af_unix: fix races in sk_peer_pid and sk_peer_cred accesses

Jann Horn reported that SO_PEERCRED and SO_PEERGROUPS implementations
are racy, as af_unix can concurrently change sk_peer_pid and sk_peer_cred.

In order to fix this issue, this patch adds a new spinlock that needs
to be used whenever these fields are read or written.

Jann also pointed out that l2cap_sock_get_peer_pid_cb() is currently
reading sk->sk_peer_pid which makes no sense, as this field
is only possibly set by AF_UNIX sockets.
We will have to clean this in a separate patch.
This could be done by reverting b48596d1dc25 "Bluetooth: L2CAP: Add get_peer_pid callback"
or implementing what was truly expected.

Fixes: 109f6e39fa07 ("af_unix: Allow SO_PEERCRED to work across namespaces.")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Jann Horn <jannh@google.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Cc: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/include/net/sock.h b/include/net/sock.h
index dc3f816..ae929e2 100644 (file)
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -488,8 +488,10 @@ struct sock {
        u8                      sk_prefer_busy_poll;
        u16                     sk_busy_poll_budget;
 #endif
+       spinlock_t              sk_peer_lock;
        struct pid              *sk_peer_pid;
        const struct cred       *sk_peer_cred;
+
        long                    sk_rcvtimeo;
        ktime_t                 sk_stamp;
 #if BITS_PER_LONG==32

diff --git a/net/core/sock.c b/net/core/sock.c
index 7060d18..c1601f7 100644 (file)
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -1376,6 +1376,16 @@ set_sndbuf:
 }
 EXPORT_SYMBOL(sock_setsockopt);
 
+static const struct cred *sk_get_peer_cred(struct sock *sk)
+{
+       const struct cred *cred;
+
+       spin_lock(&sk->sk_peer_lock);
+       cred = get_cred(sk->sk_peer_cred);
+       spin_unlock(&sk->sk_peer_lock);
+
+       return cred;
+}
 
 static void cred_to_ucred(struct pid *pid, const struct cred *cred,
                          struct ucred *ucred)
@@ -1552,7 +1562,11 @@ int sock_getsockopt(struct socket *sock, int level, int optname,
                struct ucred peercred;
                if (len > sizeof(peercred))
                        len = sizeof(peercred);
+
+               spin_lock(&sk->sk_peer_lock);
                cred_to_ucred(sk->sk_peer_pid, sk->sk_peer_cred, &peercred);
+               spin_unlock(&sk->sk_peer_lock);
+
                if (copy_to_user(optval, &peercred, len))
                        return -EFAULT;
                goto lenout;
@@ -1560,20 +1574,23 @@ int sock_getsockopt(struct socket *sock, int level, int optname,
 
        case SO_PEERGROUPS:
        {
+               const struct cred *cred;
                int ret, n;
 
-               if (!sk->sk_peer_cred)
+               cred = sk_get_peer_cred(sk);
+               if (!cred)
                        return -ENODATA;
 
-               n = sk->sk_peer_cred->group_info->ngroups;
+               n = cred->group_info->ngroups;
                if (len < n * sizeof(gid_t)) {
                        len = n * sizeof(gid_t);
+                       put_cred(cred);
                        return put_user(len, optlen) ? -EFAULT : -ERANGE;
                }
                len = n * sizeof(gid_t);
 
-               ret = groups_to_user((gid_t __user *)optval,
-                                    sk->sk_peer_cred->group_info);
+               ret = groups_to_user((gid_t __user *)optval, cred->group_info);
+               put_cred(cred);
                if (ret)
                        return ret;
                goto lenout;
@@ -1935,9 +1952,10 @@ static void __sk_destruct(struct rcu_head *head)
                sk->sk_frag.page = NULL;
        }
 
-       if (sk->sk_peer_cred)
-               put_cred(sk->sk_peer_cred);
+       /* We do not need to acquire sk->sk_peer_lock, we are the last user. */
+       put_cred(sk->sk_peer_cred);
        put_pid(sk->sk_peer_pid);
+
        if (likely(sk->sk_net_refcnt))
                put_net(sock_net(sk));
        sk_prot_free(sk->sk_prot_creator, sk);
@@ -3145,6 +3163,8 @@ void sock_init_data(struct socket *sock, struct sock *sk)
 
        sk->sk_peer_pid         =       NULL;
        sk->sk_peer_cred        =       NULL;
+       spin_lock_init(&sk->sk_peer_lock);
+
        sk->sk_write_pending    =       0;
        sk->sk_rcvlowat         =       1;
        sk->sk_rcvtimeo         =       MAX_SCHEDULE_TIMEOUT;

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index f505b89..efac598 100644 (file)
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -608,20 +608,42 @@ static void unix_release_sock(struct sock *sk, int embrion)
 
 static void init_peercred(struct sock *sk)
 {
-       put_pid(sk->sk_peer_pid);
-       if (sk->sk_peer_cred)
-               put_cred(sk->sk_peer_cred);
+       const struct cred *old_cred;
+       struct pid *old_pid;
+
+       spin_lock(&sk->sk_peer_lock);
+       old_pid = sk->sk_peer_pid;
+       old_cred = sk->sk_peer_cred;
        sk->sk_peer_pid  = get_pid(task_tgid(current));
        sk->sk_peer_cred = get_current_cred();
+       spin_unlock(&sk->sk_peer_lock);
+
+       put_pid(old_pid);
+       put_cred(old_cred);
 }
 
 static void copy_peercred(struct sock *sk, struct sock *peersk)
 {
-       put_pid(sk->sk_peer_pid);
-       if (sk->sk_peer_cred)
-               put_cred(sk->sk_peer_cred);
+       const struct cred *old_cred;
+       struct pid *old_pid;
+
+       if (sk < peersk) {
+               spin_lock(&sk->sk_peer_lock);
+               spin_lock_nested(&peersk->sk_peer_lock, SINGLE_DEPTH_NESTING);
+       } else {
+               spin_lock(&peersk->sk_peer_lock);
+               spin_lock_nested(&sk->sk_peer_lock, SINGLE_DEPTH_NESTING);
+       }
+       old_pid = sk->sk_peer_pid;
+       old_cred = sk->sk_peer_cred;
        sk->sk_peer_pid  = get_pid(peersk->sk_peer_pid);
        sk->sk_peer_cred = get_cred(peersk->sk_peer_cred);
+
+       spin_unlock(&sk->sk_peer_lock);
+       spin_unlock(&peersk->sk_peer_lock);
+
+       put_pid(old_pid);
+       put_cred(old_cred);
 }
 
 static int unix_listen(struct socket *sock, int backlog)