Commit 0896cade authored Oct 20, 2017 by Elena Reshetova Committed by Anna Schumaker Nov 17, 2017

fs, nfs: convert nfs_cache_defer_req.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 nfs_cache_defer_req.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>
Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

parent 81a090b9

fs/nfs/cache_lib.c

+3 −3

Original line number	Diff line number	Diff line
		@@ -66,7 +66,7 @@ out:
		*/
		void nfs_cache_defer_req_put(struct nfs_cache_defer_req *dreq)
		{
		if (atomic_dec_and_test(&dreq->count))
		if (refcount_dec_and_test(&dreq->count))
		kfree(dreq);
		}

		@@ -86,7 +86,7 @@ static struct cache_deferred_req nfs_dns_cache_defer(struct cache_req req)

		dreq = container_of(req, struct nfs_cache_defer_req, req);
		dreq->deferred_req.revisit = nfs_dns_cache_revisit;
		atomic_inc(&dreq->count);
		refcount_inc(&dreq->count);

		return &dreq->deferred_req;
		}
		@@ -98,7 +98,7 @@ struct nfs_cache_defer_req *nfs_cache_defer_req_alloc(void)
		dreq = kzalloc(sizeof(*dreq), GFP_KERNEL);
		if (dreq) {
		init_completion(&dreq->completion);
		atomic_set(&dreq->count, 1);
		refcount_set(&dreq->count, 1);
		dreq->req.defer = nfs_dns_cache_defer;
		}
		return dreq;

fs/nfs/cache_lib.h

+1 −1

Original line number	Diff line number	Diff line
		@@ -15,7 +15,7 @@ struct nfs_cache_defer_req {
		struct cache_req req;
		struct cache_deferred_req deferred_req;
		struct completion completion;
		atomic_t count;
		refcount_t count;
		};

		extern int nfs_cache_upcall(struct cache_detail cd, char entry_name);

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