[PATCH] ecdsa: Fix incorrect usage of vli_cmp

[Stefan Berger <stefanb@linux.ibm.com>]

Fix incorrect usage of vli_cmp when calculating the value of res.x. For
signature verification to succeed, res.x must be the same as the r
component of the signature which is in the range of [1..n-1] with 'n'
being the order of the curve. Therefore, when res.x equals n calculate
res.x = res.x - n as well. Signature verification could have previously
unnecessarily failed in extremely rare cases.

Fixes: 4e6602916bc6 ("crypto: ecdsa - Add support for ECDSA signature verification")
Cc: <stable@vger.kernel.org>
Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
---
 crypto/ecdsa.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/crypto/ecdsa.c b/crypto/ecdsa.c
index b3a8a6b572ba..674ab9275366 100644
--- a/crypto/ecdsa.c
+++ b/crypto/ecdsa.c
@@ -120,8 +120,8 @@ static int _ecdsa_verify(struct ecc_ctx *ctx, const u64 *hash, const u64 *r, con
 	/* res = u1*G + u2 * pub_key */
 	ecc_point_mult_shamir(&res, u1, &curve->g, u2, &ctx->pub_key, curve);
 
-	/* res.x = res.x mod n (if res.x > order) */
-	if (unlikely(vli_cmp(res.x, curve->n, ndigits) == 1))
+	/* res.x = res.x mod n (if res.x >= order) */
+	if (unlikely(vli_cmp(res.x, curve->n, ndigits) >= 0))
 		/* faster alternative for NIST p384, p256 & p192 */
 		vli_sub(res.x, res.x, curve->n, ndigits);
 
-- 
2.34.1