Commit 48ba2462 authored by David Howells's avatar David Howells Committed by Rusty Russell
Browse files

MODSIGN: Implement module signature checking 



Check the signature on the module against the keys compiled into the kernel or
available in a hardware key store.

Currently, only RSA keys are supported - though that's easy enough to change,
and the signature is expected to contain raw components (so not a PGP or
PKCS#7 formatted blob).

The signature blob is expected to consist of the following pieces in order:

 (1) The binary identifier for the key.  This is expected to match the
     SubjectKeyIdentifier from an X.509 certificate.  Only X.509 type
     identifiers are currently supported.

 (2) The signature data, consisting of a series of MPIs in which each is in
     the format of a 2-byte BE word sizes followed by the content data.

 (3) A 12 byte information block of the form:

	struct module_signature {
		enum pkey_algo		algo : 8;
		enum pkey_hash_algo	hash : 8;
		enum pkey_id_type	id_type : 8;
		u8			__pad;
		__be32			id_length;
		__be32			sig_length;
	};

     The three enums are defined in crypto/public_key.h.

     'algo' contains the public-key algorithm identifier (0->DSA, 1->RSA).

     'hash' contains the digest algorithm identifier (0->MD4, 1->MD5, 2->SHA1,
      etc.).

     'id_type' contains the public-key identifier type (0->PGP, 1->X.509).

     '__pad' should be 0.

     'id_length' should contain in the binary identifier length in BE form.

     'sig_length' should contain in the signature data length in BE form.

     The lengths are in BE order rather than CPU order to make dealing with
     cross-compilation easier.

Signed-off-by: default avatarDavid Howells <dhowells@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au> (minor Kconfig fix)
parent 631cc66e

init/Kconfig

+8 −0
Original line number Diff line number Diff line
@@ -1588,6 +1588,14 @@ config MODULE_SRCVERSION_ALL 
config MODULE_SIG

	bool "Module signature verification"

	depends on MODULES

	select KEYS

	select CRYPTO

	select ASYMMETRIC_KEY_TYPE

	select ASYMMETRIC_PUBLIC_KEY_SUBTYPE

	select PUBLIC_KEY_ALGO_RSA

	select ASN1

	select OID_REGISTRY

	select X509_CERTIFICATE_PARSER

	help

	  Check modules for valid signatures upon load: the signature

	  is simply appended to the module. For more information see

kernel/module_signing.c

+221 −1
Original line number Diff line number Diff line
@@ -11,13 +11,233 @@ 


#include <linux/kernel.h>

#include <linux/err.h>

#include <crypto/public_key.h>

#include <crypto/hash.h>

#include <keys/asymmetric-type.h>

#include "module-internal.h"



/*

 * Module signature information block.

 *

 * The constituents of the signature section are, in order:

 *

 *	- Signer's name

 *	- Key identifier

 *	- Signature data

 *	- Information block

 */

struct module_signature {

	enum pkey_algo		algo : 8;	/* Public-key crypto algorithm */

	enum pkey_hash_algo	hash : 8;	/* Digest algorithm */

	enum pkey_id_type	id_type : 8;	/* Key identifier type */

	u8			signer_len;	/* Length of signer's name */

	u8			key_id_len;	/* Length of key identifier */

	u8			__pad[3];

	__be32			sig_len;	/* Length of signature data */

};



/*

 * Digest the module contents.

 */

static struct public_key_signature *mod_make_digest(enum pkey_hash_algo hash,

						    const void *mod,

						    unsigned long modlen)

{

	struct public_key_signature *pks;

	struct crypto_shash *tfm;

	struct shash_desc *desc;

	size_t digest_size, desc_size;

	int ret;



	pr_devel("==>%s()\n", __func__);

	

	/* Allocate the hashing algorithm we're going to need and find out how

	 * big the hash operational data will be.

	 */

	tfm = crypto_alloc_shash(pkey_hash_algo[hash], 0, 0);

	if (IS_ERR(tfm))

		return (PTR_ERR(tfm) == -ENOENT) ? ERR_PTR(-ENOPKG) : ERR_CAST(tfm);



	desc_size = crypto_shash_descsize(tfm) + sizeof(*desc);

	digest_size = crypto_shash_digestsize(tfm);



	/* We allocate the hash operational data storage on the end of our

	 * context data and the digest output buffer on the end of that.

	 */

	ret = -ENOMEM;

	pks = kzalloc(digest_size + sizeof(*pks) + desc_size, GFP_KERNEL);

	if (!pks)

		goto error_no_pks;



	pks->pkey_hash_algo	= hash;

	pks->digest		= (u8 *)pks + sizeof(*pks) + desc_size;

	pks->digest_size	= digest_size;



	desc = (void *)pks + sizeof(*pks);

	desc->tfm   = tfm;

	desc->flags = CRYPTO_TFM_REQ_MAY_SLEEP;



	ret = crypto_shash_init(desc);

	if (ret < 0)

		goto error;



	ret = crypto_shash_finup(desc, mod, modlen, pks->digest);

	if (ret < 0)

		goto error;



	crypto_free_shash(tfm);

	pr_devel("<==%s() = ok\n", __func__);

	return pks;



error:

	kfree(pks);

error_no_pks:

	crypto_free_shash(tfm);

	pr_devel("<==%s() = %d\n", __func__, ret);

	return ERR_PTR(ret);

}



/*

 * Extract an MPI array from the signature data.  This represents the actual

 * signature.  Each raw MPI is prefaced by a BE 2-byte value indicating the

 * size of the MPI in bytes.

