linux/certs/blacklist.c

// SPDX-License-Identifier: GPL-2.0-or-later
/* System hash blacklist.
 *
 * Copyright (C) 2016 Red Hat, Inc. All Rights Reserved.
 * Written by David Howells (dhowells@redhat.com)
 */

#define pr_fmt(fmt) "blacklist: "fmt
#include <linux/module.h>
#include <linux/slab.h>
#include <linux/key.h>
#include <linux/key-type.h>
#include <linux/sched.h>
#include <linux/ctype.h>
#include <linux/err.h>
#include <linux/seq_file.h>
#include <linux/uidgid.h>
#include <keys/asymmetric-type.h>
#include <keys/system_keyring.h>
#include "blacklist.h"

/*
 * According to crypto/asymmetric_keys/x509_cert_parser.c:x509_note_pkey_algo(),
 * the size of the currently longest supported hash algorithm is 512 bits,
 * which translates into 128 hex characters.
 */
#define MAX_HASH_LEN	128

#define BLACKLIST_KEY_PERM (KEY_POS_SEARCH | KEY_POS_VIEW | \
			    KEY_USR_SEARCH | KEY_USR_VIEW)

static const char tbs_prefix[] = "tbs";
static const char bin_prefix[] = "bin";

static struct key *blacklist_keyring;

#ifdef CONFIG_SYSTEM_REVOCATION_LIST
extern __initconst const u8 revocation_certificate_list[];
extern __initconst const unsigned long revocation_certificate_list_size;
#endif

/*
 * The description must be a type prefix, a colon and then an even number of
 * hex digits.  The hash is kept in the description.
 */
static int blacklist_vet_description(const char *desc)
{
	int i, prefix_len, tbs_step = 0, bin_step = 0;

	/* The following algorithm only works if prefix lengths match. */
	BUILD_BUG_ON(sizeof(tbs_prefix) != sizeof(bin_prefix));
	prefix_len = sizeof(tbs_prefix) - 1;
	for (i = 0; *desc; desc++, i++) {
		if (*desc == ':') {
			if (tbs_step == prefix_len)
				goto found_colon;
			if (bin_step == prefix_len)
				goto found_colon;
			return -EINVAL;
		}
		if (i >= prefix_len)
			return -EINVAL;
		if (*desc == tbs_prefix[i])
			tbs_step++;
		if (*desc == bin_prefix[i])
			bin_step++;
	}
	return -EINVAL;

found_colon:
	desc++;
	for (i = 0; *desc && i < MAX_HASH_LEN; desc++, i++) {
		if (!isxdigit(*desc) || isupper(*desc))
			return -EINVAL;
	}
	if (*desc)
		/* The hash is greater than MAX_HASH_LEN. */
		return -ENOPKG;

	/* Checks for an even number of hexadecimal characters. */
	if (i == 0 || i & 1)
		return -EINVAL;
	return 0;
}

static int blacklist_key_instantiate(struct key *key,
		struct key_preparsed_payload *prep)
{
#ifdef CONFIG_SYSTEM_BLACKLIST_AUTH_UPDATE
	int err;
#endif

	/* Sets safe default permissions for keys loaded by user space. */
	key->perm = BLACKLIST_KEY_PERM;

	/*
	 * Skips the authentication step for builtin hashes, they are not
	 * signed but still trusted.
	 */
	if (key->flags & (1 << KEY_FLAG_BUILTIN))
		goto out;

#ifdef CONFIG_SYSTEM_BLACKLIST_AUTH_UPDATE
	/*
	 * Verifies the description's PKCS#7 signature against the builtin
	 * trusted keyring.
	 */
	err = verify_pkcs7_signature(key->description,
			strlen(key->description), prep->data, prep->datalen,
			NULL, VERIFYING_UNSPECIFIED_SIGNATURE, NULL, NULL);
	if (err)
		return err;
#else
	/*
	 * It should not be possible to come here because the keyring doesn't
	 * have KEY_USR_WRITE and the only other way to call this function is
	 * for builtin hashes.
	 */
	WARN_ON_ONCE(1);
	return -EPERM;
#endif

