netlink: annotate data races around nlk->bound

While existing code is correct, KCSAN is reporting
a data-race in netlink_insert / netlink_sendmsg [1]

It is correct to read nlk->bound without a lock, as netlink_autobind()
will acquire all needed locks.

[1]
BUG: KCSAN: data-race in netlink_insert / netlink_sendmsg

write to 0xffff8881031c8b30 of 1 bytes by task 18752 on cpu 0:
 netlink_insert+0x5cc/0x7f0 net/netlink/af_netlink.c:597
 netlink_autobind+0xa9/0x150 net/netlink/af_netlink.c:842
 netlink_sendmsg+0x479/0x7c0 net/netlink/af_netlink.c:1892
 sock_sendmsg_nosec net/socket.c:703 [inline]
 sock_sendmsg net/socket.c:723 [inline]
 ____sys_sendmsg+0x360/0x4d0 net/socket.c:2392
 ___sys_sendmsg net/socket.c:2446 [inline]
 __sys_sendmsg+0x1ed/0x270 net/socket.c:2475
 __do_sys_sendmsg net/socket.c:2484 [inline]
 __se_sys_sendmsg net/socket.c:2482 [inline]
 __x64_sys_sendmsg+0x42/0x50 net/socket.c:2482
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x3d/0x90 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff8881031c8b30 of 1 bytes by task 18751 on cpu 1:
 netlink_sendmsg+0x270/0x7c0 net/netlink/af_netlink.c:1891
 sock_sendmsg_nosec net/socket.c:703 [inline]
 sock_sendmsg net/socket.c:723 [inline]
 __sys_sendto+0x2a8/0x370 net/socket.c:2019
 __do_sys_sendto net/socket.c:2031 [inline]
 __se_sys_sendto net/socket.c:2027 [inline]
 __x64_sys_sendto+0x74/0x90 net/socket.c:2027
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x3d/0x90 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

value changed: 0x00 -> 0x01

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 18751 Comm: syz-executor.0 Not tainted 5.14.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011

Fixes: da314c9923fe ("netlink: Replace rhash_portid with bound")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index 24b7cf4..ada47e5 100644 (file)
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -594,7 +594,10 @@ static int netlink_insert(struct sock *sk, u32 portid)
 
        /* We need to ensure that the socket is hashed and visible. */
        smp_wmb();
-       nlk_sk(sk)->bound = portid;
+       /* Paired with lockless reads from netlink_bind(),
+        * netlink_connect() and netlink_sendmsg().
+        */
+       WRITE_ONCE(nlk_sk(sk)->bound, portid);
 
 err:
        release_sock(sk);
@@ -1012,7 +1015,8 @@ static int netlink_bind(struct socket *sock, struct sockaddr *addr,
        if (nlk->ngroups < BITS_PER_LONG)
                groups &= (1UL << nlk->ngroups) - 1;
 
-       bound = nlk->bound;
+       /* Paired with WRITE_ONCE() in netlink_insert() */
+       bound = READ_ONCE(nlk->bound);
        if (bound) {
                /* Ensure nlk->portid is up-to-date. */
                smp_rmb();
@@ -1098,8 +1102,9 @@ static int netlink_connect(struct socket *sock, struct sockaddr *addr,
 
        /* No need for barriers here as we return to user-space without
         * using any of the bound attributes.
+        * Paired with WRITE_ONCE() in netlink_insert().
         */
-       if (!nlk->bound)
+       if (!READ_ONCE(nlk->bound))
                err = netlink_autobind(sock);
 
        if (err == 0) {
@@ -1888,7 +1893,8 @@ static int netlink_sendmsg(struct socket *sock, struct msghdr *msg, size_t len)
                dst_group = nlk->dst_group;
        }
 
-       if (!nlk->bound) {
+       /* Paired with WRITE_ONCE() in netlink_insert() */
+       if (!READ_ONCE(nlk->bound)) {
                err = netlink_autobind(sock);
                if (err)
                        goto out;