[syzbot] [net?] KCSAN: data-race in fib_table_lookup / fib_table_lookup (4)
From: syzbot <hidden>
Date: 2026-01-27 04:27:26
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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@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 ================================================================== --- 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 syzkaller@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 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