+static int test_ctx_set_ct_flag(BN_CTX *c)
+{
+ int st = 0;
+ size_t i;
+ BIGNUM *b[15];
+
+ BN_CTX_start(c);
+ for (i = 0; i < OSSL_NELEM(b); i++) {
+ if (!TEST_ptr(b[i] = BN_CTX_get(c)))
+ goto err;
+ if (i % 2 == 1)
+ BN_set_flags(b[i], BN_FLG_CONSTTIME);
+ }
+
+ st = 1;
+ err:
+ BN_CTX_end(c);
+ return st;
+}
+
+static int test_ctx_check_ct_flag(BN_CTX *c)
+{
+ int st = 0;
+ size_t i;
+ BIGNUM *b[30];
+
+ BN_CTX_start(c);
+ for (i = 0; i < OSSL_NELEM(b); i++) {
+ if (!TEST_ptr(b[i] = BN_CTX_get(c)))
+ goto err;
+ if (!TEST_false(BN_get_flags(b[i], BN_FLG_CONSTTIME)))
+ goto err;
+ }
+
+ st = 1;
+ err:
+ BN_CTX_end(c);
+ return st;
+}
+
+static int test_ctx_consttime_flag(void)
+{
+ /*-
+ * The constant-time flag should not "leak" among BN_CTX frames:
+ *
+ * - test_ctx_set_ct_flag() starts a frame in the given BN_CTX and
+ * sets the BN_FLG_CONSTTIME flag on some of the BIGNUMs obtained
+ * from the frame before ending it.
+ * - test_ctx_check_ct_flag() then starts a new frame and gets a
+ * number of BIGNUMs from it. In absence of leaks, none of the
+ * BIGNUMs in the new frame should have BN_FLG_CONSTTIME set.
+ *
+ * In actual BN_CTX usage inside libcrypto the leak could happen at
+ * any depth level in the BN_CTX stack, with varying results
+ * depending on the patterns of sibling trees of nested function
+ * calls sharing the same BN_CTX object, and the effect of
+ * unintended BN_FLG_CONSTTIME on the called BN_* functions.
+ *
+ * This simple unit test abstracts away this complexity and verifies
+ * that the leak does not happen between two sibling functions
+ * sharing the same BN_CTX object at the same level of nesting.
+ *
+ */
+ BN_CTX *nctx = NULL;
+ BN_CTX *sctx = NULL;
+ size_t i = 0;
+ int st = 0;
+
+ if (!TEST_ptr(nctx = BN_CTX_new())
+ || !TEST_ptr(sctx = BN_CTX_secure_new()))
+ goto err;
+
+ for (i = 0; i < 2; i++) {
+ BN_CTX *c = i == 0 ? nctx : sctx;
+ if (!TEST_true(test_ctx_set_ct_flag(c))
+ || !TEST_true(test_ctx_check_ct_flag(c)))
+ goto err;
+ }
+
+ st = 1;
+ err:
+ BN_CTX_free(nctx);
+ BN_CTX_free(sctx);
+ return st;
+}
+