 *

 * RSA signatures only have one MPI, so currently we only read one.

 */

static int mod_extract_mpi_array(struct public_key_signature *pks,

				 const void *data, size_t len)

{

	size_t nbytes;

	MPI mpi;



	if (len < 3)

		return -EBADMSG;

	nbytes = ((const u8 *)data)[0] << 8 | ((const u8 *)data)[1];

	data += 2;

	len -= 2;

	if (len != nbytes)

		return -EBADMSG;



	mpi = mpi_read_raw_data(data, nbytes);

	if (!mpi)

		return -ENOMEM;

	pks->mpi[0] = mpi;

	pks->nr_mpi = 1;

	return 0;

}



/*

 * Request an asymmetric key.

 */

static struct key *request_asymmetric_key(const char *signer, size_t signer_len,

					  const u8 *key_id, size_t key_id_len)

{

	key_ref_t key;

	size_t i;

	char *id, *q;



	pr_devel("==>%s(,%zu,,%zu)\n", __func__, signer_len, key_id_len);



	/* Construct an identifier. */

	id = kmalloc(signer_len + 2 + key_id_len * 2 + 1, GFP_KERNEL);

	if (!id)

		return ERR_PTR(-ENOKEY);



	memcpy(id, signer, signer_len);



	q = id + signer_len;

	*q++ = ':';

	*q++ = ' ';

	for (i = 0; i < key_id_len; i++) {

		*q++ = hex_asc[*key_id >> 4];

		*q++ = hex_asc[*key_id++ & 0x0f];

	}



	*q = 0;



	pr_debug("Look up: \"%s\"\n", id);



	key = keyring_search(make_key_ref(modsign_keyring, 1),

			     &key_type_asymmetric, id);

	if (IS_ERR(key))

		pr_warn("Request for unknown module key '%s' err %ld\n",

			id, PTR_ERR(key));

	kfree(id);



	if (IS_ERR(key)) {

		switch (PTR_ERR(key)) {

			/* Hide some search errors */

		case -EACCES:

		case -ENOTDIR:

		case -EAGAIN:

			return ERR_PTR(-ENOKEY);

		default:

			return ERR_CAST(key);

		}

	}



	pr_devel("<==%s() = 0 [%x]\n", __func__, key_serial(key_ref_to_ptr(key)));

	return key_ref_to_ptr(key);

}



/*

 * Verify the signature on a module.

 */

int mod_verify_sig(const void *mod, unsigned long modlen,

		   const void *sig, unsigned long siglen)

{

	return -ENOKEY;

	struct public_key_signature *pks;

	struct module_signature ms;

	struct key *key;

	size_t sig_len;

	int ret;



	pr_devel("==>%s(,%lu,,%lu,)\n", __func__, modlen, siglen);



	if (siglen <= sizeof(ms))

		return -EBADMSG;



	memcpy(&ms, sig + (siglen - sizeof(ms)), sizeof(ms));

	siglen -= sizeof(ms);



	sig_len = be32_to_cpu(ms.sig_len);

	if (sig_len >= siglen ||

	    siglen - sig_len != (size_t)ms.signer_len + ms.key_id_len)

		return -EBADMSG;



	/* For the moment, only support RSA and X.509 identifiers */

	if (ms.algo != PKEY_ALGO_RSA ||

	    ms.id_type != PKEY_ID_X509)

		return -ENOPKG;



	if (ms.hash >= PKEY_HASH__LAST ||

	    !pkey_hash_algo[ms.hash])

		return -ENOPKG;



	key = request_asymmetric_key(sig, ms.signer_len,

				     sig + ms.signer_len, ms.key_id_len);

	if (IS_ERR(key))

		return PTR_ERR(key);



	pks = mod_make_digest(ms.hash, mod, modlen);

	if (IS_ERR(pks)) {

		ret = PTR_ERR(pks);

		goto error_put_key;

	}



	ret = mod_extract_mpi_array(pks, sig + ms.signer_len + ms.key_id_len,

				    sig_len);

	if (ret < 0)

		goto error_free_pks;



	ret = verify_signature(key, pks);

	pr_devel("verify_signature() = %d\n", ret);



error_free_pks:

	mpi_free(pks->rsa.s);

	kfree(pks);

error_put_key:

	key_put(key);

	pr_devel("<==%s() = %d\n", __func__, ret);

	return ret;	

}

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