ovl: avoid deadlock on directory ioctl

The function ovl_dir_real_file() currently uses the inode lock to serialize
writes to the od->upperfile field.

However, this function will get called by ovl_ioctl_set_flags(), which
utilizes the inode lock too.  In this case ovl_dir_real_file() will try to
claim a lock that is owned by a function in its call stack, which won't get
released before ovl_dir_real_file() returns.

Fix by replacing the open coded compare and exchange by an explicit atomic
op.

Fixes: 61536be ("ovl: support [S|G]ETFLAGS and FS[S|G]ETXATTR ioctls for directories")
Cc: stable@vger.kernel.org # v5.10
Reported-by: Icenowy Zheng <icenowy@aosc.io>
Tested-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/readdir.c

@@ -865,7 +865,7 @@ struct file *ovl_dir_real_file(const struct file *file, bool want_upper)
 
 	struct ovl_dir_file *od = file->private_data;
 	struct dentry *dentry = file->f_path.dentry;
-	struct file *realfile = od->realfile;
+	struct file *old, *realfile = od->realfile;
 
 	if (!OVL_TYPE_UPPER(ovl_path_type(dentry)))
 		return want_upper ? NULL : realfile;
@@ -874,29 +874,20 @@ struct file *ovl_dir_real_file(const struct file *file, bool want_upper)
 	 * Need to check if we started out being a lower dir, but got copied up
 	 */
 	if (!od->is_upper) {
-		struct inode *inode = file_inode(file);
-
 		realfile = READ_ONCE(od->upperfile);
 		if (!realfile) {
 			struct path upperpath;
 
 			ovl_path_upper(dentry, &upperpath);
 			realfile = ovl_dir_open_realfile(file, &upperpath);
+			if (IS_ERR(realfile))
+				return realfile;
 
-			inode_lock(inode);
-			if (!od->upperfile) {
-				if (IS_ERR(realfile)) {
-					inode_unlock(inode);
-					return realfile;
-				}
-				smp_store_release(&od->upperfile, realfile);
-			} else {
-				/* somebody has beaten us to it */
-				if (!IS_ERR(realfile))
-					fput(realfile);
-				realfile = od->upperfile;
+			old = cmpxchg_release(&od->upperfile, NULL, realfile);
+			if (old) {
+				fput(realfile);
+				realfile = old;
 			}
-			inode_unlock(inode);
 		}
 	}