[PATCH v1] ksmbd: Fix user namespace mapping

A kernel daemon should not rely on the current thread, which is unknown
and might be malicious.  Before this security fix,
ksmbd_override_fsids() didn't correctly override FS UID/GID which means
that arbitrary user space threads could trick the kernel to impersonate
arbitrary users or groups for file system access checks, leading to
file system access bypass.

This was found while investigating truncate support for Landlock:
https://lore.kernel.org/r/CAKYAXd8fpMJ7guizOjHgxEyyjoUwPsx3jLOPZP=wPYcbhkVXqA@mail.gmail.com (local)

Fixes: e2f34481b24d ("cifsd: add server-side procedures for SMB3")
Cc: Hyunchul Lee <hyc.lee@gmail.com>
Cc: Namjae Jeon <linkinjeon@kernel.org>
Cc: Steve French <redacted>
Cc: stable@vger.kernel.org
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Link: https://lore.kernel.org/r/20220929100447.108468-1-mic@digikod.net (local)
---
 fs/ksmbd/smb_common.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/ksmbd/smb_common.c b/fs/ksmbd/smb_common.c
index 7f8ab14fb8ec..d96da872d70a 100644
--- a/fs/ksmbd/smb_common.c
+++ b/fs/ksmbd/smb_common.c

@@ -4,6 +4,8 @@
  *   Copyright (C) 2018 Namjae Jeon <linkinjeon@kernel.org>
  */
 
+#include <linux/user_namespace.h>
+
 #include "smb_common.h"
 #include "server.h"
 #include "misc.h"

@@ -625,8 +627,8 @@ int ksmbd_override_fsids(struct ksmbd_work *work)
 	if (!cred)
 		return -ENOMEM;
 
-	cred->fsuid = make_kuid(current_user_ns(), uid);
-	cred->fsgid = make_kgid(current_user_ns(), gid);
+	cred->fsuid = make_kuid(&init_user_ns, uid);
+	cred->fsgid = make_kgid(&init_user_ns, gid);
 
 	gi = groups_alloc(0);
 	if (!gi) {

base-commit: f76349cf41451c5c42a99f18a9163377e4b364ff
-- 
2.37.2