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.
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