Avoid negative zero.

Whenever setting negative to one (or when it could potentially be one),
always use BN_set_negative() since it checks for a zero valued bignum and
will not permit negative to be set in this case. Since BN_is_zero()
currently relies on top == 0, call BN_set_negative() after top has been
set (or bn_correct_top() has been called).

This fixes a long standing issue where -0 and +0 have been permitted,
however multiple code paths (such as BN_cmp()) fail to treat these as
equivalent.

Prompted by Guido Vranken who is adding negative zero fuzzing to oss-fuzz.

ok tb@

1 files changed, 3 insertions, 3 deletions

diff --git a/src/lib/libcrypto/bn/bn_mul.c b/src/lib/libcrypto/bn/bn_mul.c
index bd679108db..38c01dad18 100644
--- a/src/lib/libcrypto/bn/bn_mul.c
+++ b/src/lib/libcrypto/bn/bn_mul.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: bn_mul.c,v 1.30 2023/01/23 12:17:57 jsing Exp $ */
+/* $OpenBSD: bn_mul.c,v 1.31 2023/02/13 04:25:37 jsing Exp $ */
 /* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
  * All rights reserved.
  *
@@ -1003,10 +1003,10 @@ BN_mul(BIGNUM *r, const BIGNUM *a, const BIGNUM *b, BN_CTX *ctx)
         }
 
         rr->top = rn;
-        rr->neg = a->neg ^ b->neg;
-
         bn_correct_top(rr);
 
+        BN_set_negative(rr, a->neg ^ b->neg);
+
         if (r != rr)
                 BN_copy(r, rr);
  done: