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From 3118eb64934499d93db3230748a452351d1d9a65 Mon Sep 17 00:00:00 2001
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From: Tomas Mraz <tomas@openssl.org>
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Date: Mon, 28 Feb 2022 18:26:21 +0100
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Subject: [PATCH] Fix possible infinite loop in BN_mod_sqrt()
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The calculation in some cases does not finish for non-prime p.
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This fixes CVE-2022-0778.
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Based on patch by David Benjamin <davidben@google.com>.
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Reviewed-by: Paul Dale <pauli@openssl.org>
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Reviewed-by: Matt Caswell <matt@openssl.org>
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---
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crypto/bn/bn_sqrt.c | 30 ++++++++++++++++++------------
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1 file changed, 18 insertions(+), 12 deletions(-)
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diff --git a/crypto/bn/bn_sqrt.c b/crypto/bn/bn_sqrt.c
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index 1723d5ded5a8..53b0f559855c 100644
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--- a/crypto/bn/bn_sqrt.c
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+++ b/crypto/bn/bn_sqrt.c
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@@ -14,7 +14,8 @@ BIGNUM *BN_mod_sqrt(BIGNUM *in, const BIGNUM *a, const BIGNUM *p, BN_CTX *ctx)
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/*
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* Returns 'ret' such that ret^2 == a (mod p), using the Tonelli/Shanks
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* algorithm (cf. Henri Cohen, "A Course in Algebraic Computational Number
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- * Theory", algorithm 1.5.1). 'p' must be prime!
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+ * Theory", algorithm 1.5.1). 'p' must be prime, otherwise an error or
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+ * an incorrect "result" will be returned.
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*/
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{
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BIGNUM *ret = in;
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@@ -301,18 +302,23 @@ BIGNUM *BN_mod_sqrt(BIGNUM *in, const BIGNUM *a, const BIGNUM *p, BN_CTX *ctx)
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goto vrfy;
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}
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- /* find smallest i such that b^(2^i) = 1 */
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- i = 1;
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- if (!BN_mod_sqr(t, b, p, ctx))
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- goto end;
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- while (!BN_is_one(t)) {
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- i++;
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- if (i == e) {
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- BNerr(BN_F_BN_MOD_SQRT, BN_R_NOT_A_SQUARE);
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- goto end;
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+ /* Find the smallest i, 0 < i < e, such that b^(2^i) = 1. */
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+ for (i = 1; i < e; i++) {
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+ if (i == 1) {
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+ if (!BN_mod_sqr(t, b, p, ctx))
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+ goto end;
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+
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+ } else {
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+ if (!BN_mod_mul(t, t, t, p, ctx))
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+ goto end;
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}
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- if (!BN_mod_mul(t, t, t, p, ctx))
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- goto end;
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+ if (BN_is_one(t))
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+ break;
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+ }
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+ /* If not found, a is not a square or p is not prime. */
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+ if (i >= e) {
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+ BNerr(BN_F_BN_MOD_SQRT, BN_R_NOT_A_SQUARE);
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+ goto end;
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}
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/* t := y^2^(e - i - 1) */
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