out:
	return generic_key_instantiate(key, prep);
}

static int blacklist_key_update(struct key *key,
		struct key_preparsed_payload *prep)
{
	return -EPERM;
}

static void blacklist_describe(const struct key *key, struct seq_file *m)
{
	seq_puts(m, key->description);
}

static struct key_type key_type_blacklist = {
	.name			= "blacklist",
	.vet_description	= blacklist_vet_description,
	.instantiate		= blacklist_key_instantiate,
	.update			= blacklist_key_update,
	.describe		= blacklist_describe,
};

static char *get_raw_hash(const u8 *hash, size_t hash_len,
		enum blacklist_hash_type hash_type)
{
	size_t type_len;
	const char *type_prefix;
	char *buffer, *p;

	switch (hash_type) {
	case BLACKLIST_HASH_X509_TBS:
		type_len = sizeof(tbs_prefix) - 1;
		type_prefix = tbs_prefix;
		break;
	case BLACKLIST_HASH_BINARY:
		type_len = sizeof(bin_prefix) - 1;
		type_prefix = bin_prefix;
		break;
	default:
		WARN_ON_ONCE(1);
		return ERR_PTR(-EINVAL);
	}
	buffer = kmalloc(type_len + 1 + hash_len * 2 + 1, GFP_KERNEL);
	if (!buffer)
		return ERR_PTR(-ENOMEM);
	p = memcpy(buffer, type_prefix, type_len);
	p += type_len;
	*p++ = ':';
	bin2hex(p, hash, hash_len);
	p += hash_len * 2;
	*p = '\0';
	return buffer;
}

/**
 * mark_raw_hash_blacklisted - Add a hash to the system blacklist
 * @hash: The hash as a hex string with a type prefix (eg. "tbs:23aa429783")
 */
static int mark_raw_hash_blacklisted(const char *hash)
{
	key_ref_t key;

	key = key_create(make_key_ref(blacklist_keyring, true),
			 "blacklist",
			 hash,
			 NULL,
			 0,
			 BLACKLIST_KEY_PERM,
			 KEY_ALLOC_NOT_IN_QUOTA |
			 KEY_ALLOC_BUILT_IN);
	if (IS_ERR(key)) {
		if (PTR_ERR(key) == -EEXIST)
			pr_warn("Duplicate blacklisted hash %s\n", hash);
		else
			pr_err("Problem blacklisting hash %s: %pe\n", hash, key);
		return PTR_ERR(key);
	}
	return 0;
}

int mark_hash_blacklisted(const u8 *hash, size_t hash_len,
		enum blacklist_hash_type hash_type)
{
	const char *buffer;
	int err;

	buffer = get_raw_hash(hash, hash_len, hash_type);
	if (IS_ERR(buffer))
		return PTR_ERR(buffer);
	err = mark_raw_hash_blacklisted(buffer);
	kfree(buffer);
	return err;
}

/**
 * is_hash_blacklisted - Determine if a hash is blacklisted
 * @hash: The hash to be checked as a binary blob
 * @hash_len: The length of the binary hash
 * @hash_type: Type of hash
 */
int is_hash_blacklisted(const u8 *hash, size_t hash_len,
		enum blacklist_hash_type hash_type)
{
	key_ref_t kref;
	const char *buffer;
	int ret = 0;

	buffer = get_raw_hash(hash, hash_len, hash_type);
	if (IS_ERR(buffer))
		return PTR_ERR(buffer);
	kref = keyring_search(make_key_ref(blacklist_keyring, true),
			      &key_type_blacklist, buffer, false);
	if (!IS_ERR(kref)) {
		key_ref_put(kref);
		ret = -EKEYREJECTED;
	}

	kfree(buffer);
	return ret;
}
EXPORT_SYMBOL_GPL(is_hash_blacklisted);

int is_binary_blacklisted(const u8 *hash, size_t hash_len)
{
	if (is_hash_blacklisted(hash, hash_len, BLACKLIST_HASH_BINARY) ==
			-EKEYREJECTED)
		return -EPERM;

	return 0;
}
EXPORT_SYMBOL_GPL(is_binary_blacklisted);

