Date: Thu, 23 Jul 2015 08:03:49 +0100 From: Mark R V Murray <markm@FreeBSD.org> To: Warner Losh <imp@bsdimp.com> Cc: 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: <E20B169F-4C8A-4D11-9853-5C2EFC116450@FreeBSD.org> In-Reply-To: <FFFB06D7-164B-40B3-AFC3-A6630BCF074E@bsdimp.com> 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>
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> On 23 Jul 2015, at 00:53, Warner Losh <imp@bsdimp.com> wrote: >=20 >>>> 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. >>>=20 >>> 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. >=20 > I=E2=80=99m not sure I=E2=80=99m 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=E2=80=99s 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=E2=80=99t 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. >>>=20 >>> 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. >>=20 >> 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. >>=20 >> 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. >=20 > 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=E2=80=99t care = that much about entropy to leave performance on the table. OK - I=E2=80=99m sold! I=E2=80=99ll make a kernel option defaulting to = off. :-) M --=20 Mark R V Murray
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