unix: properly account for FDs passed over unix sockets

It is possible for a process to allocate and accumulate far more FDs than
the process' limit by sending them over a unix socket then closing them
to keep the process' fd count low.

This change addresses this problem by keeping track of the number of FDs
in flight per user and preventing non-privileged processes from having
more FDs in flight than their configured FD limit.

Reported-by: socketpair@gmail.com
Reported-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Mitigates: CVE-2013-4312 (Linux 2.0+)
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 8 insertions, 5 deletions

diff --git a/net/unix/garbage.c b/net/unix/garbage.c
index a73a226f2d33..8fcdc2283af5 100644
--- a/net/unix/garbage.c
+++ b/net/unix/garbage.c
@@ -120,11 +120,11 @@ void unix_inflight(struct file *fp)
 {
         struct sock *s = unix_get_socket(fp);
 
+        spin_lock(&unix_gc_lock);
+
         if (s) {
                 struct unix_sock *u = unix_sk(s);
 
-                spin_lock(&unix_gc_lock);
-
                 if (atomic_long_inc_return(&u->inflight) == 1) {
                         BUG_ON(!list_empty(&u->link));
                         list_add_tail(&u->link, &gc_inflight_list);
@@ -132,25 +132,28 @@ void unix_inflight(struct file *fp)
                         BUG_ON(list_empty(&u->link));
                 }
                 unix_tot_inflight++;
-                spin_unlock(&unix_gc_lock);
         }
+        fp->f_cred->user->unix_inflight++;
+        spin_unlock(&unix_gc_lock);
 }
 
 void unix_notinflight(struct file *fp)
 {
         struct sock *s = unix_get_socket(fp);
 
+        spin_lock(&unix_gc_lock);
+
         if (s) {
                 struct unix_sock *u = unix_sk(s);
 
-                spin_lock(&unix_gc_lock);
                 BUG_ON(list_empty(&u->link));
 
                 if (atomic_long_dec_and_test(&u->inflight))
                         list_del_init(&u->link);
                 unix_tot_inflight--;
-                spin_unlock(&unix_gc_lock);
         }
+        fp->f_cred->user->unix_inflight--;
+        spin_unlock(&unix_gc_lock);
 }
 
 static void scan_inflight(struct sock *x, void (*func)(struct unix_sock *),