bcache: convert cached_dev.count from atomic_t to refcount_t

atomic_t variables are currently used to implement reference
counters with the following properties:
 - counter is initialized to 1 using atomic_set()
 - a resource is freed upon counter reaching zero
 - once counter reaches zero, its further
   increments aren't allowed
 - counter schema uses basic atomic operations
   (set, inc, inc_not_zero, dec_and_test, etc.)

Such atomic variables should be converted to a newly provided
refcount_t type and API that prevents accidental counter overflows
and underflows. This is important since overflows and underflows
can lead to use-after-free situation and be exploitable.

The variable cached_dev.count is used as pure reference counter.
Convert it to refcount_t and fix up the operations.

Suggested-by: Kees Cook <keescook@chromium.org>
Reviewed-by: David Windsor <dwindsor@gmail.com>
Reviewed-by: Hans Liljestrand <ishkamiel@gmail.com>
Reviewed-by: Michael Lyle <mlyle@lyle.org>
Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h
index d77c4829c49727c2a904cc84d16ceec765cdeaa5..363ea6256b39316a41f6ab996b5e7c1c8d4fb442 100644 (file)
--- a/drivers/md/bcache/bcache.h
+++ b/drivers/md/bcache/bcache.h
@@ -184,6 +184,7 @@
 #include <linux/mutex.h>
 #include <linux/rbtree.h>
 #include <linux/rwsem.h>
+#include <linux/refcount.h>
 #include <linux/types.h>
 #include <linux/workqueue.h>
 
@@ -296,7 +297,7 @@ struct cached_dev {
        struct semaphore        sb_write_mutex;
 
        /* Refcount on the cache set. Always nonzero when we're caching. */
-       atomic_t                count;
+       refcount_t              count;
        struct work_struct      detach;
 
        /*
@@ -805,13 +806,13 @@ do {                                                                      \
 
 static inline void cached_dev_put(struct cached_dev *dc)
 {
-       if (atomic_dec_and_test(&dc->count))
+       if (refcount_dec_and_test(&dc->count))
                schedule_work(&dc->detach);
 }
 
 static inline bool cached_dev_get(struct cached_dev *dc)
 {
-       if (!atomic_inc_not_zero(&dc->count))
+       if (!refcount_inc_not_zero(&dc->count))
                return false;
 
        /* Paired with the mb in cached_dev_attach */

diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
index 72c3b2929ef02bfe1ae03756620a320251ea87ab..46134c45c6f693ae47b69080d5e9eadc6b295e00 100644 (file)
--- a/drivers/md/bcache/super.c
+++ b/drivers/md/bcache/super.c
@@ -902,7 +902,7 @@ static void cached_dev_detach_finish(struct work_struct *w)
        closure_init_stack(&cl);
 
        BUG_ON(!test_bit(BCACHE_DEV_DETACHING, &dc->disk.flags));
-       BUG_ON(atomic_read(&dc->count));
+       BUG_ON(refcount_read(&dc->count));
 
        mutex_lock(&bch_register_lock);
 
@@ -1029,7 +1029,7 @@ int bch_cached_dev_attach(struct cached_dev *dc, struct cache_set *c)
         * dc->c must be set before dc->count != 0 - paired with the mb in
         * cached_dev_get()
         */
-       atomic_set(&dc->count, 1);
+       refcount_set(&dc->count, 1);
 
        /* Block writeback thread, but spawn it */
        down_write(&dc->writeback_lock);
@@ -1041,7 +1041,7 @@ int bch_cached_dev_attach(struct cached_dev *dc, struct cache_set *c)
        if (BDEV_STATE(&dc->sb) == BDEV_STATE_DIRTY) {
                bch_sectors_dirty_init(&dc->disk);
                atomic_set(&dc->has_dirty, 1);
-               atomic_inc(&dc->count);
+               refcount_inc(&dc->count);
                bch_writeback_queue(dc);
        }
 

diff --git a/drivers/md/bcache/writeback.h b/drivers/md/bcache/writeback.h
index 34bcf49d737bdfb1fc07d8fcea8f9f4f0840da8d..7d25bff37a9bf95ada7bfdeeb3b8e3fb17aaa469 100644 (file)
--- a/drivers/md/bcache/writeback.h
+++ b/drivers/md/bcache/writeback.h
@@ -91,7 +91,7 @@ static inline void bch_writeback_add(struct cached_dev *dc)
 {
        if (!atomic_read(&dc->has_dirty) &&
            !atomic_xchg(&dc->has_dirty, 1)) {
-               atomic_inc(&dc->count);
+               refcount_inc(&dc->count);
 
                if (BDEV_STATE(&dc->sb) != BDEV_STATE_DIRTY) {
                        SET_BDEV_STATE(&dc->sb, BDEV_STATE_DIRTY);