smb: client: reject userspace cifs.spnego descriptions

cve CVE-2026-46243
commit-author Asim Viladi Oglu Manizada <manizada@pm.me>
commit 3da1fdf

cifs.spnego key descriptions contain authority-bearing fields such as
pid, uid, creduid, and upcall_target that cifs.upcall treats as
kernel-originating inputs. However, userspace can also create keys of
this type through request_key(2) or add_key(2), allowing those fields to
be supplied without CIFS origin.

Only accept cifs.spnego descriptions while CIFS is using its private
spnego_cred to request the key.

Fixes: f1d662a ("[CIFS] Add upcall files for cifs to use spnego/kerberos")
Assisted-by: avom-custom-harness:gpt-5.5-qwen3.6-mod-mix
Reviewed-by: David Howells <dhowells@redhat.com>
Signed-off-by: Asim Viladi Oglu Manizada <manizada@pm.me>
Signed-off-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Shreeya Patel <spatel@ciq.com>

Author: shreeya-patel98

fs/smb/client/cifs_spnego.c

@@ -8,6 +8,7 @@
  */
 
 #include <linux/list.h>
+#include <linux/cred.h>
 #include <linux/slab.h>
 #include <linux/string.h>
 #include <keys/user-type.h>
@@ -40,12 +41,27 @@ cifs_spnego_key_destroy(struct key *key)
 	kfree(key->payload.data[0]);
 }
 
+static int
+cifs_spnego_key_vet_description(const char *description)
+{
+	/*
+	 * cifs.spnego descriptions are authority-bearing inputs to cifs.upcall.
+	 * They are only valid when produced by CIFS while using the private
+	 * spnego_cred installed below.  Do not let userspace create this type
+	 * of key through request_key(2)/add_key(2), since the helper treats
+	 * pid/uid/creduid/upcall_target as kernel-originating fields.
+	 */
+	if (current_cred() != spnego_cred)
+		return -EPERM;
+	return 0;
+}
 
 /*
  * keytype for CIFS spnego keys
  */
 struct key_type cifs_spnego_key_type = {
 	.name		= "cifs.spnego",
+	.vet_description = cifs_spnego_key_vet_description,
 	.instantiate	= cifs_spnego_key_instantiate,
 	.destroy	= cifs_spnego_key_destroy,
 	.describe	= user_describe,