fs, nfs: convert nfs_lock_context.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_lock_context.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>

1 files changed, 6 insertions, 6 deletions

diff --git a/fs/nfs/inode.c b/fs/nfs/inode.c
index 134d9f560240..52a60e399a2d 100644
--- a/fs/nfs/inode.c
+++ b/fs/nfs/inode.c
@@ -783,7 +783,7 @@ EXPORT_SYMBOL_GPL(nfs_getattr);
 
 static void nfs_init_lock_context(struct nfs_lock_context *l_ctx)
 {
-        atomic_set(&l_ctx->count, 1);
+        refcount_set(&l_ctx->count, 1);
         l_ctx->lockowner = current->files;
         INIT_LIST_HEAD(&l_ctx->list);
         atomic_set(&l_ctx->io_count, 0);
@@ -797,7 +797,7 @@ static struct nfs_lock_context *__nfs_find_lock_context(struct nfs_open_context
         do {
                 if (pos->lockowner != current->files)
                         continue;
-                atomic_inc(&pos->count);
+                refcount_inc(&pos->count);
                 return pos;
         } while ((pos = list_entry(pos->list.next, typeof(*pos), list)) != head);
         return NULL;
@@ -836,7 +836,7 @@ void nfs_put_lock_context(struct nfs_lock_context *l_ctx)
         struct nfs_open_context *ctx = l_ctx->open_context;
         struct inode *inode = d_inode(ctx->dentry);
 
-        if (!atomic_dec_and_lock(&l_ctx->count, &inode->i_lock))
+        if (!refcount_dec_and_lock(&l_ctx->count, &inode->i_lock))
                 return;
         list_del(&l_ctx->list);
         spin_unlock(&inode->i_lock);
@@ -913,7 +913,7 @@ EXPORT_SYMBOL_GPL(alloc_nfs_open_context);
 struct nfs_open_context *get_nfs_open_context(struct nfs_open_context *ctx)
 {
         if (ctx != NULL)
-                atomic_inc(&ctx->lock_context.count);
+                refcount_inc(&ctx->lock_context.count);
         return ctx;
 }
 EXPORT_SYMBOL_GPL(get_nfs_open_context);
@@ -924,11 +924,11 @@ static void __put_nfs_open_context(struct nfs_open_context *ctx, int is_sync)
         struct super_block *sb = ctx->dentry->d_sb;
 
         if (!list_empty(&ctx->list)) {
-                if (!atomic_dec_and_lock(&ctx->lock_context.count, &inode->i_lock))
+                if (!refcount_dec_and_lock(&ctx->lock_context.count, &inode->i_lock))
                         return;
                 list_del(&ctx->list);
                 spin_unlock(&inode->i_lock);
-        } else if (!atomic_dec_and_test(&ctx->lock_context.count))
+        } else if (!refcount_dec_and_test(&ctx->lock_context.count))
                 return;
         if (inode != NULL)
                 NFS_PROTO(inode)->close_context(ctx, is_sync);