From: Tomas Mraz <tomas@openssl.foundation>
Date: Fri, 10 Jul 2026 09:09:09 +0000 (+0200)
Subject: Avoid undefined behavior adding or subtracting two BN_zero() values
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=1768a5a1be8895f3cf54d418093c337ea1f3238d;p=thirdparty%2Fopenssl.git

Avoid undefined behavior adding or subtracting two BN_zero() values

Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: Nikola Pajkovsky <nikolap@openssl.org>
MergeDate: Fri Jul 10 14:54:48 2026
(Merged from https://github.com/openssl/openssl/pull/31916)
---

diff --git a/crypto/bn/bn_add.c b/crypto/bn/bn_add.c
index 88d77c534f0..24151d0e13d 100644
--- a/crypto/bn/bn_add.c
+++ b/crypto/bn/bn_add.c
@@ -97,6 +97,8 @@ int BN_uadd(BIGNUM *r, const BIGNUM *a, const BIGNUM *b)
         return 0;
 
     r->top = max;
+    if (max == 0)
+        goto end;
 
     ap = a->d;
     bp = b->d;
@@ -116,6 +118,7 @@ int BN_uadd(BIGNUM *r, const BIGNUM *a, const BIGNUM *b)
     *rp = carry;
     r->top += (int)carry;
 
+end:
     r->neg = 0;
     bn_check_top(r);
     return 1;
@@ -143,6 +146,9 @@ int BN_usub(BIGNUM *r, const BIGNUM *a, const BIGNUM *b)
     if (bn_wexpand(r, max) == NULL)
         return 0;
 
+    if (max == 0)
+        goto end;
+
     ap = a->d;
     bp = b->d;
     rp = r->d;
@@ -162,6 +168,7 @@ int BN_usub(BIGNUM *r, const BIGNUM *a, const BIGNUM *b)
     while (max && *--rp == 0)
         max--;
 
+end:
     r->top = max;
     r->neg = 0;
     bn_pollute(r);