[PATCH 14/15] kernel: convert futex_pi_state.refcount from atomic_t to refcount_t

[Elena Reshetova]

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.

Suggested-by: Kees Cook <keesc...@chromium.org>
Reviewed-by: David Windsor <dwind...@gmail.com>
Reviewed-by: Hans Liljestrand <ishkam...@gmail.com>
Signed-off-by: Elena Reshetova <elena.reshet...@intel.com>
---
 kernel/futex.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 16dbe4c..1cc7641 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -67,6 +67,7 @@
 #include <linux/freezer.h>
 #include <linux/bootmem.h>
 #include <linux/fault-inject.h>
+#include <linux/refcount.h>
 
 #include <asm/futex.h>
 
@@ -209,7 +210,7 @@ struct futex_pi_state {
        struct rt_mutex pi_mutex;
 
        struct task_struct *owner;
-       atomic_t refcount;
+       refcount_t refcount;
 
        union futex_key key;
 } __randomize_layout;
@@ -794,7 +795,7 @@ static int refill_pi_state_cache(void)
        INIT_LIST_HEAD(&pi_state->list);
        /* pi_mutex gets initialized later */
        pi_state->owner = NULL;
-       atomic_set(&pi_state->refcount, 1);
+       refcount_set(&pi_state->refcount, 1);
        pi_state->key = FUTEX_KEY_INIT;
 
        current->pi_state_cache = pi_state;
@@ -814,7 +815,7 @@ static struct futex_pi_state *alloc_pi_state(void)
 
 static void get_pi_state(struct futex_pi_state *pi_state)
 {
-       WARN_ON_ONCE(!atomic_inc_not_zero(&pi_state->refcount));
+       refcount_inc(&pi_state->refcount);
 }
 
 /*
@@ -828,7 +829,7 @@ static void put_pi_state(struct futex_pi_state *pi_state)
        if (!pi_state)
                return;
 
-       if (!atomic_dec_and_test(&pi_state->refcount))
+       if (!refcount_dec_and_test(&pi_state->refcount))
                return;
 
        /*
@@ -852,7 +853,7 @@ static void put_pi_state(struct futex_pi_state *pi_state)
                 * refcount is at 0 - put it back to 1.
                 */
                pi_state->owner = NULL;
-               atomic_set(&pi_state->refcount, 1);
+               refcount_set(&pi_state->refcount, 1);
                current->pi_state_cache = pi_state;
        }
 }
@@ -1046,7 +1047,7 @@ static int attach_to_pi_state(u32 __user *uaddr, u32 uval,
         * and futex_wait_requeue_pi() as it cannot go to 0 and consequently
         * free pi_state before we can take a reference ourselves.
         */
-       WARN_ON(!atomic_read(&pi_state->refcount));
+       WARN_ON(!refcount_read(&pi_state->refcount));
 
        /*
         * Now that we have a pi_state, we can acquire wait_lock
-- 
2.7.4