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, 7 insertions, 4 deletions

diff --git a/src/lib/libcrypto/bn/bn_print.c b/src/lib/libcrypto/bn/bn_print.c
index 7251e4d831..7e0683b679 100644
--- a/src/lib/libcrypto/bn/bn_print.c
+++ b/src/lib/libcrypto/bn/bn_print.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: bn_print.c,v 1.37 2022/11/26 16:08:51 tb Exp $ */
+/* $OpenBSD: bn_print.c,v 1.38 2023/02/13 04:25:37 jsing Exp $ */
 /* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
  * All rights reserved.
  *
@@ -251,7 +251,8 @@ BN_hex2bn(BIGNUM **bn, const char *a)
         }
         ret->top = h;
         bn_correct_top(ret);
-        ret->neg = neg;
+
+        BN_set_negative(ret, neg);
 
         *bn = ret;
         return (num);
@@ -317,9 +318,11 @@ BN_dec2bn(BIGNUM **bn, const char *a)
                         j = 0;
                 }
         }
-        ret->neg = neg;
 
         bn_correct_top(ret);
+
+        BN_set_negative(ret, neg);
+
         *bn = ret;
         return (num);
 
@@ -344,7 +347,7 @@ BN_asc2bn(BIGNUM **bn, const char *a)
                         return 0;
         }
         if (*a == '-')
-                (*bn)->neg = 1;
+                BN_set_negative(*bn, 1);
         return 1;
 }