net, ipv4: convert fib_info.fib_clntref from atomic_t to refcount_t

refcount_t type and corresponding API should be
used instead of atomic_t when the variable is used as
a reference counter. This allows to avoid accidental
refcounter overflows that might lead to use-after-free
situations.

Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Hans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David Windsor <dwindsor@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/ip_fib.h

@@ -23,6 +23,7 @@
 #include <net/inetpeer.h>
 #include <linux/percpu.h>
 #include <linux/notifier.h>
+#include <linux/refcount.h>
 
 struct fib_config {
 	u8			fc_dst_len;
@@ -105,7 +106,7 @@ struct fib_info {
 	struct hlist_node	fib_lhash;
 	struct net		*fib_net;
 	int			fib_treeref;
-	atomic_t		fib_clntref;
+	refcount_t		fib_clntref;
 	unsigned int		fib_flags;
 	unsigned char		fib_dead;
 	unsigned char		fib_protocol;
@@ -430,12 +431,12 @@ void free_fib_info(struct fib_info *fi);
 
 static inline void fib_info_hold(struct fib_info *fi)
 {
-	atomic_inc(&fi->fib_clntref);
+	refcount_inc(&fi->fib_clntref);
 }
 
 static inline void fib_info_put(struct fib_info *fi)
 {
-	if (atomic_dec_and_test(&fi->fib_clntref))
+	if (refcount_dec_and_test(&fi->fib_clntref))
 		free_fib_info(fi);
 }
 

net/ipv4/fib_semantics.c

@@ -1253,7 +1253,7 @@ link_it:
 	}
 
 	fi->fib_treeref++;
-	atomic_inc(&fi->fib_clntref);
+	refcount_set(&fi->fib_clntref, 1);
 	spin_lock_bh(&fib_info_lock);
 	hlist_add_head(&fi->fib_hash,
 		       &fib_info_hash[fib_info_hashfn(fi)]);

net/ipv4/fib_trie.c

@@ -1463,7 +1463,7 @@ found:
 			}
 
 			if (!(fib_flags & FIB_LOOKUP_NOREF))
-				atomic_inc(&fi->fib_clntref);
+				refcount_inc(&fi->fib_clntref);
 
 			res->prefix = htonl(n->key);
 			res->prefixlen = KEYLENGTH - fa->fa_slen;