linux-kernel - [syzbot] [net?] KCSAN: data-race in fib_table_lookup / fib_table

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Message-ID: <69783ead.050a0220.c9109.0013.GAE@google.com>
Date: Mon, 26 Jan 2026 20:27:25 -0800
From: syzbot <syzbot+d24f940f770afda885cf@...kaller.appspotmail.com>
To: 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: [syzbot] [net?] KCSAN: data-race in fib_table_lookup /
 fib_table_lookup (4)

Hello,

syzbot found the following issue on:

HEAD commit:    fcb70a56f4d8 Merge tag 'vfs-6.19-rc8.fixes' of git://git.k..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=10c8a05a580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=8e27f4588a0f2183
dashboard link: https://syzkaller.appspot.com/bug?extid=d24f940f770afda885cf
compiler:       Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/f09fa28d6094/disk-fcb70a56.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/d7b2aaa28bcd/vmlinux-fcb70a56.xz
kernel image: https://storage.googleapis.com/syzbot-assets/aa72cf9601a0/bzImage-fcb70a56.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+d24f940f770afda885cf@...kaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in fib_table_lookup / fib_table_lookup

write to 0xffff88811b06a7fa of 1 bytes by task 4167 on cpu 0:
 fib_alias_accessed net/ipv4/fib_lookup.h:32 [inline]
 fib_table_lookup+0x361/0xd60 net/ipv4/fib_trie.c:1565
 fib_lookup include/net/ip_fib.h:390 [inline]
 ip_route_output_key_hash_rcu+0x378/0x1380 net/ipv4/route.c:2814
 ip_route_output_key_hash net/ipv4/route.c:2705 [inline]
 __ip_route_output_key include/net/route.h:169 [inline]
 ip_route_output_flow+0x65/0x110 net/ipv4/route.c:2932
 udp_sendmsg+0x13c3/0x15d0 net/ipv4/udp.c:1450
 inet_sendmsg+0xac/0xd0 net/ipv4/af_inet.c:859
 sock_sendmsg_nosec net/socket.c:727 [inline]
 __sock_sendmsg net/socket.c:742 [inline]
 ____sys_sendmsg+0x53a/0x600 net/socket.c:2592
 ___sys_sendmsg+0x195/0x1e0 net/socket.c:2646
 __sys_sendmmsg+0x185/0x320 net/socket.c:2735
 __do_sys_sendmmsg net/socket.c:2762 [inline]
 __se_sys_sendmmsg net/socket.c:2759 [inline]
 __x64_sys_sendmmsg+0x57/0x70 net/socket.c:2759
 x64_sys_call+0x1e28/0x3000 arch/x86/include/generated/asm/syscalls_64.h:308
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xc0/0x2a0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff88811b06a7fa of 1 bytes by task 4168 on cpu 1:
 fib_alias_accessed net/ipv4/fib_lookup.h:31 [inline]
 fib_table_lookup+0x338/0xd60 net/ipv4/fib_trie.c:1565
 fib_lookup include/net/ip_fib.h:390 [inline]
 ip_route_output_key_hash_rcu+0x378/0x1380 net/ipv4/route.c:2814
 ip_route_output_key_hash net/ipv4/route.c:2705 [inline]
 __ip_route_output_key include/net/route.h:169 [inline]
 ip_route_output_flow+0x65/0x110 net/ipv4/route.c:2932
 udp_sendmsg+0x13c3/0x15d0 net/ipv4/udp.c:1450
 inet_sendmsg+0xac/0xd0 net/ipv4/af_inet.c:859
 sock_sendmsg_nosec net/socket.c:727 [inline]
 __sock_sendmsg net/socket.c:742 [inline]
 ____sys_sendmsg+0x53a/0x600 net/socket.c:2592
 ___sys_sendmsg+0x195/0x1e0 net/socket.c:2646
 __sys_sendmmsg+0x185/0x320 net/socket.c:2735
 __do_sys_sendmmsg net/socket.c:2762 [inline]
 __se_sys_sendmmsg net/socket.c:2759 [inline]
 __x64_sys_sendmmsg+0x57/0x70 net/socket.c:2759
 x64_sys_call+0x1e28/0x3000 arch/x86/include/generated/asm/syscalls_64.h:308
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xc0/0x2a0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0x00 -> 0x01

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 4168 Comm: syz.4.206 Not tainted syzkaller #0 PREEMPT(voluntary) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
==================================================================


---
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