linux-kernel - Re: [PATCH 2/3] mm/filemap: initiate readahead even if IOCB

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Date:   Thu, 31 Jan 2019 13:06:58 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Daniel Gruss <daniel@...ss.cc>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-api@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Jann Horn <jannh@...gle.com>, Jiri Kosina <jkosina@...e.cz>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Chinner <david@...morbit.com>,
        Kevin Easton <kevin@...rana.org>,
        Matthew Wilcox <willy@...radead.org>,
        Cyril Hrubis <chrubis@...e.cz>, Tejun Heo <tj@...nel.org>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Jiri Kosina <jikos@...nel.org>
Subject: Re: [PATCH 2/3] mm/filemap: initiate readahead even if IOCB_NOWAIT is
 set for the I/O

On 1/31/19 1:04 PM, Daniel Gruss wrote:
> On 1/30/19 1:44 PM, Vlastimil Babka wrote:
>> Close that sidechannel by always initiating readahead on the cache if we
>> encounter a cache miss for preadv2(RWF_NOWAIT); with that in place, probing
>> the pagecache residency itself will actually populate the cache, making the
>> sidechannel useless.
> 
> I fear this does not really close the side channel. You can time the
> preadv2 function and infer which path it took, so you just bring it down
> to the same as using mmap and timing accesses.
> If I understood it correctly, this patch just removes the advantages of
> preadv2 over mmmap+access for the attacker.

But isn't that the same with mincore()? We can't simply remove the
possibility of mmap+access, but we are closing the simpler methods?

Vlastimil


> Cheers,
> Daniel
>