af_unix: limit recursion level

commit 25888e30319f8896fc656fc68643e6a078263060 upstream

Its easy to eat all kernel memory and trigger NMI watchdog, using an
exploit program that queues unix sockets on top of others.

lkml ref : http://lkml.org/lkml/2010/11/25/8

This mechanism is used in applications, one choice we have is to have a
recursion limit.

Other limits might be needed as well (if we queue other types of files),
since the passfd mechanism is currently limited by socket receive queue
sizes only.

Add a recursion_level to unix socket, allowing up to 4 levels.

Each time we send an unix socket through sendfd mechanism, we copy its
recursion level (plus one) to receiver. This recursion level is cleared
when socket receive queue is emptied.

[PG: slight modifications required due to absense of 7361c36c5 in 34]

Reported-by: Марк Коренберг <socketpair@gmail.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>

2 files changed, 33 insertions, 6 deletions

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 8c34e3bf877..207a119b5dc 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -1324,9 +1324,25 @@ static void unix_destruct_fds(struct sk_buff *skb)
 	sock_wfree(skb);
 }
 
+#define MAX_RECURSION_LEVEL 4
+
 static int unix_attach_fds(struct scm_cookie *scm, struct sk_buff *skb)
 {
 	int i;
+	unsigned char max_level = 0;
+	int unix_sock_count = 0;
+
+	for (i = scm->fp->count - 1; i >= 0; i--) {
+		struct sock *sk = unix_get_socket(scm->fp->fp[i]);
+
+		if (sk) {
+			unix_sock_count++;
+			max_level = max(max_level,
+					unix_sk(sk)->recursion_level);
+		}
+	}
+	if (unlikely(max_level > MAX_RECURSION_LEVEL))
+		return -ETOOMANYREFS;
 
 	/*
 	 * Need to duplicate file references for the sake of garbage
@@ -1337,10 +1353,12 @@ static int unix_attach_fds(struct scm_cookie *scm, struct sk_buff *skb)
 	if (!UNIXCB(skb).fp)
 		return -ENOMEM;
 
-	for (i = scm->fp->count-1; i >= 0; i--)
-		unix_inflight(scm->fp->fp[i]);
+	if (unix_sock_count) {
+		for (i = scm->fp->count-1; i >= 0; i--)
+			unix_inflight(scm->fp->fp[i]);
+	}
 	skb->destructor = unix_destruct_fds;
-	return 0;
+	return max_level;
 }
 
 /*
@@ -1362,6 +1380,7 @@ static int unix_dgram_sendmsg(struct kiocb *kiocb, struct socket *sock,
 	struct sk_buff *skb;
 	long timeo;
 	struct scm_cookie tmp_scm;
+	int max_level = 0;
 
 	if (NULL == siocb->scm)
 		siocb->scm = &tmp_scm;
@@ -1402,8 +1421,9 @@ static int unix_dgram_sendmsg(struct kiocb *kiocb, struct socket *sock,
 	memcpy(UNIXCREDS(skb), &siocb->scm->creds, sizeof(struct ucred));
 	if (siocb->scm->fp) {
 		err = unix_attach_fds(siocb->scm, skb);
-		if (err)
+		if (err < 0)
 			goto out_free;
+		max_level = err + 1;
 	}
 	unix_get_secdata(siocb->scm, skb);
 
@@ -1484,6 +1504,8 @@ restart:
 	}
 
 	skb_queue_tail(&other->sk_receive_queue, skb);
+	if (max_level > unix_sk(other)->recursion_level)
+		unix_sk(other)->recursion_level = max_level;
 	unix_state_unlock(other);
 	other->sk_data_ready(other, len);
 	sock_put(other);
@@ -1514,6 +1536,7 @@ static int unix_stream_sendmsg(struct kiocb *kiocb, struct socket *sock,
 	int sent = 0;
 	struct scm_cookie tmp_scm;
 	bool fds_sent = false;
+	int max_level = 0;
 
 	if (NULL == siocb->scm)
 		siocb->scm = &tmp_scm;
@@ -1578,10 +1601,11 @@ static int unix_stream_sendmsg(struct kiocb *kiocb, struct socket *sock,
 		/* Only send the fds in the first buffer */
 		if (siocb->scm->fp && !fds_sent) {
 			err = unix_attach_fds(siocb->scm, skb);
-			if (err) {
+			if (err < 0) {
 				kfree_skb(skb);
 				goto out_err;
 			}
+			max_level = err + 1;
 			fds_sent = true;
 		}
 
@@ -1598,6 +1622,8 @@ static int unix_stream_sendmsg(struct kiocb *kiocb, struct socket *sock,
 			goto pipe_err_free;
 
 		skb_queue_tail(&other->sk_receive_queue, skb);
+		if (max_level > unix_sk(other)->recursion_level)
+			unix_sk(other)->recursion_level = max_level;
 		unix_state_unlock(other);
 		other->sk_data_ready(other, size);
 		sent += size;
@@ -1814,6 +1840,7 @@ static int unix_stream_recvmsg(struct kiocb *iocb, struct socket *sock,
 		unix_state_lock(sk);
 		skb = skb_dequeue(&sk->sk_receive_queue);
 		if (skb == NULL) {
+			unix_sk(sk)->recursion_level = 0;
 			if (copied >= target)
 				goto unlock;
 
diff --git a/net/unix/garbage.c b/net/unix/garbage.c
index ef5aa55c6b9..493e0e67685 100644
--- a/net/unix/garbage.c
+++ b/net/unix/garbage.c
@@ -96,7 +96,7 @@ static DECLARE_WAIT_QUEUE_HEAD(unix_gc_wait);
 unsigned int unix_tot_inflight;
 
 
-static struct sock *unix_get_socket(struct file *filp)
+struct sock *unix_get_socket(struct file *filp)
 {
 	struct sock *u_sock = NULL;
 	struct inode *inode = filp->f_path.dentry->d_inode;