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, 5 insertions, 3 deletions

diff --git a/src/lib/libcrypto/bn/bn_add.c b/src/lib/libcrypto/bn/bn_add.c
index dc525db084..8ae9bbe854 100644
--- a/src/lib/libcrypto/bn/bn_add.c
+++ b/src/lib/libcrypto/bn/bn_add.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: bn_add.c,v 1.21 2023/02/02 18:39:26 jsing Exp $ */
+/* $OpenBSD: bn_add.c,v 1.22 2023/02/13 04:25:37 jsing Exp $ */
 /* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
  * All rights reserved.
  *
@@ -381,7 +381,8 @@ BN_add(BIGNUM *r, const BIGNUM *a, const BIGNUM *b)
                 }
         }
 
-        r->neg = r_neg;
+        BN_set_negative(r, r_neg);
+
         return ret;
 }
 
@@ -409,6 +410,7 @@ BN_sub(BIGNUM *r, const BIGNUM *a, const BIGNUM *b)
                 }
         }
 
-        r->neg = r_neg;
+        BN_set_negative(r, r_neg);
+
         return ret;
 }