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Date:   Thu, 20 Jun 2019 11:22:09 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Minchan Kim <minchan@kernel.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
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        oleg@redhat.com, christian@brauner.io, oleksandr@redhat.com,
        hdanton@sina.com, lizeb@google.com
Subject: Re: [PATCH v2 4/5] mm: introduce MADV_PAGEOUT
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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?

> Could you elborate it a bit more what's your concern?

If you manage to unmap from a remote process then you can measure delays
implied from the refault and that information can be used to infer what
the remote application is doing.
-- 
Michal Hocko
SUSE Labs