Avoid -0 in BN_div_word().

Currently, the use of BN_div_word() can result in -0 - avoid this by
setting negative again, at the end of the computation.

Should fix oss-fuzz 56667.

ok tb@

diff --git a/lib/libcrypto/bn/bn_word.c b/lib/libcrypto/bn/bn_word.c
index c900f9e..68d5c2a 100644 (file)
--- a/lib/libcrypto/bn/bn_word.c
+++ b/lib/libcrypto/bn/bn_word.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: bn_word.c,v 1.19 2023/03/11 14:13:11 jsing Exp $ */
+/* $OpenBSD: bn_word.c,v 1.20 2023/03/11 14:14:54 jsing Exp $ */
 /* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
  * All rights reserved.
  *
@@ -131,6 +131,10 @@ BN_div_word(BIGNUM *a, BN_ULONG w)
        if ((a->top > 0) && (a->d[a->top - 1] == 0))
                a->top--;
        ret >>= j;
+
+       /* Set negative again, to handle -0 case. */
+       BN_set_negative(a, a->neg);
+
        return (ret);
 }