Fix Excessive time spent checking DH q parameter value (#1121)

aws · Aug 1, 2023 · 1bb574f · 1bb574f
1 parent a818ddf
commit 1bb574f
Show file tree

Hide file tree

Showing 2 changed files with 30 additions and 1 deletion.
diff --git a/crypto/dh_extra/dh_test.cc b/crypto/dh_extra/dh_test.cc
@@ -140,6 +140,27 @@ TEST(DHTest, OversizedModulus) {
   ASSERT_EQ(DH_R_MODULUS_TOO_LARGE, ERR_GET_REASON(error));
 }
 
+TEST(DHTest, LargeQ) {
+  bssl::UniquePtr<DH> a(DH_new());
+  ASSERT_TRUE(a);
+  ASSERT_TRUE(DH_generate_parameters_ex(a.get(), 64, DH_GENERATOR_5, nullptr));
+
+  bssl::UniquePtr<BIGNUM> q(BN_new());
+  ASSERT_TRUE(q);
+  BN_set_word(q.get(), 2039L);
+
+  a.get()->q = q.release();
+
+  ASSERT_TRUE(DH_generate_key(a.get()));
+
+  ASSERT_TRUE(BN_copy(a.get()->q, a.get()->p));
+  ASSERT_TRUE(BN_add(a.get()->q, a.get()->q, BN_value_one()));
+
+  int check_result;
+  ASSERT_TRUE(DH_check(a.get(), &check_result));
+  ASSERT_TRUE(check_result & DH_CHECK_INVALID_Q_VALUE);
+}
+
 // The following parameters are taken from RFC 5114, section 2.2. This is not a
 // safe prime. Do not use these parameters.
 static const uint8_t kRFC5114_2048_224P[] = {

diff --git a/crypto/fipsmodule/dh/check.c b/crypto/fipsmodule/dh/check.c
@@ -120,7 +120,7 @@ int DH_check(const DH *dh, int *out_flags) {
   //   for 3, p mod 12 == 5
   //   for 5, p mod 10 == 3 or 7
   // should hold.
-  int ok = 0, r;
+  int ok = 0, r, q_good = 0;
   BN_CTX *ctx = NULL;
   BN_ULONG l;
   BIGNUM *t1 = NULL, *t2 = NULL;
@@ -148,6 +148,14 @@ int DH_check(const DH *dh, int *out_flags) {
   }
 
   if (dh->q) {
+    if (BN_ucmp(dh->p, dh->q) > 0) {
+      q_good = 1;
+    } else {
+      *out_flags |= DH_CHECK_INVALID_Q_VALUE;
+    }
+  }
+
+  if (q_good) {
     if (BN_cmp(dh->g, BN_value_one()) <= 0) {
       *out_flags |= DH_CHECK_NOT_SUITABLE_GENERATOR;
     } else if (BN_cmp(dh->g, dh->p) >= 0) {