[PATCH 10/11] fs, nfs: convert nfs_lock_context.count from atomic_t to refcount_t

[Elena Reshetova]

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 <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>
---
 fs/nfs/inode.c         | 12 ++++++------
 include/linux/nfs_fs.h |  3 ++-
 2 files changed, 8 insertions(+), 7 deletions(-)

diff --git a/fs/nfs/inode.c b/fs/nfs/inode.c
index 134d9f5..52a60e3 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);
diff --git a/include/linux/nfs_fs.h b/include/linux/nfs_fs.h
index a0282ce..51e9124 100644
--- a/include/linux/nfs_fs.h
+++ b/include/linux/nfs_fs.h
@@ -22,6 +22,7 @@
 #include <linux/mm.h>
 #include <linux/pagemap.h>
 #include <linux/rbtree.h>
+#include <linux/refcount.h>
 #include <linux/rwsem.h>
 #include <linux/wait.h>
 
@@ -55,7 +56,7 @@ struct nfs_access_entry {
 };
 
 struct nfs_lock_context {
-       atomic_t count;
+       refcount_t count;
        struct list_head list;
        struct nfs_open_context *open_context;
        fl_owner_t lockowner;
-- 
2.7.4