verify_pefile.c

// SPDX-License-Identifier: GPL-2.0-or-later
/* Parse a signed PE binary
 *
 * Copyright (C) 2014 Red Hat, Inc. All Rights Reserved.
 * Written by David Howells (dhowells@redhat.com)
 */

#define pr_fmt(fmt) "PEFILE: "fmt
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/slab.h>
#include <linux/err.h>
#include <linux/pe.h>
#include <linux/asn1.h>
#include <linux/verification.h>
#include <crypto/hash.h>
#include "verify_pefile.h"

/*
 * Parse a PE binary.
 */
static int pefile_parse_binary(const void *pebuf, unsigned int pelen,
			       struct pefile_context *ctx)
{
	const struct mz_hdr *mz = pebuf;
	const struct pe_hdr *pe;
	const struct pe32_opt_hdr *pe32;
	const struct pe32plus_opt_hdr *pe64;
	const struct data_directory *ddir;
	const struct data_dirent *dde;
	const struct section_header *secs, *sec;
	size_t cursor, datalen = pelen;

	kenter("");

#define chkaddr(base, x, s)						\
	do {								\
		if ((x) < base || (s) >= datalen || (x) > datalen - (s)) \
			return -ELIBBAD;				\
	} while (0)

	chkaddr(0, 0, sizeof(*mz));
	if (mz->magic != MZ_MAGIC)
		return -ELIBBAD;
	cursor = sizeof(*mz);

	chkaddr(cursor, mz->peaddr, sizeof(*pe));
	pe = pebuf + mz->peaddr;
	if (pe->magic != PE_MAGIC)
		return -ELIBBAD;
	cursor = mz->peaddr + sizeof(*pe);

	chkaddr(0, cursor, sizeof(pe32->magic));
	pe32 = pebuf + cursor;
	pe64 = pebuf + cursor;

	switch (pe32->magic) {
	case PE_OPT_MAGIC_PE32:
		chkaddr(0, cursor, sizeof(*pe32));
		ctx->image_checksum_offset =
			(unsigned long)&pe32->csum - (unsigned long)pebuf;
		ctx->header_size = pe32->header_size;
		cursor += sizeof(*pe32);
		ctx->n_data_dirents = pe32->data_dirs;
		break;

	case PE_OPT_MAGIC_PE32PLUS:
		chkaddr(0, cursor, sizeof(*pe64));
		ctx->image_checksum_offset =
			(unsigned long)&pe64->csum - (unsigned long)pebuf;
		ctx->header_size = pe64->header_size;
		cursor += sizeof(*pe64);
		ctx->n_data_dirents = pe64->data_dirs;
		break;

	default:
		pr_debug("Unknown PEOPT magic = %04hx\n", pe32->magic);
		return -ELIBBAD;
	}

	pr_debug("checksum @ %x\n", ctx->image_checksum_offset);
	pr_debug("header size = %x\n", ctx->header_size);

	if (cursor >= ctx->header_size || ctx->header_size >= datalen)
		return -ELIBBAD;

	if (ctx->n_data_dirents > (ctx->header_size - cursor) / sizeof(*dde))
		return -ELIBBAD;

	ddir = pebuf + cursor;
	cursor += sizeof(*dde) * ctx->n_data_dirents;

	ctx->cert_dirent_offset =
		(unsigned long)&ddir->certs - (unsigned long)pebuf;
	ctx->certs_size = ddir->certs.size;

	if (!ddir->certs.virtual_address || !ddir->certs.size) {
		pr_debug("Unsigned PE binary\n");
		return -ENODATA;
	}

	chkaddr(ctx->header_size, ddir->certs.virtual_address,
		ddir->certs.size);
	ctx->sig_offset = ddir->certs.virtual_address;
	ctx->sig_len = ddir->certs.size;
	pr_debug("cert = %x @%x [%*ph]\n",
		 ctx->sig_len, ctx->sig_offset,
		 ctx->sig_len, pebuf + ctx->sig_offset);

	ctx->n_sections = pe->sections;
	if (ctx->n_sections > (ctx->header_size - cursor) / sizeof(*sec))
		return -ELIBBAD;
	ctx->secs = secs = pebuf + cursor;

	return 0;
}

/*
 * Check and strip the PE wrapper from around the signature and check that the
 * remnant looks something like PKCS#7.
 */
static int pefile_strip_sig_wrapper(const void *pebuf,
				    struct pefile_context *ctx)
{
	struct win_certificate wrapper;
	const u8 *pkcs7;
	unsigned len;

	if (ctx->sig_len < sizeof(wrapper)) {
		pr_debug("Signature wrapper too short\n");
		return -ELIBBAD;
	}

	memcpy(&wrapper, pebuf + ctx->sig_offset, sizeof(wrapper));
	pr_debug("sig wrapper = { %x, %x, %x }\n",
		 wrapper.length, wrapper.revision, wrapper.cert_type);

