[PATCH 6.18.y] ksmbd: prevent share root removal through empty paths

[Sasha Levin <sashal@kernel.org>]

From: Namjae Jeon <linkinjeon@kernel.org>

[ Upstream commit 9fd1fcbbf84986a7bcf601a2b90540899069d263 ]

A FileLinkInformation request can supply "/" as the new link name.
smb2_get_name() strips the trailing slash and passes an empty string to
ksmbd_vfs_kern_path_start_removing().

ksmbd_vfs_path_lookup() resolves an empty pathname as the configured share
path, without LOOKUP_BENEATH. With ReplaceIfExists set, smb2_create_link()
can therefore remove the share root from its parent before link creation
fails. An authenticated client with a writable share and DELETE access to
a file can trigger this if the share root is empty and the mapped user
has write and search permission on its parent directory.

Return -EINVAL for empty pathnames when for_remove is set, before
resolving the share path or acquiring removal resources. Ordinary lookups
continue to accept empty pathnames for opening the share root.

Fixes: 74d7970febf7 ("ksmbd: fix racy issue from using ->d_parent and ->d_name")
Cc: stable@vger.kernel.org
Reported-by: Zachariah Mithani (AploSec) <zachariah@aplosai.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
[ sashal: Replaced for_remove with the existing do_lock parameter. ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/smb/server/vfs.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/smb/server/vfs.c b/fs/smb/server/vfs.c
index c18ab8431e0d2..29b204776b961 100644
--- a/fs/smb/server/vfs.c
+++ b/fs/smb/server/vfs.c
@@ -78,6 +78,10 @@ static int ksmbd_vfs_path_lookup(struct ksmbd_share_config *share_conf,
 	struct dentry *d;
 
 	if (pathname[0] == '\0') {
+		/* The share root must never be removed. */
+		if (do_lock)
+			return -EINVAL;
+
 		pathname = share_conf->path;
 		root_share_path = NULL;
 	} else {
-- 
2.53.0