netdev - [PATCH v2 net] af_unix: Annotate data-race around unix

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Date: Thu, 23 May 2024 00:40:02 +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 v2 net] af_unix: Annotate data-race around unix_sk(sk)->addr.

Once unix_sk(sk)->addr is assigned under net->unx.table.locks and
unix_sk(sk)->bindlock, *(unix_sk(sk)->addr) and unix_sk(sk)->path are
fully set up, and unix_sk(sk)->addr is never changed.

unix_getname() and unix_copy_addr() access the two fields locklessly,
and commit ae3b564179bf ("missing barriers in some of unix_sock ->addr
and ->path accesses") added smp_store_release() and smp_load_acquire()
pairs.

In other functions, we still read unix_sk(sk)->addr locklessly to check
if the socket is bound, and KCSAN complains about it.  [0]

Given these functions have no dependency for *(unix_sk(sk)->addr) and
unix_sk(sk)->path, READ_ONCE() is enough to annotate the data-race.

Note that it is safe to access unix_sk(sk)->addr locklessly if the socket
is found in the hash table.  For example, the lockless read of otheru->addr
in unix_stream_connect() is safe.

Note also that newu->addr there is of the child socket that is still not
accessible from userspace, and smp_store_release() publishes the address
in case the socket is accept()ed and unix_getname() / unix_copy_addr()
is called.

[0]:
BUG: KCSAN: data-race in unix_bind / unix_listen

write (marked) to 0xffff88805f8d1840 of 8 bytes by task 13723 on cpu 0:
 __unix_set_addr_hash net/unix/af_unix.c:329 [inline]
 unix_bind_bsd net/unix/af_unix.c:1241 [inline]
 unix_bind+0x881/0x1000 net/unix/af_unix.c:1319
 __sys_bind+0x194/0x1e0 net/socket.c:1847
 __do_sys_bind net/socket.c:1858 [inline]
 __se_sys_bind net/socket.c:1856 [inline]
 __x64_sys_bind+0x40/0x50 net/socket.c:1856
 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 0xffff88805f8d1840 of 8 bytes by task 13724 on cpu 1:
 unix_listen+0x72/0x180 net/unix/af_unix.c:734
 __sys_listen+0xdc/0x160 net/socket.c:1881
 __do_sys_listen net/socket.c:1890 [inline]
 __se_sys_listen net/socket.c:1888 [inline]
 __x64_sys_listen+0x2e/0x40 net/socket.c:1888
 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: 0x0000000000000000 -> 0xffff88807b5b1b40

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 13724 Comm: syz-executor.4 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: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: syzkaller <syzkaller@...glegroups.com>
Signed-off-by: Kuniyuki Iwashima <kuniyu@...zon.com>
---
v2: Add explanation why lockless read of otheru->addr is safe in
    unix_stream_connect()

v1: https://lore.kernel.org/netdev/20240518000148.27947-1-kuniyu@amazon.com/
---
 net/unix/af_unix.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index ca101690e740..92a88ac070ca 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -731,7 +731,7 @@ static int unix_listen(struct socket *sock, int backlog)
 	if (sock->type != SOCK_STREAM && sock->type != SOCK_SEQPACKET)
 		goto out;	/* Only stream/seqpacket sockets accept */
 	err = -EINVAL;
-	if (!u->addr)
+	if (!READ_ONCE(u->addr))
 		goto out;	/* No listens on an unbound socket */
 	unix_state_lock(sk);
 	if (sk->sk_state != TCP_CLOSE && sk->sk_state != TCP_LISTEN)
@@ -1369,7 +1369,7 @@ static int unix_dgram_connect(struct socket *sock, struct sockaddr *addr,
 
 		if ((test_bit(SOCK_PASSCRED, &sock->flags) ||
 		     test_bit(SOCK_PASSPIDFD, &sock->flags)) &&
-		    !unix_sk(sk)->addr) {
+		    !READ_ONCE(unix_sk(sk)->addr)) {
 			err = unix_autobind(sk);
 			if (err)
 				goto out;
@@ -1481,7 +1481,8 @@ static int unix_stream_connect(struct socket *sock, struct sockaddr *uaddr,
 		goto out;
 
 	if ((test_bit(SOCK_PASSCRED, &sock->flags) ||
-	     test_bit(SOCK_PASSPIDFD, &sock->flags)) && !u->addr) {
+	     test_bit(SOCK_PASSPIDFD, &sock->flags)) &&
+	    !READ_ONCE(u->addr)) {
 		err = unix_autobind(sk);
 		if (err)
 			goto out;
@@ -1951,7 +1952,8 @@ static int unix_dgram_sendmsg(struct socket *sock, struct msghdr *msg,
 	}
 
 	if ((test_bit(SOCK_PASSCRED, &sock->flags) ||
-	     test_bit(SOCK_PASSPIDFD, &sock->flags)) && !u->addr) {
+	     test_bit(SOCK_PASSPIDFD, &sock->flags)) &&
+	    !READ_ONCE(u->addr)) {
 		err = unix_autobind(sk);
 		if (err)
 			goto out;
-- 
2.30.2