Fix a -0 corner case in BN_div_internal()

If the numerator is negative, the numerator and divisor are the same
length (in words) and the absolute value of the divisor > the absolute
value of the numerator, the "no_branch" case produces -0 since negative
has already been set. Call BN_set_negative() at the end of the function
to avoid this.

ok tb@

diff --git a/lib/libcrypto/bn/bn_div.c b/lib/libcrypto/bn/bn_div.c
index 7ada277..686b957 100644 (file)
--- a/lib/libcrypto/bn/bn_div.c
+++ b/lib/libcrypto/bn/bn_div.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: bn_div.c,v 1.37 2023/02/13 04:25:37 jsing Exp $ */
+/* $OpenBSD: bn_div.c,v 1.38 2023/02/14 18:19:27 jsing Exp $ */
 /* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
  * All rights reserved.
  *
@@ -237,7 +237,7 @@ static int
 BN_div_internal(BIGNUM *quotient, BIGNUM *remainder, const BIGNUM *numerator,
     const BIGNUM *divisor, BN_CTX *ctx, int ct)
 {
-       int norm_shift, i, loop;
+       int norm_shift, i, loop, r_neg;
        BIGNUM *tmp, wnum, *snum, *sdiv, *res;
        BN_ULONG *resp, *wnump;
        BN_ULONG d0, d1;
@@ -341,7 +341,7 @@ BN_div_internal(BIGNUM *quotient, BIGNUM *remainder, const BIGNUM *numerator,
        if (!bn_wexpand(res, (loop + 1)))
                goto err;
        res->top = loop - no_branch;
-       BN_set_negative(res, numerator->neg ^ divisor->neg);
+       r_neg = numerator->neg ^ divisor->neg;
        resp = &(res->d[loop - 1]);
 
        /* space for temp */
@@ -420,6 +420,8 @@ BN_div_internal(BIGNUM *quotient, BIGNUM *remainder, const BIGNUM *numerator,
        if (no_branch)
                bn_correct_top(res);
 
+       BN_set_negative(res, r_neg);
+
  done:
        ret = 1;
  err: