[PATCH v2] ovl: keep err zero after successful ovl_cache_get()

ovl_iterate_merged() stores PTR_ERR(cache) in err before checking
IS_ERR(cache). On success err holds the truncated cache pointer and
can be returned as a bogus non-zero error.

The syzbot reproducer reaches this through overlay-on-overlay readdir:

  getdents64
    iterate_dir(outer overlay file)
      ovl_iterate_merged()
        ovl_cache_get()
          ovl_dir_read_merged()
            ovl_dir_read()
              iterate_dir(inner overlay file)
                ovl_iterate_merged()

Only compute PTR_ERR(cache) on the error path.

Fixes: d25e4b739f83 ("ovl: refactor ovl_iterate() and port to cred guard")
Reported-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=a16fb0cce329a320661c
Cc: [email protected]
Signed-off-by: Nirmoy Das <[email protected]>
---
v2:
 - Drop the now-redundant 'int err = 0' initializer and the trailing
   'return err' in ovl_iterate_merged(); err is only used inside the
   loop's update-check, so the function can just return 0 on success.
   (Amir Goldstein)
 - Link to v1:
   https://lore.kernel.org/all/[email protected]/

 fs/overlayfs/readdir.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
index 1dcc75b3a90f9..e7fe29cb6028b 100644
--- a/fs/overlayfs/readdir.c
+++ b/fs/overlayfs/readdir.c
@@ -838,15 +838,14 @@ static int ovl_iterate_merged(struct file *file, struct dir_context *ctx)
 	struct ovl_dir_file *od = file->private_data;
 	struct dentry *dentry = file->f_path.dentry;
 	struct ovl_cache_entry *p;
-	int err = 0;
+	int err;
 
 	if (!od->cache) {
 		struct ovl_dir_cache *cache;
 
 		cache = ovl_cache_get(dentry);
-		err = PTR_ERR(cache);
 		if (IS_ERR(cache))
-			return err;
+			return PTR_ERR(cache);
 
 		od->cache = cache;
 		ovl_seek_cursor(od, ctx->pos);
@@ -869,7 +868,7 @@ static int ovl_iterate_merged(struct file *file, struct dir_context *ctx)
 		od->cursor = p->l_node.next;
 		ctx->pos++;
 	}
-	return err;
+	return 0;
 }
 
 static bool ovl_need_adjust_d_ino(struct file *file)
-- 
2.43.0