netdev - [PATCH v1 net] af_unix: Annotate data-races around sk->sk

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Date: Sat, 18 May 2024 10:13:46 +0900
From: Kuniyuki Iwashima <kuniyu@...zon.com>
To: "David S. Miller" <davem@...emloft.net>, Eric Dumazet
	<edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni
	<pabeni@...hat.com>
CC: Kuniyuki Iwashima <kuniyu@...zon.com>, Kuniyuki Iwashima
	<kuni1840@...il.com>, <netdev@...r.kernel.org>, syzkaller
	<syzkaller@...glegroups.com>
Subject: [PATCH v1 net] af_unix: Annotate data-races around sk->sk_hash.

syzkaller reported data-race of sk->sk_hash in unix_autobind() [0],
and the same ones exist in unix_bind_bsd() and unix_bind_abstract().

The three bind() functions prefetch sk->sk_hash locklessly and
use it later after validating that unix_sk(sk)->addr is NULL under
unix_sk(sk)->bindlock.

The prefetched sk->sk_hash is the hash value of unbound socket set
in unix_create1() and does not change until bind() completes.

There could be a chance that sk->sk_hash changes after the lockless
read.  However, in such a case, non-NULL unix_sk(sk)->addr is visible
under unix_sk(sk)->bindlock, and bind() returns -EINVAL without using
the prefetched value.

The KCSAN splat is false-positive, but let's use WRITE_ONCE() and
READ_ONCE() to silence it.

[0]:
BUG: KCSAN: data-race in unix_autobind / unix_autobind

write to 0xffff888034a9fb88 of 4 bytes by task 4468 on cpu 0:
 __unix_set_addr_hash net/unix/af_unix.c:331 [inline]
 unix_autobind+0x47a/0x7d0 net/unix/af_unix.c:1185
 unix_dgram_connect+0x7e3/0x890 net/unix/af_unix.c:1373
 __sys_connect_file+0xd7/0xe0 net/socket.c:2048
 __sys_connect+0x114/0x140 net/socket.c:2065
 __do_sys_connect net/socket.c:2075 [inline]
 __se_sys_connect net/socket.c:2072 [inline]
 __x64_sys_connect+0x40/0x50 net/socket.c:2072
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0x4f/0x110 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x46/0x4e

read to 0xffff888034a9fb88 of 4 bytes by task 4465 on cpu 1:
 unix_autobind+0x28/0x7d0 net/unix/af_unix.c:1134
 unix_dgram_connect+0x7e3/0x890 net/unix/af_unix.c:1373
 __sys_connect_file+0xd7/0xe0 net/socket.c:2048
 __sys_connect+0x114/0x140 net/socket.c:2065
 __do_sys_connect net/socket.c:2075 [inline]
 __se_sys_connect net/socket.c:2072 [inline]
 __x64_sys_connect+0x40/0x50 net/socket.c:2072
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0x4f/0x110 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x46/0x4e

value changed: 0x000000e4 -> 0x000001e3

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 4465 Comm: syz-executor.0 Not tainted 6.8.0-12822-gcd51db110a7e #12
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.0-0-gd239552ce722-prebuilt.qemu.org 04/01/2014

Fixes: afd20b9290e1 ("af_unix: Replace the big lock with small locks.")
Reported-by: syzkaller <syzkaller@...glegroups.com>
Signed-off-by: Kuniyuki Iwashima <kuniyu@...zon.com>
---
 net/unix/af_unix.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 92a88ac070ca..e92b45e21664 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -327,8 +327,7 @@ static void __unix_set_addr_hash(struct net *net, struct sock *sk,
 {
 	__unix_remove_socket(sk);
 	smp_store_release(&unix_sk(sk)->addr, addr);
-
-	sk->sk_hash = hash;
+	WRITE_ONCE(sk->sk_hash, hash);
 	__unix_insert_socket(net, sk);
 }
 
@@ -1131,7 +1130,7 @@ static struct sock *unix_find_other(struct net *net,
 
 static int unix_autobind(struct sock *sk)
 {
-	unsigned int new_hash, old_hash = sk->sk_hash;
+	unsigned int new_hash, old_hash = READ_ONCE(sk->sk_hash);
 	struct unix_sock *u = unix_sk(sk);
 	struct net *net = sock_net(sk);
 	struct unix_address *addr;
@@ -1195,7 +1194,7 @@ static int unix_bind_bsd(struct sock *sk, struct sockaddr_un *sunaddr,
 {
 	umode_t mode = S_IFSOCK |
 	       (SOCK_INODE(sk->sk_socket)->i_mode & ~current_umask());
-	unsigned int new_hash, old_hash = sk->sk_hash;
+	unsigned int new_hash, old_hash = READ_ONCE(sk->sk_hash);
 	struct unix_sock *u = unix_sk(sk);
 	struct net *net = sock_net(sk);
 	struct mnt_idmap *idmap;
@@ -1261,7 +1260,7 @@ static int unix_bind_bsd(struct sock *sk, struct sockaddr_un *sunaddr,
 static int unix_bind_abstract(struct sock *sk, struct sockaddr_un *sunaddr,
 			      int addr_len)
 {
-	unsigned int new_hash, old_hash = sk->sk_hash;
+	unsigned int new_hash, old_hash = READ_ONCE(sk->sk_hash);
 	struct unix_sock *u = unix_sk(sk);
 	struct net *net = sock_net(sk);
 	struct unix_address *addr;
-- 
2.30.2