[PATCH] fix a mount write count leak in ksmbd_vfs_kern_path_locked() (was Re: [RFC] MNT_WRITE_HOLD mess)

[Al Viro <viro@zeniv.linux.org.uk>]

If the call of ksmbd_vfs_lock_parent() fails, we drop the parent_path
references and return an error.  We need to drop the write access we
just got on parent_path->mnt before we drop the mount reference - callers
assume that ksmbd_vfs_kern_path_locked() returns with mount write
access grabbed if and only if it has returned 0.

Fixes: 864fb5d37163 "ksmbd: fix possible deadlock in smb2_open"
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---
diff --git a/fs/smb/server/vfs.c b/fs/smb/server/vfs.c
index 0f3aad12e495..d3437f6644e3 100644
--- a/fs/smb/server/vfs.c
+++ b/fs/smb/server/vfs.c
@@ -1282,6 +1282,7 @@ int ksmbd_vfs_kern_path_locked(struct ksmbd_work *work, char *name,
 
 		err = ksmbd_vfs_lock_parent(parent_path->dentry, path->dentry);
 		if (err) {
+			mnt_drop_write(parent_path->mnt);
 			path_put(path);
 			path_put(parent_path);
 		}