[PATCH 6.12 0889/1102] smb: client: reject userspace cifs.idmap descriptions

6.12-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Aohan Mei <redacted>

commit d9d7eeb0cea5b55b82888f443622fd8d4ee064f3 upstream.

cifs.idmap key descriptions carry authority-bearing fields (owner and
group SIDs and uid/gid values in "os:"/"gs:"/"oi:"/"gi:" form) that the
cifs.idmap upcall helper treats as kernel-originating inputs.  Unlike
its sibling cifs.spnego, the cifs.idmap key type has no vet_description
hook, so userspace can create keys of this type through
request_key(2)/add_key(2) and supply those fields without CIFS origin.
A request_key(2) call with a non-NULL callout then drives a root
usermodehelper upcall (/sbin/request-key -> cifs.idmap) that consumes
the unvetted description in root context.

Only accept cifs.idmap descriptions while CIFS is using its private
root_cred to request the key.  id_to_sid()/sid_to_id() already run
under override_creds(root_cred), so the kernel-originated path is
unaffected.

This mirrors commit 3da1fdf4efbc ("smb: client: reject userspace
cifs.spnego descriptions"), which applied the same restriction to
cifs.spnego.

Fixes: 4d79dba0e007 ("cifs: Add idmap key and related data structures and functions (try #17 repost)")
Reported-by: TencentOS Corvus AI <redacted>
Cc: stable@vger.kernel.org
Assisted-by: CodeBuddy:Kimi-K3
Signed-off-by: Aohan Mei <redacted>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Paulo Alcantara <pc@manguebit.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 fs/smb/client/cifsacl.c |   15 +++++++++++++++
 1 file changed, 15 insertions(+)

--- a/fs/smb/client/cifsacl.c
+++ b/fs/smb/client/cifsacl.c

@@ -101,8 +101,23 @@ cifs_idmap_key_destroy(struct key *key)
 		kfree(key->payload.data[0]);
 }
 
+static int
+cifs_idmap_key_vet_description(const char *description)
+{
+	/*
+	 * cifs.idmap descriptions are authority-bearing inputs to the
+	 * cifs.idmap upcall helper.  Only allow the kernel to create this
+	 * type of key using the private root_cred installed in
+	 * init_cifs_idmap; reject userspace request_key(2)/add_key(2).
+	 */
+	if (current_cred() != root_cred)
+		return -EPERM;
+	return 0;
+}
+
 static struct key_type cifs_idmap_key_type = {
 	.name        = "cifs.idmap",
+	.vet_description = cifs_idmap_key_vet_description,
 	.instantiate = cifs_idmap_key_instantiate,
 	.destroy     = cifs_idmap_key_destroy,
 	.describe    = user_describe,