[PATCH16/17] pefile: Digest the PE binary and compare to the PKCS#7 data

David Howells dhowells at redhat.com
Wed Jul 9 08:17:07 PDT 2014


Digest the signed parts of the PE binary, canonicalising the section table
before we need it, and then compare the the resulting digest to the one in the
PKCS#7 signed content.

Signed-off-by: David Howells <dhowells at redhat.com>
Acked-by: Vivek Goyal <vgoyal at redhat.com>
Reviewed-by: Kees Cook <keescook at chromium.org>
---

 crypto/asymmetric_keys/verify_pefile.c |  197 ++++++++++++++++++++++++++++++++
 1 file changed, 197 insertions(+)


diff --git a/crypto/asymmetric_keys/verify_pefile.c b/crypto/asymmetric_keys/verify_pefile.c
index b975918..029a365 100644
--- a/crypto/asymmetric_keys/verify_pefile.c
+++ b/crypto/asymmetric_keys/verify_pefile.c
@@ -185,6 +185,192 @@ static int pefile_strip_sig_wrapper(const void *pebuf,
 	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(",%u", 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(hash_algo_name[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:
+	kfree(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
@@ -252,6 +438,17 @@ int verify_pefile_signature(const void *pebuf, unsigned pelen,
 	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);
+	if (ret < 0)
+		goto error;
+
+	ret = pkcs7_verify(pkcs7);
+	if (ret < 0)
+		goto error;
+
 	ret = -ENOANO; // Not yet complete
 
 error:




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