	/* Both pesign and sbsign round up the length of certificate table
	 * (in optional header data directories) to 8 byte alignment.
	 */
	if (round_up(wrapper.length, 8) != ctx->sig_len) {
		pr_debug("Signature wrapper len wrong\n");
		return -ELIBBAD;
	}
	if (wrapper.revision != WIN_CERT_REVISION_2_0) {
		pr_debug("Signature is not revision 2.0\n");
		return -ENOTSUPP;
	}
	if (wrapper.cert_type != WIN_CERT_TYPE_PKCS_SIGNED_DATA) {
		pr_debug("Signature certificate type is not PKCS\n");
		return -ENOTSUPP;
	}

	/* It looks like the pkcs signature length in wrapper->length and the
	 * size obtained from the data dir entries, which lists the total size
	 * of certificate table, are both aligned to an octaword boundary, so
	 * we may have to deal with some padding.
	 */
	ctx->sig_len = wrapper.length;
	ctx->sig_offset += sizeof(wrapper);
	ctx->sig_len -= sizeof(wrapper);
	if (ctx->sig_len < 4) {
		pr_debug("Signature data missing\n");
		return -EKEYREJECTED;
	}

	/* What's left should be a PKCS#7 cert */
	pkcs7 = pebuf + ctx->sig_offset;
	if (pkcs7[0] != (ASN1_CONS_BIT | ASN1_SEQ))
		goto not_pkcs7;

	switch (pkcs7[1]) {
	case 0 ... 0x7f:
		len = pkcs7[1] + 2;
		goto check_len;
	case ASN1_INDEFINITE_LENGTH:
		return 0;
	case 0x81:
		len = pkcs7[2] + 3;
		goto check_len;
	case 0x82:
		len = ((pkcs7[2] << 8) | pkcs7[3]) + 4;
		goto check_len;
	case 0x83 ... 0xff:
		return -EMSGSIZE;
	default:
		goto not_pkcs7;
	}

check_len:
	if (len <= ctx->sig_len) {
		/* There may be padding */
		ctx->sig_len = len;
		return 0;
	}
not_pkcs7:
	pr_debug("Signature data not PKCS#7\n");
	return -ELIBBAD;
}

/*
 * Compare two sections for canonicalisation.
 */
static int pefile_compare_shdrs(const void *a, const void *b)
{
	const struct section_header *shdra = a;
	const struct section_header *shdrb = b;
	int rc;

	if (shdra->data_addr > shdrb->data_addr)
		return 1;
	if (shdrb->data_addr > shdra->data_addr)
		return -1;

	if (shdra->virtual_address > shdrb->virtual_address)
		return 1;
	if (shdrb->virtual_address > shdra->virtual_address)
		return -1;

	rc = strcmp(shdra->name, shdrb->name);
	if (rc != 0)
		return rc;

	if (shdra->virtual_size > shdrb->virtual_size)
		return 1;
	if (shdrb->virtual_size > shdra->virtual_size)
		return -1;

	if (shdra->raw_data_size > shdrb->raw_data_size)
		return 1;
	if (shdrb->raw_data_size > shdra->raw_data_size)
		return -1;

	return 0;
}

/*
 * Load the contents of the PE binary into the digest, leaving out the image
 * checksum and the certificate data block.
 */
static int pefile_digest_pe_contents(const void *pebuf, unsigned int pelen,
				     struct pefile_context *ctx,
				     struct shash_desc *desc)
{
	unsigned *canon, tmp, loop, i, hashed_bytes;
	int ret;

	/* Digest the header and data directory, but leave out the image
	 * checksum and the data dirent for the signature.
	 */
	ret = crypto_shash_update(desc, pebuf, ctx->image_checksum_offset);
	if (ret < 0)
		return ret;

	tmp = ctx->image_checksum_offset + sizeof(uint32_t);
	ret = crypto_shash_update(desc, pebuf + tmp,
				  ctx->cert_dirent_offset - tmp);
	if (ret < 0)
		return ret;

	tmp = ctx->cert_dirent_offset + sizeof(struct data_dirent);
	ret = crypto_shash_update(desc, pebuf + tmp, ctx->header_size - tmp);
	if (ret < 0)
		return ret;

	canon = kcalloc(ctx->n_sections, sizeof(unsigned), GFP_KERNEL);
	if (!canon)
		return -ENOMEM;

