Re: [PATCH 2/3] mm/filemap: initiate readahead even if IOCB_NOWAIT is set for the I/O

[Vlastimil Babka]

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
>