[PATCH 5.15.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: Moved the empty-name check from the shared lookup helper to smb2_create_link() because the helper lacks a for_remove parameter. ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/ksmbd/smb2pdu.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/ksmbd/smb2pdu.c b/fs/ksmbd/smb2pdu.c
index 4c42205fe5261..0daafda3f59e6 100644
--- a/fs/ksmbd/smb2pdu.c
+++ b/fs/ksmbd/smb2pdu.c
@@ -5711,7 +5711,8 @@ static int smb2_create_link(struct ksmbd_work *work,
 	link_name = smb2_get_name(file_info->FileName,
 				  le32_to_cpu(file_info->FileNameLength),
 				  local_nls);
-	if (IS_ERR(link_name) || S_ISDIR(file_inode(filp)->i_mode)) {
+	if (IS_ERR(link_name) || link_name[0] == '\0' ||
+	    S_ISDIR(file_inode(filp)->i_mode)) {
 		rc = -EINVAL;
 		goto out;
 	}
-- 
2.53.0