Fix possible filp_cachep memory corruption

In commit 31e6b01f ("fs: rcu-walk for path lookup") we started doing
path lookup using RCU, which then falls back to a careful non-RCU lookup
in case of problems (LOOKUP_REVAL).  So do_filp_open() has this "re-do
the lookup carefully" looping case.

However, that means that we must not release the open-intent file data
if we are going to loop around and use it once more!

Fix this by moving the release of the open-intent data to the function
that allocates it (do_filp_open() itself) rather than the helper
functions that can get called multiple times (finish_open() and
do_last()).  This makes the logic for the lifetime of that field much
more obvious, and avoids the possible double free.
Reported-by: NJ. R. Okajima <hooanon05@yahoo.co.jp>
Acked-by: NAl Viro <viro@zeniv.linux.org.uk>
Cc: Nick Piggin <npiggin@kernel.dk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

fs/namei.c

@@ -561,10 +561,14 @@ static inline int nameidata_drop_rcu_last_maybe(struct nameidata *nd)
  */
 void release_open_intent(struct nameidata *nd)
 {
-	if (nd->intent.open.file->f_path.dentry == NULL)
-		put_filp(nd->intent.open.file);
-	else
-		fput(nd->intent.open.file);
+	struct file *file = nd->intent.open.file;
+
+	if (file && !IS_ERR(file)) {
+		if (file->f_path.dentry == NULL)
+			put_filp(file);
+		else
+			fput(file);
+	}
 }
 
 /*
@@ -2265,8 +2269,6 @@ static struct file *finish_open(struct nameidata *nd,
 	return filp;
 
 exit:
-	if (!IS_ERR(nd->intent.open.file))
-		release_open_intent(nd);
 	path_put(&nd->path);
 	return ERR_PTR(error);
 }
@@ -2389,8 +2391,6 @@ static struct file *do_last(struct nameidata *nd, struct path *path,
 exit_dput:
 	path_put_conditional(path, nd);
 exit:
-	if (!IS_ERR(nd->intent.open.file))
-		release_open_intent(nd);
 	path_put(&nd->path);
 	return ERR_PTR(error);
 }
@@ -2477,6 +2477,7 @@ struct file *do_filp_open(int dfd, const char *pathname,
 	}
 	audit_inode(pathname, nd.path.dentry);
 	filp = finish_open(&nd, open_flag, acc_mode);
+	release_open_intent(&nd);
 	return filp;
 
 creat:
@@ -2553,6 +2554,7 @@ struct file *do_filp_open(int dfd, const char *pathname,
 		path_put(&nd.root);
 	if (filp == ERR_PTR(-ESTALE) && !(flags & LOOKUP_REVAL))
 		goto reval;
+	release_open_intent(&nd);
 	return filp;
 
 exit_dput:
@@ -2560,8 +2562,6 @@ struct file *do_filp_open(int dfd, const char *pathname,
 out_path:
 	path_put(&nd.path);
 out_filp:
-	if (!IS_ERR(nd.intent.open.file))
-		release_open_intent(&nd);
 	filp = ERR_PTR(error);
 	goto out;
 }

fs/open.c

@@ -790,6 +790,8 @@ struct file *nameidata_to_filp(struct nameidata *nd)
 
 	/* Pick up the filp from the open intent */
 	filp = nd->intent.open.file;
+	nd->intent.open.file = NULL;
+
 	/* Has the filesystem initialised the file for us? */
 	if (filp->f_path.dentry == NULL) {
 		path_get(&nd->path);