Add ossl_bn_is_word_fixed_top()

Also correct some BN_FLG_FIXED_TOP flag handling.

Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Neil Horman <nhorman@openssl.org>

(cherry picked from commit 2d285fa873)

(Merged from https://github.com/openssl/openssl/pull/24317)

crypto/bn/bn_lib.c

@@ -769,6 +769,7 @@ int ossl_bn_mask_bits_fixed_top(BIGNUM *a, int n)
         a->top = w + 1;
         a->d[w] &= ~(BN_MASK2 << b);
     }
+    a->flags |= BN_FLG_FIXED_TOP;
     return 1;
 }
 
@@ -956,6 +957,22 @@ int BN_is_word(const BIGNUM *a, const BN_ULONG w)
     return BN_abs_is_word(a, w) && (!w || !a->neg);
 }
 
+int ossl_bn_is_word_fixed_top(const BIGNUM *a, BN_ULONG w)
+{
+    int res, i;
+    const BN_ULONG *ap = a->d;
+
+    if (a->neg || a->top == 0)
+        return 0;
+
+    res = constant_time_select_int(constant_time_eq_bn(ap[0], w), 1, 0);
+
+    for (i = 1; i < a->top; i++)
+        res = constant_time_select_int(constant_time_is_zero_bn(ap[i]),
+                                       res, 0);
+    return res;
+}
+
 int BN_is_odd(const BIGNUM *a)
 {
     return (a->top > 0) && (a->d[0] & 1);

crypto/bn/bn_local.h

@@ -676,6 +676,5 @@ static ossl_inline BIGNUM *bn_expand(BIGNUM *a, int bits)
 
 int ossl_bn_check_prime(const BIGNUM *w, int checks, BN_CTX *ctx,
                         int do_trial_division, BN_GENCB *cb);
-int ossl_bn_mask_bits_fixed_top(BIGNUM *a, int n);
 
 #endif

crypto/bn/bn_rand.c

@@ -322,7 +322,7 @@ int BN_generate_dsa_nonce(BIGNUM *out, const BIGNUM *range,
             goto end;
 
         /* Clear out the top bits and rejection filter into range */
-        BN_set_flags(out, BN_FLG_CONSTTIME | BN_FLG_FIXED_TOP);
+        BN_set_flags(out, BN_FLG_CONSTTIME);
         ossl_bn_mask_bits_fixed_top(out, BN_num_bits(range));
 
         if (BN_ucmp(out, range) < 0) {

crypto/bn/bn_shift.c

@@ -156,6 +156,9 @@ int BN_rshift(BIGNUM *r, const BIGNUM *a, int n)
         return 0;
     }
 
+    bn_check_top(r);
+    bn_check_top(a);
+
     ret = bn_rshift_fixed_top(r, a, n);
 
     bn_correct_top(r);
@@ -177,9 +180,6 @@ int bn_rshift_fixed_top(BIGNUM *r, const BIGNUM *a, int n)
     BN_ULONG *t, *f;
     BN_ULONG l, m, mask;
 
-    bn_check_top(r);
-    bn_check_top(a);
-
     assert(n >= 0);
 
     nw = n / BN_BITS2;

include/crypto/bn.h

@@ -87,6 +87,8 @@ int bn_lshift_fixed_top(BIGNUM *r, const BIGNUM *a, int n);
 int bn_rshift_fixed_top(BIGNUM *r, const BIGNUM *a, int n);
 int bn_div_fixed_top(BIGNUM *dv, BIGNUM *rem, const BIGNUM *m,
                      const BIGNUM *d, BN_CTX *ctx);
+int ossl_bn_mask_bits_fixed_top(BIGNUM *a, int n);
+int ossl_bn_is_word_fixed_top(const BIGNUM *a, BN_ULONG w);
 
 #define BN_PRIMETEST_COMPOSITE                    0
 #define BN_PRIMETEST_COMPOSITE_WITH_FACTOR        1

include/internal/constant_time.h

@@ -150,6 +150,17 @@ static ossl_inline BN_ULONG constant_time_lt_bn(BN_ULONG a, BN_ULONG b)
 {
     return constant_time_msb_bn(a ^ ((a ^ b) | ((a - b) ^ b)));
 }
+
+static ossl_inline BN_ULONG constant_time_is_zero_bn(BN_ULONG a)
+{
+    return constant_time_msb_bn(~a & (a - 1));
+}
+
+static ossl_inline BN_ULONG constant_time_eq_bn(BN_ULONG a,
+                                                BN_ULONG b)
+{
+    return constant_time_is_zero_bn(a ^ b);
+}
 #endif
 
 static ossl_inline unsigned int constant_time_ge(unsigned int a,