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>

diff --git a/include/linux/sched.h b/include/linux/sched.h
index edad7a43edea141b3ada89f7f12edac2add7b68e..fbf25f19b3b5a07101cf9fb77f67776c06867238 100644 (file)
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -830,6 +830,7 @@ struct user_struct {
        unsigned long mq_bytes; /* How many bytes can be allocated to mqueue? */
 #endif
        unsigned long locked_shm; /* How many pages of mlocked shm ? */
+       unsigned long unix_inflight;    /* How many files in flight in unix sockets */
 
 #ifdef CONFIG_KEYS
        struct key *uid_keyring;        /* UID specific keyring */

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index ef05cd9403d4bb2c2ad0a3faddc71de611d892df..e3f85bc8b135d8feb13d60432e5f6cdb1064fbd6 100644 (file)
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -1513,6 +1513,21 @@ static void unix_destruct_scm(struct sk_buff *skb)
        sock_wfree(skb);
 }
 
+/*
+ * The "user->unix_inflight" variable is protected by the garbage
+ * collection lock, and we just read it locklessly here. If you go
+ * over the limit, there might be a tiny race in actually noticing
+ * it across threads. Tough.
+ */
+static inline bool too_many_unix_fds(struct task_struct *p)
+{
+       struct user_struct *user = current_user();
+
+       if (unlikely(user->unix_inflight > task_rlimit(p, RLIMIT_NOFILE)))
+               return !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN);
+       return false;
+}
+
 #define MAX_RECURSION_LEVEL 4
 
 static int unix_attach_fds(struct scm_cookie *scm, struct sk_buff *skb)
@@ -1521,6 +1536,9 @@ static int unix_attach_fds(struct scm_cookie *scm, struct sk_buff *skb)
        unsigned char max_level = 0;
        int unix_sock_count = 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]);
 
@@ -1542,10 +1560,8 @@ static int unix_attach_fds(struct scm_cookie *scm, struct sk_buff *skb)
        if (!UNIXCB(skb).fp)
                return -ENOMEM;
 
-       if (unix_sock_count) {
-               for (i = scm->fp->count - 1; i >= 0; i--)
-                       unix_inflight(scm->fp->fp[i]);
-       }
+       for (i = scm->fp->count - 1; i >= 0; i--)
+               unix_inflight(scm->fp->fp[i]);
        return max_level;
 }
 

diff --git a/net/unix/garbage.c b/net/unix/garbage.c
index a73a226f2d33f07261f606fa84ef4db8b91ed6fc..8fcdc2283af50c5caecd409b79cae6028ca2c836 100644 (file)
--- 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 *),