Re: [PATCH v3 10/10] kfence: add test suite

["Paul E. McKenney" <paulmck@kernel.org>]

On Mon, Sep 21, 2020 at 03:26:11PM +0200, Marco Elver wrote:
> Add KFENCE test suite, testing various error detection scenarios. Makes
> use of KUnit for test organization. Since KFENCE's interface to obtain
> error reports is via the console, the test verifies that KFENCE outputs
> expected reports to the console.
> 
> Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
> Co-developed-by: Alexander Potapenko <glider@google.com>
> Signed-off-by: Alexander Potapenko <glider@google.com>
> Signed-off-by: Marco Elver <elver@google.com>

[ . . . ]

> +/* Test SLAB_TYPESAFE_BY_RCU works. */
> +static void test_memcache_typesafe_by_rcu(struct kunit *test)
> +{
> +	const size_t size = 32;
> +	struct expect_report expect = {
> +		.type = KFENCE_ERROR_UAF,
> +		.fn = test_memcache_typesafe_by_rcu,
> +	};
> +
> +	setup_test_cache(test, size, SLAB_TYPESAFE_BY_RCU, NULL);
> +	KUNIT_EXPECT_TRUE(test, test_cache); /* Want memcache. */
> +
> +	expect.addr = test_alloc(test, size, GFP_KERNEL, ALLOCATE_ANY);
> +	*expect.addr = 42;
> +
> +	rcu_read_lock();
> +	test_free(expect.addr);
> +	KUNIT_EXPECT_EQ(test, *expect.addr, (char)42);
> +	rcu_read_unlock();

It won't happen very often, but memory really could be freed at this point,
especially in CONFIG_RCU_STRICT_GRACE_PERIOD=y kernels ...

> +	/* No reports yet, memory should not have been freed on access. */
> +	KUNIT_EXPECT_FALSE(test, report_available());

... so the above statement needs to go before the rcu_read_unlock().

> +	rcu_barrier(); /* Wait for free to happen. */

But you are quite right that the memory is not -guaranteed- to be freed
until we get here.

							Thanx, Paul