[PATCH 2/4] crypto: ecc - Fix off-by-one missing to clear most significant digit

[Jonas Rebmann]

From: Stefan Berger <stefanb at linux.ibm.com>

Fix an off-by-one error where the most significant digit was not
initialized leading to signature verification failures by the testmgr.

Example: If a curve requires ndigits (=9) and diff (=2) indicates that
2 digits need to be set to zero then start with digit 'ndigits - diff' (=7)
and clear 'diff' digits starting from there, so 7 and 8.

Reported-by: Venkat Rao Bagalkote <venkat88 at linux.vnet.ibm.com>
Closes: https://lore.kernel.org/linux-crypto/619bc2de-b18a-4939-a652-9ca886bf6349@linux.ibm.com/T/#m045d8812409ce233c17fcdb8b88b6629c671f9f4
Fixes: 2fd2a82ccbfc ("crypto: ecdsa - Use ecc_digits_from_bytes to create hash digits array")
Signed-off-by: Stefan Berger <stefanb at linux.ibm.com>
Tested-by: Venkat Rao Bagalkote <venkat88 at linux.vnet.ibm.com>
Signed-off-by: Herbert Xu <herbert at gondor.apana.org.au>
(cherry picked from linux commit 1dcf865d3bf5bff45e93cb2410911b3428dacb78)
Reported-by: Lukas Wunner <lukas at wunner.de>
Signed-off-by: Jonas Rebmann <jre at pengutronix.de>
---
 crypto/ecc.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/crypto/ecc.c b/crypto/ecc.c
index 77068ebfbd..01003d8a38 100644
--- a/crypto/ecc.c
+++ b/crypto/ecc.c
@@ -1,4 +1,4 @@
-// SPDX-Comment: Origin-URL: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/crypto/ecc.c?id=c6ab5c915da460c0397960af3c308386c3f3247b
+// SPDX-Comment: Origin-URL: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/crypto/ecc.c?id=1dcf865d3bf5bff45e93cb2410911b3428dacb78
 /*
  * Copyright (c) 2013, 2014 Kenneth MacKay. All rights reserved.
  * Copyright (c) 2019 Vitaly Chikunov <vt at altlinux.org>
@@ -66,7 +66,7 @@ void ecc_digits_from_bytes(const u8 *in, unsigned int nbytes,
 	/* diff > 0: not enough input bytes: set most significant digits to 0 */
 	if (diff > 0) {
 		ndigits -= diff;
-		memset(&out[ndigits - 1], 0, diff * sizeof(u64));
+		memset(&out[ndigits], 0, diff * sizeof(u64));
 	}
 
 	if (o) {

-- 
2.54.0.129.g3edf2eeba9