Re: [PATCH] mm: fix use-after free of page_ext after race with memory-offline

[Michal Hocko <mhocko@suse.com>]

On Mon 18-07-22 19:28:13, Charan Teja Kalla wrote:
> Thanks Michal for the comments!!
> 
> On 7/18/2022 5:20 PM, Michal Hocko wrote:
> >> The above mentioned race is just one example __but the problem persists
> >> in the other paths too involving page_ext->flags access(eg:
> >> page_is_idle())__. Since offline waits till the last reference on the
> >> page goes down i.e. any path that took the refcount on the page can make
> >> the memory offline operation to wait. Eg: In the migrate_pages()
> >> operation, we do take the extra refcount on the pages that are under
> >> migration and then we do copy page_owner by accessing page_ext. For
> >>
> >> Fix those paths where offline races with page_ext access by maintaining
> >> synchronization with rcu lock.
> > Please be much more specific about the synchronization. How does RCU
> > actually synchronize the offlining and access? Higher level description
> > of all the actors would be very helpful not only for the review but also
> > for future readers.
> 
> I will improve the commit message about this synchronization change
> using RCU's.

Thanks! The most imporant part is how the exclusion is actual achieved
because that is not really clear at first sight

CPU1					CPU2
lookup_page_ext(PageA)			offlining
					  offline_page_ext
					    __free_page_ext(addrA)
					      get_entry(addrA)
					      ms->page_ext = NULL
					      synchronize_rcu()
					      free_page_ext
					        free_pages_exact (now addrA is unusable)
					
  rcu_read_lock()
  entryA = get_entry(addrA)
    base + page_ext_size * index # an address not invalidated by the freeing path
  do_something(entryA)
  rcu_read_unlock()

CPU1 never checks ms->page_ext so it cannot bail out early when the
thing is torn down. Or maybe I am missing something. I am not familiar
with page_ext much.

> > Also, more specifically
> > [...]
> >> diff --git a/mm/page_ext.c b/mm/page_ext.c
> >> index 3dc715d..5ccd3ee 100644
> >> --- a/mm/page_ext.c
> >> +++ b/mm/page_ext.c
> >> @@ -299,8 +299,9 @@ static void __free_page_ext(unsigned long pfn)
> >>  	if (!ms || !ms->page_ext)
> >>  		return;
> >>  	base = get_entry(ms->page_ext, pfn);
> >> -	free_page_ext(base);
> >>  	ms->page_ext = NULL;
> >> +	synchronize_rcu();
> >> +	free_page_ext(base);
> >>  }
> > So you are imposing the RCU grace period for each page_ext! This can get
> > really expensive. Have you tried to measure the effect?

I was wrong here! This is for each memory section which is not as
terrible as every single page_ext. This can be still quite a lot memory
sections in a single memory block (e.g. on ppc memory sections are
ridiculously small).

> I didn't really measure the effect. Let me measure it and post these in V2.

I think it would be much more optimal to split the operation into 2
phases. Invalidate all the page_ext metadata then synchronize_rcu and
only then free them all. I am not very familiar with page_ext so I am
not sure this is easy to be done. Maybe page_ext = NULL can be done in
the first stage.

> > Is there any reason why page_ext is freed during offlining rather when
> > it is hotremoved?
> 
> This is something I am struggling to get the answer. IMO, this is even
> wrong design where I don't have page_ext but page. Moving the freeing of
> page_ext to hotremove path actually solves the problem but somehow this
> idea didn't liked[1].  copying the excerpt here:

yes, it certainly adds subtlety to the page_ext thingy. I do agree that
even situation around struct page is not all that great wrt
synchronization. We have pfn_to_online_page which even when racy doesn't
give you a garbage because hotremove happens very rarely or so long
after offlining that the race window is essentially impractically too
long for any potential damage. We would have to change a lot to make it
work "properly". I am not optimistic this is actually feasible.

> > 3) Change the design where the page_ext is valid as long as the struct
> > page is alive.
> 
> :/ Doesn't spark joy."

I would be wondering why. It should only take to move the callback to
happen at hotremove. So it shouldn't be very involved of a change. I can
imagine somebody would be relying on releasing resources when offlining
memory but is that really the case?

-- 
Michal Hocko
SUSE Labs