Re: [PATCH v3] sched/numa: fix unsafe get_task_struct() in task_numa_assign()

[Oleg Nesterov <oleg@redhat.com>]

On 10/22, Peter Zijlstra wrote:
>
> So I worry about cache aliasing (not an issue on x86), so by touching
> 'random' pages that might be freed and reissued to back userspace, we
> could be accessing the one page through multiple virtual mappings which
> therefore result in aliases.

Or this page can be vmalloc'ed. Yes, but we do not care. Although this
was one of the reasons why the 2nd version of xxx() checks ->sighand at
the end, even if this is not needed correctness-wise.

Let's look at the code again,

	struct task_struct *xxx(struct task_struct **ptask)
	{
		struct task_struct *task;
		struct sighand_struct *sighand;

	retry:
		task = ACCESS_ONCE(*ptask);
		if (!task)
			return task;

		if (IS_ENABLED(CONFIG_DEBUG_PAGEALLOC)) {
			if (probe_kernel_read(&sighand, &task->sighand, sizeof(sighand)))
				goto retry;
		} else {
			sighand = task->sighand;
		}

(this if/else should go into a separare helper)

		/*
		 * Pairs with atomic_dec_and_test() in put_task_struct(task).
		 * If we have read the freed/reused memory, we must see that
		 * the pointer was updated.
		 */
		smp_rmb();
		if (unlikely(task != ACCESS_ONCE(*ptask)))
			goto retry;

At this point we know that task_struct was not freed. Otherwise, since
this function assumes that "*ptask" must be cleared or updated before
the final put_task_struct(), we must notice that *ptask differs.

This means that we have read the correct value of ->sighand and the check
below is correct too. Even if ->sighand is not stable and can be already
NULL right after probe_kernel_read(), this doesn't matter.

And this also means that aliasing is not a problem. If it was freed we
could read the random value, but in this case we are not even going to
look at result.

		/*
		 * release_task(task) was already called; potentially before
		 * the caller took rcu_read_lock() and in this case it can be
		 * freed before rcu_read_unlock().
		 */
		if (!sighand)
			return NULL;
		return task;
	}

> SDBR avoids this issue by guaranteeing the page is not reissued for
> another purpose.

Yes, this is true.

> I'm not sure I can convince myself SLUB is correct here. How do we avoid
> cache aliasing.

Hmm. so perhaps I misunderstood your concern...

Do you mean that on !x86 a plain LOAD can "corrupt" the memory as it seen
from another vaddr?

If yes, this is another argument for a helper which reads the potentially
freed freed slab memory. get_freepointer_safe() can use it too and it can
be reimplemented in arch/xxx/include if necessary.

Or I missed your point completely?

Oleg.