seccomp: Switch from atomic_t to recount_t

This switches the seccomp usage tracking from atomic_t to refcount_t to
gain refcount overflow protections.

Cc: Elena Reshetova <elena.reshetova@intel.com>
Cc: David Windsor <dwindsor@gmail.com>
Cc: Hans Liljestrand <hans.liljestrand@aalto.fi>
Signed-off-by: Kees Cook <keescook@chromium.org>

kernel/seccomp.c

@@ -13,7 +13,7 @@
  *        of Berkeley Packet Filters/Linux Socket Filters.
  */
 
-#include <linux/atomic.h>
+#include <linux/refcount.h>
 #include <linux/audit.h>
 #include <linux/compat.h>
 #include <linux/coredump.h>
@@ -56,7 +56,7 @@
  * to a task_struct (other than @usage).
  */
 struct seccomp_filter {
-	atomic_t usage;
+	refcount_t usage;
 	struct seccomp_filter *prev;
 	struct bpf_prog *prog;
 };
@@ -378,7 +378,7 @@ static struct seccomp_filter *seccomp_prepare_filter(struct sock_fprog *fprog)
 		return ERR_PTR(ret);
 	}
 
-	atomic_set(&sfilter->usage, 1);
+	refcount_set(&sfilter->usage, 1);
 
 	return sfilter;
 }
@@ -465,7 +465,7 @@ void get_seccomp_filter(struct task_struct *tsk)
 	if (!orig)
 		return;
 	/* Reference count is bounded by the number of total processes. */
-	atomic_inc(&orig->usage);
+	refcount_inc(&orig->usage);
 }
 
 static inline void seccomp_filter_free(struct seccomp_filter *filter)
@@ -481,7 +481,7 @@ void put_seccomp_filter(struct task_struct *tsk)
 {
 	struct seccomp_filter *orig = tsk->seccomp.filter;
 	/* Clean up single-reference branches iteratively. */
-	while (orig && atomic_dec_and_test(&orig->usage)) {
+	while (orig && refcount_dec_and_test(&orig->usage)) {
 		struct seccomp_filter *freeme = orig;
 		orig = orig->prev;
 		seccomp_filter_free(freeme);