cred: Do not default to init_cred in prepare_kernel_cred()

A common exploit pattern for ROP attacks is to abuse prepare_kernel_cred()
in order to construct escalated privileges[1]. Instead of providing a
short-hand argument (NULL) to the "daemon" argument to indicate using
init_cred as the base cred, require that "daemon" is always set to
an actual task. Replace all existing callers that were passing NULL
with &init_task.

Future attacks will need to have sufficiently powerful read/write
primitives to have found an appropriately privileged task and written it
to the ROP stack as an argument to succeed, which is similarly difficult
to the prior effort needed to escalate privileges before struct cred
existed: locate the current cred and overwrite the uid member.

This has the added benefit of meaning that prepare_kernel_cred() can no
longer exceed the privileges of the init task, which may have changed from
the original init_cred (e.g. dropping capabilities from the bounding set).

[1] https://google.com/search?q=commit_creds(prepare_kernel_cred(0))

Cc: "Eric W. Biederman" <[e-mail]>
Cc: David Howells <[e-mail]>
Cc: "Rafael J. Wysocki" <[e-mail]>
Cc: Steve French <[e-mail]>
Cc: Ronnie Sahlberg <[e-mail]>
Cc: Shyam Prasad N <[e-mail]>
Cc: Tom Talpey <[e-mail]>
Cc: Namjae Jeon <[e-mail]>
Cc: Trond Myklebust <[e-mail]>
Cc: Anna Schumaker <[e-mail]>
Cc: Chuck Lever <[e-mail]>
Cc: Jeff Layton <[e-mail]>
Cc: "David S. Miller" <[e-mail]>
Cc: Eric Dumazet <[e-mail]>
Cc: Jakub Kicinski <[e-mail]>
Cc: Paolo Abeni <[e-mail]>
Cc: "Michal Koutný" <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Signed-off-by: Kees Cook <[e-mail]>
Acked-by: Luis Chamberlain <[e-mail]>
Reviewed-by: Sergey Senozhatsky <[e-mail]>
Acked-by: Russ Weight <[e-mail]>
Acked-by: Greg Kroah-Hartman <[e-mail]>
Acked-by: Paulo Alcantara (SUSE) <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

drivers/base/firmware_loader/main.c

···
 	 * called by a driver when serving an unrelated request from userland, we use
 	 * the kernel credentials to read the file.
 	 */
-	kern_cred = prepare_kernel_cred(NULL);
+	kern_cred = prepare_kernel_cred(&init_task);
 	if (!kern_cred) {
 		ret = -ENOMEM;
 		goto out;

fs/cifs/cifs_spnego.c

···
 	 * spnego upcalls.
 	 */
 
-	cred = prepare_kernel_cred(NULL);
+	cred = prepare_kernel_cred(&init_task);
 	if (!cred)
 		return -ENOMEM;
 

fs/cifs/cifsacl.c

···
 	 * this is used to prevent malicious redirections from being installed
 	 * with add_key().
 	 */
-	cred = prepare_kernel_cred(NULL);
+	cred = prepare_kernel_cred(&init_task);
 	if (!cred)
 		return -ENOMEM;
 

fs/ksmbd/smb_common.c

···
 	if (share->force_gid != KSMBD_SHARE_INVALID_GID)
 		gid = share->force_gid;
 
-	cred = prepare_kernel_cred(NULL);
+	cred = prepare_kernel_cred(&init_task);
 	if (!cred)
 		return -ENOMEM;
 

fs/nfs/flexfilelayout/flexfilelayout.c

···
 		gid = make_kgid(&init_user_ns, id);
 
 		if (gfp_flags & __GFP_FS)
-			kcred = prepare_kernel_cred(NULL);
+			kcred = prepare_kernel_cred(&init_task);
 		else {
 			unsigned int nofs_flags = memalloc_nofs_save();
-			kcred = prepare_kernel_cred(NULL);
+			kcred = prepare_kernel_cred(&init_task);
 			memalloc_nofs_restore(nofs_flags);
 		}
 		rc = -ENOMEM;

fs/nfs/nfs4idmap.c

···
 	printk(KERN_NOTICE "NFS: Registering the %s key type\n",
 		key_type_id_resolver.name);
 
-	cred = prepare_kernel_cred(NULL);
+	cred = prepare_kernel_cred(&init_task);
 	if (!cred)
 		return -ENOMEM;
 

fs/nfsd/nfs4callback.c

···
 	} else {
 		struct cred *kcred;
 
-		kcred = prepare_kernel_cred(NULL);
+		kcred = prepare_kernel_cred(&init_task);
 		if (!kcred)
 			return NULL;
 

kernel/cred.c

···
  * override a task's own credentials so that work can be done on behalf of that
  * task that requires a different subjective context.
  *
- * @daemon is used to provide a base for the security record, but can be NULL.
- * If @daemon is supplied, then the security data will be derived from that;
- * otherwise they'll be set to 0 and no groups, full capabilities and no keys.
+ * @daemon is used to provide a base cred, with the security data derived from
+ * that; if this is "&init_task", they'll be set to 0, no groups, full
+ * capabilities, and no keys.
  *
  * The caller may change these controls afterwards if desired.
  *
···
 	const struct cred *old;
 	struct cred *new;
 
+	if (WARN_ON_ONCE(!daemon))
+		return NULL;
+
 	new = kmem_cache_alloc(cred_jar, GFP_KERNEL);
 	if (!new)
 		return NULL;
 
 	kdebug("prepare_kernel_cred() alloc %p", new);
 
-	if (daemon)
-		old = get_task_cred(daemon);
-	else
-		old = get_cred(&init_cred);
-
+	old = get_task_cred(daemon);
 	validate_creds(old);
 
 	*new = *old;

net/dns_resolver/dns_key.c

···
 	 * this is used to prevent malicious redirections from being installed
 	 * with add_key().
 	 */
-	cred = prepare_kernel_cred(NULL);
+	cred = prepare_kernel_cred(&init_task);
 	if (!cred)
 		return -ENOMEM;