[PATCH 6.6.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-path check from the shared lookup helper to smb2_create_link() because the helper lacks for_remove. ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/smb/server/smb2pdu.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/smb/server/smb2pdu.c b/fs/smb/server/smb2pdu.c
index aaccef2e3f3bb..1573edebee2af 100644
--- a/fs/smb/server/smb2pdu.c
+++ b/fs/smb/server/smb2pdu.c
@@ -6303,6 +6303,12 @@ static int smb2_create_link(struct ksmbd_work *work,
 		goto out;
 	}
 
+	/* The share root must never be removed. */
+	if (link_name[0] == '\0') {
+		rc = -EINVAL;
+		goto out;
+	}
+
 	ksmbd_debug(SMB, "link name is %s\n", link_name);
 	target_name = file_path(filp, pathname, PATH_MAX);
 	if (IS_ERR(target_name)) {
-- 
2.53.0