#ifdef CONFIG_SYSTEM_REVOCATION_LIST
/**
 * add_key_to_revocation_list - Add a revocation certificate to the blacklist
 * @data: The data blob containing the certificate
 * @size: The size of data blob
 */
int add_key_to_revocation_list(const char *data, size_t size)
{
	key_ref_t key;

	key = key_create_or_update(make_key_ref(blacklist_keyring, true),
				   "asymmetric",
				   NULL,
				   data,
				   size,
				   KEY_POS_VIEW | KEY_POS_READ | KEY_POS_SEARCH
				   | KEY_USR_VIEW,
				   KEY_ALLOC_NOT_IN_QUOTA | KEY_ALLOC_BUILT_IN
				   | KEY_ALLOC_BYPASS_RESTRICTION);

	if (IS_ERR(key)) {
		pr_err("Problem with revocation key (%ld)\n", PTR_ERR(key));
		return PTR_ERR(key);
	}

	return 0;
}

/**
 * is_key_on_revocation_list - Determine if the key for a PKCS#7 message is revoked
 * @pkcs7: The PKCS#7 message to check
 */
int is_key_on_revocation_list(struct pkcs7_message *pkcs7)
{
	int ret;

	ret = pkcs7_validate_trust(pkcs7, blacklist_keyring);

	if (ret == 0)
		return -EKEYREJECTED;

	return -ENOKEY;
}
#endif

static int restrict_link_for_blacklist(struct key *dest_keyring,
		const struct key_type *type, const union key_payload *payload,
		struct key *restrict_key)
{
	if (type == &key_type_blacklist)
		return 0;
	return -EOPNOTSUPP;
}

/*
 * Initialise the blacklist
 *
 * The blacklist_init() function is registered as an initcall via
 * device_initcall().  As a result if the blacklist_init() function fails for
 * any reason the kernel continues to execute.  While cleanly returning -ENODEV
 * could be acceptable for some non-critical kernel parts, if the blacklist
 * keyring fails to load it defeats the certificate/key based deny list for
 * signed modules.  If a critical piece of security functionality that users
 * expect to be present fails to initialize, panic()ing is likely the right
 * thing to do.
 */
static int __init blacklist_init(void)
{
	const char *const *bl;
	struct key_restriction *restriction;

	if (register_key_type(&key_type_blacklist) < 0)
		panic("Can't allocate system blacklist key type\n");

	restriction = kzalloc(sizeof(*restriction), GFP_KERNEL);
	if (!restriction)
		panic("Can't allocate blacklist keyring restriction\n");
	restriction->check = restrict_link_for_blacklist;

	blacklist_keyring =
		keyring_alloc(".blacklist",
			      GLOBAL_ROOT_UID, GLOBAL_ROOT_GID, current_cred(),
			      KEY_POS_VIEW | KEY_POS_READ | KEY_POS_SEARCH |
			      KEY_POS_WRITE |
			      KEY_USR_VIEW | KEY_USR_READ | KEY_USR_SEARCH
#ifdef CONFIG_SYSTEM_BLACKLIST_AUTH_UPDATE
			      | KEY_USR_WRITE
#endif
			      , KEY_ALLOC_NOT_IN_QUOTA |
			      KEY_ALLOC_SET_KEEP,
			      restriction, NULL);
	if (IS_ERR(blacklist_keyring))
		panic("Can't allocate system blacklist keyring\n");

	for (bl = blacklist_hashes; *bl; bl++)
		if (mark_raw_hash_blacklisted(*bl) < 0)
			pr_err("- blacklisting failed\n");
	return 0;
}

/*
 * Must be initialised before we try and load the keys into the keyring.
 */
device_initcall(blacklist_init);

#ifdef CONFIG_SYSTEM_REVOCATION_LIST
/*
 * Load the compiled-in list of revocation X.509 certificates.
 */
static __init int load_revocation_certificate_list(void)
{
	if (revocation_certificate_list_size)
		pr_notice("Loading compiled-in revocation X.509 certificates\n");

	return x509_load_certificate_list(revocation_certificate_list,
					  revocation_certificate_list_size,
					  blacklist_keyring);
}
late_initcall(load_revocation_certificate_list);
#endif