[PATCH v1 net-next] ipv4: fib: Annotate access to struct fib_alias.fa_state.

[Kuniyuki Iwashima <kuniyu@google.com>]

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 <kuniyu@google.com>
---
 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