[PATCH 4.14 26/34] missing barriers in some of unix_sock ->addr and ->path accesses

Greg Kroah-Hartman
2019-03-18T09:40:17Z

From: Al Viro <viro@zeniv.linux.org.uk>

[ Upstream commit ae3b564179bfd06f32d051b9e5d72ce4b2a07c37 ]

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
net/unix/af_unix.c | 57 +++++++++++++++++++++++++++++----------------------
net/unix/diag.c | 3 +-
security/lsm_audit.c | 10 +++++---
3 files changed, 41 insertions(+), 29 deletions(-)

--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -892,7 +892,7 @@ retry:
addr->hash ^= sk->sk_type;

__unix_remove_socket(sk);
- u->addr = addr;
+ smp_store_release(&u->addr, addr);
__unix_insert_socket(&unix_socket_table[addr->hash], sk);
spin_unlock(&unix_table_lock);
err = 0;
@@ -1062,7 +1062,7 @@ static int unix_bind(struct socket *sock

err = 0;
__unix_remove_socket(sk);
- u->addr = addr;
+ smp_store_release(&u->addr, addr);
__unix_insert_socket(list, sk);

out_unlock:
@@ -1333,15 +1333,29 @@ restart:
RCU_INIT_POINTER(newsk->sk_wq, &newu->peer_wq);
otheru = unix_sk(other);

- /* copy address information from listening to new sock*/
- if (otheru->addr) {
- refcount_inc(&otheru->addr->refcnt);
- newu->addr = otheru->addr;
- }
+ /* copy address information from listening to new sock
+ *
+ * The contents of *(otheru->addr) and otheru->path
+ * are seen fully set up here, since we have found
+ * otheru in hash under unix_table_lock. Insertion
+ * into the hash chain we'd found it in had been done
+ * in an earlier critical area protected by unix_table_lock,
+ * the same one where we'd set *(otheru->addr) contents,
+ * as well as otheru->path and otheru->addr itself.
+ *
+ * Using smp_store_release() here to set newu->addr
+ * is enough to make those stores, as well as stores
+ * to newu->path visible to anyone who gets newu->addr
+ * by smp_load_acquire(). IOW, the same warranties
+ * as for unix_sock instances bound in unix_bind() or
+ * in unix_autobind().
+ */
if (otheru->path.dentry) {
path_get(&otheru->path);
newu->path = otheru->path;
}
+ refcount_inc(&otheru->addr->refcnt);
+ smp_store_release(&newu->addr, otheru->addr);

/* Set credentials */
copy_peercred(sk, other);
@@ -1455,7 +1469,7 @@ out:
static int unix_getname(struct socket *sock, struct sockaddr *uaddr, int *uaddr_len, int peer)
{
struct sock *sk = sock->sk;
- struct unix_sock *u;
+ struct unix_address *addr;
DECLARE_SOCKADDR(struct sockaddr_un *, sunaddr, uaddr);
int err = 0;

@@ -1470,19 +1484,15 @@ static int unix_getname(struct socket *s
sock_hold(sk);
}

- u = unix_sk(sk);
- unix_state_lock(sk);
- if (!u->addr) {
+ addr = smp_load_acquire(&unix_sk(sk)->addr);
+ if (!addr) {
sunaddr->sun_family = AF_UNIX;
sunaddr->sun_path[0] = 0;
*uaddr_len = sizeof(short);
} else {
- struct unix_address *addr = u->addr;
-
*uaddr_len = addr->len;
memcpy(sunaddr, addr->name, *uaddr_len);
}
- unix_state_unlock(sk);
sock_put(sk);
out:
return err;
@@ -2075,11 +2085,11 @@ static int unix_seqpacket_recvmsg(struct

static void unix_copy_addr(struct msghdr *msg, struct sock *sk)
{
- struct unix_sock *u = unix_sk(sk);
+ struct unix_address *addr = smp_load_acquire(&unix_sk(sk)->addr);

- if (u->addr) {
- msg->msg_namelen = u->addr->len;
- memcpy(msg->msg_name, u->addr->name, u->addr->len);
+ if (addr) {
+ msg->msg_namelen = addr->len;
+ memcpy(msg->msg_name, addr->name, addr->len);
}
}

@@ -2583,15 +2593,14 @@ static int unix_open_file(struct sock *s
if (!ns_capable(sock_net(sk)->user_ns, CAP_NET_ADMIN))
return -EPERM;

- unix_state_lock(sk);
+ if (!smp_load_acquire(&unix_sk(sk)->addr))
+ return -ENOENT;
+
path = unix_sk(sk)->path;
- if (!path.dentry) {
- unix_state_unlock(sk);
+ if (!path.dentry)
return -ENOENT;
- }

path_get(&path);
- unix_state_unlock(sk);

fd = get_unused_fd_flags(O_CLOEXEC);
if (fd < 0)
@@ -2831,7 +2840,7 @@ static int unix_seq_show(struct seq_file
(s->sk_state == TCP_ESTABLISHED ? SS_CONNECTING : SS_DISCONNECTING),
sock_i_ino(s));

- if (u->addr) {
+ if (u->addr) { // under unix_table_lock here
int i, len;
seq_putc(seq, ' ');

--- a/net/unix/diag.c
+++ b/net/unix/diag.c
@@ -10,7 +10,8 @@

static int sk_diag_dump_name(struct sock *sk, struct sk_buff *nlskb)
{
- struct unix_address *addr = unix_sk(sk)->addr;
+ /* might or might not have unix_table_lock */
+ struct unix_address *addr = smp_load_acquire(&unix_sk(sk)->addr);

if (!addr)
return 0;
--- a/security/lsm_audit.c
+++ b/security/lsm_audit.c
@@ -321,6 +321,7 @@ static void dump_common_audit_data(struc
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(struc
#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);