Date: Wed, 22 Jul 2015 21:59:39 -1000 (HST) From: Jeff Roberson <jroberson@jroberson.net> To: Mark R V Murray <markm@FreeBSD.org> Cc: Warner Losh <imp@bsdimp.com>, src-committers <src-committers@freebsd.org>, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r284959 - in head: . share/man/man4 share/man/man9 sys/conf sys/dev/glxsb sys/dev/hifn sys/dev/random sys/dev/rndtest sys/dev/safe sys/dev/syscons sys/dev/ubsec sys/dev/virtio/random sy... Message-ID: <alpine.BSF.2.20.1507222157170.1071@desktop> In-Reply-To: <E20B169F-4C8A-4D11-9853-5C2EFC116450@FreeBSD.org> References: <201506301700.t5UH0jPq001498@svn.freebsd.org> <alpine.BSF.2.20.1507221138360.1071@desktop> <FFAED695-145A-45F5-988D-B843EF5F544B@FreeBSD.org> <alpine.BSF.2.20.1507221249120.1071@desktop> <FFFB06D7-164B-40B3-AFC3-A6630BCF074E@bsdimp.com> <E20B169F-4C8A-4D11-9853-5C2EFC116450@FreeBSD.org>
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On Thu, 23 Jul 2015, Mark R V Murray wrote: > >> On 23 Jul 2015, at 00:53, Warner Losh <imp@bsdimp.com> wrote: >> >>>>> Neither filesystem operations nor allocations are random events. They are trivially influenced by user code. A malicious attacker could create repeated patterns of allocations or filesystem activity through the syscall path to degrade your random sample source. >>>> >>>> I?m not sure I accept that - Fortuna is very careful about using non-reversible hashing in it?s accumulation, and countering such degradation is one of the algorithm?s strong points. There is perhaps risk of *no* entropy, but even the per-event timing jitter will be providing this, if nothing else. >> >> I?m not sure I?m happy about this answer. Do you have some research backing up such cavalier claims? > > It was not my intention to sound cavalier. Apologies. > > Fortuna was developed to account for many sources of entropy, good and bad alike, and Jeff?s observation is an attack on that design. I accept that the randomness of these events is poor, but they are high-rate, and this product of high-rate*low entropy is what I seek. I pulled out numbers with dtrace, and basic statistics showed that the harvesting was not useless. I completely understand that under the right circumstances these numbers might be lousy - please read the Fortuna design document to understand why this doesn?t matter. *ALL* entropy inputs to Fortuna are considered attackable, including the dedicated hardware sources. > > I have also read cryptanalyses of Fortuna, not all of them to be sure, and so far the design appears strong. The best attack that I have seen (very academic) suggests an improvement which I may incorporate. > >>>>> Perhaps more importantly to me, this is an unacceptable performance burden for the allocator. At a minimum it should compile out by default. Great care has been taken to reduce the fast path of the allocator to the minimum number of cycles and even cache misses. >>>> >>>> As currently set up in etc/rc.d/* by default, there is a simple check at each UMA harvesting opportunity, and no further action. I asked Robert Watson if this was burdensome, and he said it was not. >>> >>> I find this burdensome. You can easily add a macro around the calls or hide them in an inline with a default to off. Even a function call that checks a global and does nothing else is a handful of new cache misses. A microbenchmark will not realize the full cost of this. You will instead get the dozen or so instructions of overhead which I still find objectionable. >>> >>> Kip's observations about packet cycle budgets in high-performance applications are accurate and this is something we have put great care into over time. >> >> A certain video streaming company will be pushing the envelope to get to 100Gbps very soon. Even a few extra instructions on every packet / allocation will be a killer. Especially if one is an almost guaranteed cache miss. This most certainly will be burdensome. There absolutely must be a way to turn this off at compile time. We don?t care that much about entropy to leave performance on the table. > > OK - I?m sold! I?ll make a kernel option defaulting to off. :-) There are other sources that occur less frequently than millions of times per-second that may still provide some usefull entropy while being less performance critical under normal conditions. For example, context switches, traps, etc. I could also imagine wiring up a pmc counter to something like cache misses or branch mispredicts that would be more difficult to game, especially if the counter was cycled irregularly. Thanks, Jeff > > M > -- > Mark R V Murray > >
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