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, 2 insertions, 2 deletions

diff --git a/src/lib/libcrypto/bn/bn_mpi.c b/src/lib/libcrypto/bn/bn_mpi.c
index 9ad28b96c9..e3b9ba0dd9 100644
--- a/src/lib/libcrypto/bn/bn_mpi.c
+++ b/src/lib/libcrypto/bn/bn_mpi.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: bn_mpi.c,v 1.11 2022/11/26 16:08:51 tb Exp $ */
+/* $OpenBSD: bn_mpi.c,v 1.12 2023/02/13 04:25:37 jsing Exp $ */
 /* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
  * All rights reserved.
  *
@@ -127,7 +127,7 @@ BN_mpi2bn(const unsigned char *d, int n, BIGNUM *ain)
                         BN_free(a);
                 return (NULL);
         }
-        a->neg = neg;
+        BN_set_negative(a, neg);
         if (neg) {
                 BN_clear_bit(a, BN_num_bits(a) - 1);
         }