[syzbot] [net?] KCSAN: data-race in fib_table_lookup / fib_table_lookup (4)

[syzbot]

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+d24f94...@syzkaller.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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