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:

quoted

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