af_unix: limit recursion level

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.

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>

1 files changed, 2 insertions, 0 deletions

diff --git a/include/net/af_unix.h b/include/net/af_unix.h
index 90c9e2872f27..18e5c3f67580 100644
--- a/include/net/af_unix.h
+++ b/include/net/af_unix.h
@@ -10,6 +10,7 @@ extern void unix_inflight(struct file *fp);
 extern void unix_notinflight(struct file *fp);
 extern void unix_gc(void);
 extern void wait_for_unix_gc(void);
+extern struct sock *unix_get_socket(struct file *filp);
 
 #define UNIX_HASH_SIZE  256
 
@@ -56,6 +57,7 @@ struct unix_sock {
         spinlock_t              lock;
         unsigned int            gc_candidate : 1;
         unsigned int            gc_maybe_cycle : 1;
+        unsigned char           recursion_level;
         struct socket_wq        peer_wq;
 };
 #define unix_sk(__sk) ((struct unix_sock *)__sk)