syzbot reported that struct fib_alias.fa_state can be modified locklessly by RCU readers. [0] Let's use READ_ONCE()/WRITE_ONCE() properly. [0]: 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 Reported-by: syzbot+d24f940f770afda885cf@syzkaller.appspotmail.com Closes: https://lore.kernel.org/netdev/69783ead.050a0220.c9109.0013.GAE@google.com/ Signed-off-by: Kuniyuki Iwashima --- net/ipv4/fib_lookup.h | 6 ++++-- net/ipv4/fib_trie.c | 4 ++-- 2 files changed, 6 insertions(+), 4 deletions(-) diff --git a/net/ipv4/fib_lookup.h b/net/ipv4/fib_lookup.h index f9b9e26c32c1..0b72796dd1ad 100644 --- a/net/ipv4/fib_lookup.h +++ b/net/ipv4/fib_lookup.h @@ -28,8 +28,10 @@ struct fib_alias { /* Don't write on fa_state unless needed, to keep it shared on all cpus */ static inline void fib_alias_accessed(struct fib_alias *fa) { - if (!(fa->fa_state & FA_S_ACCESSED)) - fa->fa_state |= FA_S_ACCESSED; + u8 fa_state = READ_ONCE(fa->fa_state); + + if (!(fa_state & FA_S_ACCESSED)) + WRITE_ONCE(fa->fa_state, fa_state | FA_S_ACCESSED); } /* Exported by fib_semantics.c */ diff --git a/net/ipv4/fib_trie.c b/net/ipv4/fib_trie.c index 7e2c17fec3fc..1308213791f1 100644 --- a/net/ipv4/fib_trie.c +++ b/net/ipv4/fib_trie.c @@ -1280,7 +1280,7 @@ int fib_table_insert(struct net *net, struct fib_table *tb, new_fa->fa_dscp = fa->fa_dscp; new_fa->fa_info = fi; new_fa->fa_type = cfg->fc_type; - state = fa->fa_state; + state = READ_ONCE(fa->fa_state); new_fa->fa_state = state & ~FA_S_ACCESSED; new_fa->fa_slen = fa->fa_slen; new_fa->tb_id = tb->tb_id; @@ -1745,7 +1745,7 @@ int fib_table_delete(struct net *net, struct fib_table *tb, fib_remove_alias(t, tp, l, fa_to_delete); - if (fa_to_delete->fa_state & FA_S_ACCESSED) + if (READ_ONCE(fa_to_delete->fa_state) & FA_S_ACCESSED) rt_cache_flush(cfg->fc_nlinfo.nl_net); fib_release_info(fa_to_delete->fa_info); -- 2.52.0.457.g6b5491de43-goog