Re: [PATCH v6 8/9] kfence: add test suite

Jann Horn
2020-10-30T02:50:29Z

On Thu, Oct 29, 2020 at 2:17 PM Marco Elver <elver@google.com> 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.
[...]
> diff --git a/mm/kfence/kfence_test.c b/mm/kfence/kfence_test.c
[...]
> +static void *test_alloc(struct kunit *test, size_t size, gfp_t gfp, enum allocation_policy policy)
> +{
> + void *alloc;
> + unsigned long timeout, resched_after;
[...]
> + /*
> + * 100x the sample interval should be more than enough to ensure we get
> + * a KFENCE allocation eventually.
> + */
> + timeout = jiffies + msecs_to_jiffies(100 * CONFIG_KFENCE_SAMPLE_INTERVAL);
> + /*
> + * Especially for non-preemption kernels, ensure the allocation-gate
> + * timer has time to catch up.
> + */
> + resched_after = jiffies + msecs_to_jiffies(CONFIG_KFENCE_SAMPLE_INTERVAL);
> + do {
[...]
> + if (time_after(jiffies, resched_after))
> + cond_resched();

You probably meant to recalculate resched_after after the call to
cond_resched()?

> + } while (time_before(jiffies, timeout));
> +
> + KUNIT_ASSERT_TRUE_MSG(test, false, "failed to allocate from KFENCE");
> + return NULL; /* Unreachable. */
> +}
[...]
> +/*
> + * KFENCE is unable to detect an OOB if the allocation's alignment requirements
> + * leave a gap between the object and the guard page. Specifically, an
> + * allocation of e.g. 73 bytes is aligned on 8 and 128 bytes for SLUB or SLAB
> + * respectively. Therefore it is impossible for the allocated object to adhere
> + * to either of the page boundaries.

Should this be "to the left page boundary" instead of "to either of
the page boundaries"?

> + * However, we test that an access to memory beyond the gap result in KFENCE

*results

> + * detecting an OOB access.
> + */
> +static void test_kmalloc_aligned_oob_read(struct kunit *test)