580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=9e5198eaf003f1d1
dashboard link: https://syzkaller.appspot.com/bug?extid=cb809def1baaac68ab92
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=139c51c2580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=11b2961a580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/aec210aa3af6/disk-1d528e79.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/e7d58dbcc713/vmlinux-1d528e79.xz
kernel image: https://storage.googleapis.com/syzbot-assets/c22e61a8447d/bzImage-1d528e79.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+cb809def1baaac68ab92@syzkaller.appspotmail.com

------------[ cut here ]------------
kernel BUG at net/ipv6/ip6_fib.c:1217!
Oops: invalid opcode: 0000 [#1] SMP KASAN PTI
CPU: 0 UID: 0 PID: 6010 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
RIP: 0010:fib6_add_rt2node+0x3433/0x3470 net/ipv6/ip6_fib.c:1217
Code: ff ff 48 8b 4c 24 08 80 e1 07 80 c1 03 38 c1 0f 8c a4 f9 ff ff 48 8b 7c 24 08 e8 08 4d 28 f8 e9 95 f9 ff ff e8 0e 64 c2 f7 90 <0f> 0b 44 89 f9 80 e1 07 80 c1 03 38 c1 0f 8c 7a fa ff ff 4c 89 ff
RSP: 0018:ffffc900030a7660 EFLAGS: 00010293
RAX: ffffffff89fee772 RBX: 0000000000000001 RCX: ffff88807a2abd00
RDX: 0000000000000000 RSI: 0000000000000002 RDI: 0000000000000001
RBP: ffffc900030a7850 R08: ffffc900030a7910 R09: 0000000000000050
R10: 0000000000000000 R11: fffff52000614f24 R12: ffff888074779818
R13: ffff888074779800 R14: dffffc0000000000 R15: 0000000000000002
FS:  000055556dfd5500(0000) GS:ffff8881260b1000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000000 CR3: 0000000074636000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 fib6_add+0x8da/0x18a0 net/ipv6/ip6_fib.c:1532
 __ip6_ins_rt net/ipv6/route.c:1351 [inline]
 ip6_route_add+0xde/0x1b0 net/ipv6/route.c:3946
 ipv6_route_ioctl+0x35c/0x480 net/ipv6/route.c:4571
 inet6_ioctl+0x219/0x280 net/ipv6/af_inet6.c:577
 sock_do_ioctl+0xdc/0x300 net/socket.c:1245
 sock_ioctl+0x576/0x790 net/socket.c:1366
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:597 [inline]
 __se_sys_ioctl+0xfc/0x170 fs/ioctl.c:583
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xfa/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fbf1918f749
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffe117d8c08 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007fbf193e5fa0 RCX: 00007fbf1918f749
RDX: 0000200000000000 RSI: 000000000000890b RDI: 0000000000000007
RBP: 00007fbf19213f91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fbf193e5fa0 R14: 00007fbf193e5fa0 R15: 0000000000000003
 </TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:fib6_add_rt2node+0x3433/0x3470 net/ipv6/ip6_fib.c:1217
Code: ff ff 48 8b 4c 24 08 80 e1 07 80 c1 03 38 c1 0f 8c a4 f9 ff ff 48 8b 7c 24 08 e8 08 4d 28 f8 e9 95 f9 ff ff e8 0e 64 c2 f7 90 <0f> 0b 44 89 f9 80 e1 07 80 c1 03 38 c1 0f 8c 7a fa ff ff 4c 89 ff
RSP: 0018:ffffc900030a7660 EFLAGS: 00010293
RAX: ffffffff89fee772 RBX: 0000000000000001 RCX: ffff88807a2abd00
RDX: 0000000000000000 RSI: 0000000000000002 RDI: 0000000000000001
RBP: ffffc900030a7850 R08: ffffc900030a7910 R09: 0000000000000050
R10: 0000000000000000 R11: fffff52000614f24 R12: ffff888074779818
R13: ffff888074779800 R14: dffffc0000000000 R15: 0000000000000002
FS:  000055556dfd5500(0000) GS:ffff8881260b1000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000000 CR3: 0000000074636000 CR4: 00000000003526f0


---
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Re: [syzbot] [net?] kernel BUG in fib6_add_rt2node (2)

[David Ahern]

On 1/6/26 10:58 AM, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    1d528e794f3d Merge branch 'bpf-fix-bpf_d_path-helper-proto..
> git tree:       bpf
> console output: https://syzkaller.appspot.com/x/log.txt?x=159c51c2580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=9e5198eaf003f1d1

This config at

commit 59ba823e689f832f389ea6af6e7ae5842b3c860a (HEAD -> net-next,
net-next/main)
Merge: 76de4e1594b7 f2a3b12b305c
Author: Jakub Kicinski <kuba@kernel.org>
Date:   Thu Jan 8 11:37:07 2026 -0800

    Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net

> dashboard link: https://syzkaller.appspot.com/bug?extid=cb809def1baaac68ab92
> 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=139c51c2580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=11b2961a580000

and this reproducer does not show a problem for me.

Forwarded: Test for cb809def1baaac68ab92

[syzbot]

For archival purposes, forwarding an incoming command email to
linux-kernel@vger.kernel.org.

***

Subject: Test for cb809def1baaac68ab92
Author: syoshida@redhat.com

#syz test

diff --git a/net/ipv6/ip6_fib.c b/net/ipv6/ip6_fib.c
index 2111af022d94..c6439e30e892 100644
--- a/net/ipv6/ip6_fib.c
+++ b/net/ipv6/ip6_fib.c
@@ -1138,7 +1138,8 @@ static int fib6_add_rt2node(struct fib6_node *fn, struct fib6_info *rt,
 					fib6_set_expires(iter, rt->expires);
 					fib6_add_gc_list(iter);
 				}
-				if (!(rt->fib6_flags & (RTF_ADDRCONF | RTF_PREFIX_RT))) {
+				if (!(rt->fib6_flags & (RTF_ADDRCONF | RTF_PREFIX_RT)) &&
+				    !iter->fib6_nh->fib_nh_gw_family) {
 					iter->fib6_flags &= ~RTF_ADDRCONF;
 					iter->fib6_flags &= ~RTF_PREFIX_RT;
 				}