	/* We have to canonicalise the section table, so we perform an
	 * insertion sort.
	 */
	canon[0] = 0;
	for (loop = 1; loop < ctx->n_sections; loop++) {
		for (i = 0; i < loop; i++) {
			if (pefile_compare_shdrs(&ctx->secs[canon[i]],
						 &ctx->secs[loop]) > 0) {
				memmove(&canon[i + 1], &canon[i],
					(loop - i) * sizeof(canon[0]));
				break;
			}
		}
		canon[i] = loop;
	}

	hashed_bytes = ctx->header_size;
	for (loop = 0; loop < ctx->n_sections; loop++) {
		i = canon[loop];
		if (ctx->secs[i].raw_data_size == 0)
			continue;
		ret = crypto_shash_update(desc,
					  pebuf + ctx->secs[i].data_addr,
					  ctx->secs[i].raw_data_size);
		if (ret < 0) {
			kfree(canon);
			return ret;
		}
		hashed_bytes += ctx->secs[i].raw_data_size;
	}
	kfree(canon);

	if (pelen > hashed_bytes) {
		tmp = hashed_bytes + ctx->certs_size;
		ret = crypto_shash_update(desc,
					  pebuf + hashed_bytes,
					  pelen - tmp);
		if (ret < 0)
			return ret;
	}

	return 0;
}

/*
 * Digest the contents of the PE binary, leaving out the image checksum and the
 * certificate data block.
 */
static int pefile_digest_pe(const void *pebuf, unsigned int pelen,
			    struct pefile_context *ctx)
{
	struct crypto_shash *tfm;
	struct shash_desc *desc;
	size_t digest_size, desc_size;
	void *digest;
	int ret;

	kenter(",%s", ctx->digest_algo);

	/* Allocate the hashing algorithm we're going to need and find out how
	 * big the hash operational data will be.
	 */
	tfm = crypto_alloc_shash(ctx->digest_algo, 0, 0);
	if (IS_ERR(tfm))
		return (PTR_ERR(tfm) == -ENOENT) ? -ENOPKG : PTR_ERR(tfm);

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

	if (digest_size != ctx->digest_len) {
		pr_debug("Digest size mismatch (%zx != %x)\n",
			 digest_size, ctx->digest_len);
		ret = -EBADMSG;
		goto error_no_desc;
	}
	pr_debug("Digest: desc=%zu size=%zu\n", desc_size, digest_size);

	ret = -ENOMEM;
	desc = kzalloc(desc_size + digest_size, GFP_KERNEL);
	if (!desc)
		goto error_no_desc;

	desc->tfm   = tfm;
	ret = crypto_shash_init(desc);
	if (ret < 0)
		goto error;

	ret = pefile_digest_pe_contents(pebuf, pelen, ctx, desc);
	if (ret < 0)
		goto error;

	digest = (void *)desc + desc_size;
	ret = crypto_shash_final(desc, digest);
	if (ret < 0)
		goto error;

	pr_debug("Digest calc = [%*ph]\n", ctx->digest_len, digest);

	/* Check that the PE file digest matches that in the MSCODE part of the
	 * PKCS#7 certificate.
	 */
	if (memcmp(digest, ctx->digest, ctx->digest_len) != 0) {
		pr_debug("Digest mismatch\n");
		ret = -EKEYREJECTED;
	} else {
		pr_debug("The digests match!\n");
	}

error:
	kfree_sensitive(desc);
error_no_desc:
	crypto_free_shash(tfm);
	kleave(" = %d", ret);
	return ret;
}

/**
 * verify_pefile_signature - Verify the signature on a PE binary image
 * @pebuf: Buffer containing the PE binary image
 * @pelen: Length of the binary image
 * @trust_keys: Signing certificate(s) to use as starting points
 * @usage: The use to which the key is being put.
 *
 * Validate that the certificate chain inside the PKCS#7 message inside the PE
 * binary image intersects keys we already know and trust.
 *
 * Returns, in order of descending priority:
 *
 *  (*) -ELIBBAD if the image cannot be parsed, or:
 *
 *  (*) -EKEYREJECTED if a signature failed to match for which we have a valid
 *	key, or:
 *
 *  (*) 0 if at least one signature chain intersects with the keys in the trust
 *	keyring, or:
 *
 *  (*) -ENODATA if there is no signature present.
 *
 *  (*) -ENOPKG if a suitable crypto module couldn't be found for a check on a
 *	chain.
 *
 *  (*) -ENOKEY if we couldn't find a match for any of the signature chains in
 *	the message.
 *
 * May also return -ENOMEM.
 */
int verify_pefile_signature(const void *pebuf, unsigned pelen,
			    struct key *trusted_keys,
			    enum key_being_used_for usage)
{
	struct pefile_context ctx;
	int ret;

	kenter("");

	memset(&ctx, 0, sizeof(ctx));
	ret = pefile_parse_binary(pebuf, pelen, &ctx);
	if (ret < 0)
		return ret;

	ret = pefile_strip_sig_wrapper(pebuf, &ctx);
	if (ret < 0)
		return ret;

	ret = verify_pkcs7_signature(NULL, 0,
				     pebuf + ctx.sig_offset, ctx.sig_len,
				     trusted_keys, usage,
				     mscode_parse, &ctx);
	if (ret < 0)
		goto error;

	pr_debug("Digest: %u [%*ph]\n",
		 ctx.digest_len, ctx.digest_len, ctx.digest);

	/* Generate the digest and check against the PKCS7 certificate
	 * contents.
	 */
	ret = pefile_digest_pe(pebuf, pelen, &ctx);

error:
	kfree_sensitive(ctx.digest);
	return ret;
}