net/unix: drop obsolete fd-recursion limits

All unix sockets now account inflight FDs to the respective sender.
This was introduced in:

    commit 712f4aad
    Author: willy tarreau <w@1wt.eu>
    Date:   Sun Jan 10 07:54:56 2016 +0100

        unix: properly account for FDs passed over unix sockets

and further refined in:

    commit 415e3d3e
    Author: Hannes Frederic Sowa <hannes@stressinduktion.org>
    Date:   Wed Feb 3 02:11:03 2016 +0100

        unix: correctly track in-flight fds in sending process user_struct

Hence, regardless of the stacking depth of FDs, the total number of
inflight FDs is limited, and accounted. There is no known way for a
local user to exceed those limits or exploit the accounting.

Furthermore, the GC logic is independent of the recursion/stacking depth
as well. It solely depends on the total number of inflight FDs,
regardless of their layout.

Lastly, the current `recursion_level' suffers a TOCTOU race, since it
checks and inherits depths only at queue time. If we consider `A<-B' to
mean `queue-B-on-A', the following sequence circumvents the recursion
level easily:

    A<-B
       B<-C
          C<-D
             ...
               Y<-Z

resulting in:

    A<-B<-C<-...<-Z

With all of this in mind, lets drop the recursion limit. It has no
additional security value, anymore. On the contrary, it randomly
confuses message brokers that try to forward file-descriptors, since
any sendmsg(2) call can fail spuriously with ETOOMANYREFS if a client
maliciously modifies the FD while inflight.

Cc: Alban Crequy <alban.crequy@collabora.co.uk>
Cc: Simon McVittie <simon.mcvittie@collabora.co.uk>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Tom Gundersen <teg@jklm.no>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/af_unix.h

@@ -58,7 +58,6 @@ struct unix_sock {
 	struct list_head	link;
 	atomic_long_t		inflight;
 	spinlock_t		lock;
-	unsigned char		recursion_level;
 	unsigned long		gc_flags;
 #define UNIX_GC_CANDIDATE	0
 #define UNIX_GC_MAYBE_CYCLE	1

net/unix/af_unix.c

@@ -1528,26 +1528,13 @@ static inline bool too_many_unix_fds(struct task_struct *p)
 	return false;
 }
 
-#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;
 
 	if (too_many_unix_fds(current))
 		return -ETOOMANYREFS;
 
-	for (i = scm->fp->count - 1; i >= 0; i--) {
-		struct sock *sk = unix_get_socket(scm->fp->fp[i]);
-
-		if (sk)
-			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
 	 * collection.  Otherwise a socket in the fps might become a
@@ -1559,7 +1546,7 @@ static int unix_attach_fds(struct scm_cookie *scm, struct sk_buff *skb)
 
 	for (i = scm->fp->count - 1; i >= 0; i--)
 		unix_inflight(scm->fp->user, scm->fp->fp[i]);
-	return max_level;
+	return 0;
 }
 
 static int unix_scm_to_skb(struct scm_cookie *scm, struct sk_buff *skb, bool send_fds)
@@ -1649,7 +1636,6 @@ static int unix_dgram_sendmsg(struct socket *sock, struct msghdr *msg,
 	struct sk_buff *skb;
 	long timeo;
 	struct scm_cookie scm;
-	int max_level;
 	int data_len = 0;
 	int sk_locked;
 
@@ -1701,7 +1687,6 @@ static int unix_dgram_sendmsg(struct socket *sock, struct msghdr *msg,
 	err = unix_scm_to_skb(&scm, skb, true);
 	if (err < 0)
 		goto out_free;
-	max_level = err + 1;
 
 	skb_put(skb, len - data_len);
 	skb->data_len = data_len;
@@ -1819,8 +1804,6 @@ static int unix_dgram_sendmsg(struct socket *sock, struct msghdr *msg,
 		__net_timestamp(skb);
 	maybe_add_creds(skb, sock, other);
 	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);
 	sock_put(other);
@@ -1855,7 +1838,6 @@ static int unix_stream_sendmsg(struct socket *sock, struct msghdr *msg,
 	int sent = 0;
 	struct scm_cookie scm;
 	bool fds_sent = false;
-	int max_level;
 	int data_len;
 
 	wait_for_unix_gc();
@@ -1905,7 +1887,6 @@ static int unix_stream_sendmsg(struct socket *sock, struct msghdr *msg,
 			kfree_skb(skb);
 			goto out_err;
 		}
-		max_level = err + 1;
 		fds_sent = true;
 
 		skb_put(skb, size - data_len);
@@ -1925,8 +1906,6 @@ static int unix_stream_sendmsg(struct socket *sock, struct msghdr *msg,
 
 		maybe_add_creds(skb, sock, other);
 		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);
 		sent += size;
@@ -2324,7 +2303,6 @@ static int unix_stream_read_generic(struct unix_stream_read_state *state,
 		last_len = last ? last->len : 0;
 again:
 		if (skb == NULL) {
-			unix_sk(sk)->recursion_level = 0;
 			if (copied >= target)
 				goto unlock;