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[195.135.220.15]) by mx.google.com with ESMTPS id k6si2364654edd.269.2019.01.31.04.06.59 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 31 Jan 2019 04:06:59 -0800 (PST) Received-SPF: pass (google.com: domain of vbabka@suse.cz designates 195.135.220.15 as permitted sender) client-ip=195.135.220.15; Authentication-Results: mx.google.com; spf=pass (google.com: domain of vbabka@suse.cz designates 195.135.220.15 as permitted sender) smtp.mailfrom=vbabka@suse.cz X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 327A4AC4B; Thu, 31 Jan 2019 12:06:59 +0000 (UTC) Subject: Re: [PATCH 2/3] mm/filemap: initiate readahead even if IOCB_NOWAIT is set for the I/O To: Daniel Gruss , Andrew Morton , Linus Torvalds Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-api@vger.kernel.org, Peter Zijlstra , Greg KH , Jann Horn , Jiri Kosina , Dominique Martinet , Andy Lutomirski , Dave Chinner , Kevin Easton , Matthew Wilcox , Cyril Hrubis , Tejun Heo , "Kirill A . Shutemov" , Jiri Kosina References: <20190130124420.1834-1-vbabka@suse.cz> <20190130124420.1834-3-vbabka@suse.cz> From: Vlastimil Babka Openpgp: preference=signencrypt Message-ID: Date: Thu, 31 Jan 2019 13:06:58 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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 >