text: /* Parse a signed PE binary
   *
   * Copyright (C) 2014 Red Hat, Inc. All Rights Reserved.
   * Written by David Howells (dhowells@redhat.com)
   *
   * This program is free software; you can redistribute it and/or
   * modify it under the terms of the GNU General Public Licence
   * as published by the Free Software Foundation; either version
   * 2 of the Licence, or (at your option) any later version.
   */
  
  #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 -EKEYREJECTED;
  	}
  
  	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;
  	desc->flags = CRYPTO_TFM_REQ_MAY_SLEEP;
  	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:
  	kzfree(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:
   *
   *  (*) -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:
  	kzfree(ctx.digest);
  	return ret;
  }
  