Re: [PATCH v2 4/5] mm: introduce MADV_PAGEOUT

[Minchan Kim <minchan@kernel.org>]

On Thu, Jun 20, 2019 at 11:22:09AM +0200, Michal Hocko wrote:
> On Thu 20-06-19 17:40:40, Minchan Kim wrote:
> > > > > Pushing out a shared page cache
> > > > > is possible even now but this interface gives a much easier tool to
> > > > > evict shared state and perform all sorts of timing attacks. Unless I am
> > > > > missing something we should be doing something similar to mincore and
> > > > > ignore shared pages without a writeable access or at least document why
> > > > > we do not care.
> > > > 
> > > > I'm not sure IIUC side channel attach. As you mentioned, without this syscall,
> > > > 1. they already can do that simply by memory hogging
> > > 
> > > This is way much more harder for practical attacks because the reclaim
> > > logic is not fully under the attackers control. Having a direct tool to
> > > reclaim memory directly then just opens doors to measure the other
> > > consumers of that memory and all sorts of side channel.
> > 
> > Not sure it's much more harder. It's really easy on my experience.
> > Just creating new memory hogger and consume memory step by step until
> > you newly allocated pages will be reclaimed.
> 
> You can contain an untrusted application into a memcg and it will only
> reclaim its own working set.
> 
> > > > 2. If we need fix MADV_PAGEOUT, that means we need to fix MADV_DONTNEED, too?
> > > 
> > > nope because MADV_DONTNEED doesn't unmap from other processes.
> > 
> > Hmm, I don't understand. MADV_PAGEOUT doesn't unmap from other
> > processes, either.
> 
> Either I am confused or missing something. shrink_page_list does
> try_to_unmap and that unmaps from all processes, right?

You don't miss it. It seems now I undetstand what you pointed out.
What you meant is attacker can see what page was faulting-in from other processes
via measuring access delay from his address space and MADV_PAGEOUT makes it more
easiler. Thus, it's an issue regardless of recent mincore fix. Right?
Then, okay, I will add can_do_mincore similar check for the MADV_PAGEOUT syscall
if others have different ideas.

Thanks.