linux-kernel - [PATCH 09/23] Fix signature verification for shorter signatures

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20120522230328.24007.96707.stgit@warthog.procyon.org.uk>
Date:	Wed, 23 May 2012 00:03:28 +0100
From:	David Howells <dhowells@...hat.com>
To:	rusty@...tcorp.com.au, kyle@...artin.ca
Cc:	linux-kernel@...r.kernel.org,
	linux-security-module@...r.kernel.org, keyrings@...ux-nfs.org,
	Milan Broz <mbroz@...hat.com>,
	David Howells <dhowells@...hat.com>
Subject: [PATCH 09/23] Fix signature verification for shorter signatures

gpg can produce a signature file where length of signature is less than the
modulus size because the amount of space an MPI takes up is kept as low as
possible by discarding leading zeros.  This regularly happens for several
modules during the build.

Fix it by relaxing check in RSA verification code.

Thanks to Tomas Mraz and Miloslav Trmac for help.

Signed-off-by: Milan Broz <mbroz@...hat.com>
Signed-off-by: David Howells <dhowells@...hat.com>
---

 security/keys/crypto/crypto_rsa.c |   14 +++++++++++---
 1 files changed, 11 insertions(+), 3 deletions(-)


diff --git a/security/keys/crypto/crypto_rsa.c b/security/keys/crypto/crypto_rsa.c
index beb5181..cc5cd95 100644
--- a/security/keys/crypto/crypto_rsa.c
+++ b/security/keys/crypto/crypto_rsa.c
@@ -219,15 +219,23 @@ static int RSA_verify_signature(const struct public_key *key,
 	kenter("");
 
 	/* (1) Check the signature size against the public key modulus size */
-	k = (mpi_get_nbits(key->rsa.n) + 7) / 8;
+	k = mpi_get_nbits(key->rsa.n);
+	tsize = mpi_get_nbits(sig->rsa.s);
 
-	tsize = (mpi_get_nbits(sig->rsa.s) + 7) / 8;
+	/* According to RFC 4880 sec 3.2, length of MPI is computed starting
+	 * from most significant bit.  So the RFC 3447 sec 8.2.2 size check
+	 * must be relaxed to conform with shorter signatures - so we fail here
+	 * only if signature length is longer than modulus size.
+	 */
 	pr_devel("step 1: k=%zu size(S)=%zu\n", k, tsize);
-	if (tsize != k) {
+	if (k < tsize) {
 		ret = -EBADMSG;
 		goto error;
 	}
 
+	/* Round up and convert to octets */
+	k = (k + 7) / 8;
+
 	/* (2b) Apply the RSAVP1 verification primitive to the public key */
 	ret = RSAVP1(key, sig->rsa.s, &m);
 	if (ret < 0)

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/