missing barriers in some of unix_sock ->addr and ->path accesses

Several u->addr and u->path users are not holding any locks in
common with unix_bind().  unix_state_lock() is useless for those
purposes.

u->addr is assign-once and *(u->addr) is fully set up by the time
we set u->addr (all under unix_table_lock).  u->path is also
set in the same critical area, also before setting u->addr, and
any unix_sock with ->path filled will have non-NULL ->addr.

So setting ->addr with smp_store_release() is all we need for those
"lockless" users - just have them fetch ->addr with smp_load_acquire()
and don't even bother looking at ->path if they see NULL ->addr.

Users of ->addr and ->path fall into several classes now:
    1) ones that do smp_load_acquire(u->addr) and access *(u->addr)
and u->path only if smp_load_acquire() has returned non-NULL.
    2) places holding unix_table_lock.  These are guaranteed that
*(u->addr) is seen fully initialized.  If unix_sock is in one of the
"bound" chains, so's ->path.
    3) unix_sock_destructor() using ->addr is safe.  All places
that set u->addr are guaranteed to have seen all stores *(u->addr)
while holding a reference to u and unix_sock_destructor() is called
when (atomic) refcount hits zero.
    4) unix_release_sock() using ->path is safe.  unix_bind()
is serialized wrt unix_release() (normally - by struct file
refcount), and for the instances that had ->path set by unix_bind()
unix_release_sock() comes from unix_release(), so they are fine.
Instances that had it set in unix_stream_connect() either end up
attached to a socket (in unix_accept()), in which case the call
chain to unix_release_sock() and serialization are the same as in
the previous case, or they never get accept'ed and unix_release_sock()
is called when the listener is shut down and its queue gets purged.
In that case the listener's queue lock provides the barriers needed -
unix_stream_connect() shoves our unix_sock into listener's queue
under that lock right after having set ->path and eventual
unix_release_sock() caller picks them from that queue under the
same lock right before calling unix_release_sock().
    5) unix_find_other() use of ->path is pointless, but safe -
it happens with successful lookup by (abstract) name, so ->path.dentry
is guaranteed to be NULL there.

earlier-variant-reviewed-by: "Paul E. McKenney" <paulmck@linux.ibm.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 6 insertions, 4 deletions

diff --git a/security/lsm_audit.c b/security/lsm_audit.c
index f84001019356..33028c098ef3 100644
--- a/security/lsm_audit.c
+++ b/security/lsm_audit.c
@@ -321,6 +321,7 @@ static void dump_common_audit_data(struct audit_buffer *ab,
 		if (a->u.net->sk) {
 			struct sock *sk = a->u.net->sk;
 			struct unix_sock *u;
+			struct unix_address *addr;
 			int len = 0;
 			char *p = NULL;
 
@@ -351,14 +352,15 @@ static void dump_common_audit_data(struct audit_buffer *ab,
 #endif
 			case AF_UNIX:
 				u = unix_sk(sk);
+				addr = smp_load_acquire(&u->addr);
+				if (!addr)
+					break;
 				if (u->path.dentry) {
 					audit_log_d_path(ab, " path=", &u->path);
 					break;
 				}
-				if (!u->addr)
-					break;
-				len = u->addr->len-sizeof(short);
-				p = &u->addr->name->sun_path[0];
+				len = addr->len-sizeof(short);
+				p = &addr->name->sun_path[0];
 				audit_log_format(ab, " path=");
 				if (*p)
 					audit_log_untrustedstring(ab, p);