verify_pefile: relax wrapper length check

The PE Format Specification (section "The Attribute Certificate Table
(Image Only)") states that `dwLength` is to be rounded up to 8-byte
alignment when used for traversal.  Therefore, the field is not required
to be an 8-byte multiple in the first place.

Accordingly, pesign has not performed this alignment since version
0.110.  This causes kexec failure on pesign'd binaries with "PEFILE:
Signature wrapper len wrong".  Update the comment and relax the check.
Signed-off-by: Robbie Harwood <rharwood@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Jarkko Sakkinen <jarkko@kernel.org>
cc: Eric Biederman <ebiederm@xmission.com>
cc: Herbert Xu <herbert@gondor.apana.org.au>
cc: keyrings@vger.kernel.org
cc: linux-crypto@vger.kernel.org
cc: kexec@lists.infradead.org
Link: https://learn.microsoft.com/en-us/windows/win32/debug/pe-format#the-attribute-certificate-table-image-only
Link: https://github.com/rhboot/pesign
Link: https://lore.kernel.org/r/20230220171254.592347-2-rharwood@redhat.com/ # v2

crypto/asymmetric_keys/verify_pefile.c

@@ -135,11 +135,15 @@ static int pefile_strip_sig_wrapper(const void *pebuf,
 	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.
+	/* sbsign rounds up the length of certificate table (in optional
+	 * header data directories) to 8 byte alignment.  However, the PE
+	 * specification states that while entries are 8-byte aligned, this is
+	 * not included in their length, and as a result, pesign has not
+	 * rounded up since 0.110.
 	 */
-	if (round_up(wrapper.length, 8) != ctx->sig_len) {
-		pr_debug("Signature wrapper len wrong\n");
+	if (wrapper.length > ctx->sig_len) {
+		pr_debug("Signature wrapper bigger than sig len (%x > %x)\n",
+			 ctx->sig_len, wrapper.length);
 		return -ELIBBAD;
 	}
 	if (wrapper.revision != WIN_CERT_REVISION_2_0) {