From: "Theodore Ts'o" <tytso@mit.edu> To: PaX Team <pageexec@freemail.hu> Cc: kernel-hardening@lists.openwall.com, David Brown <david.brown@linaro.org>, emese Revfy <re.emese@gmail.com>, Andrew Morton <akpm@linux-foundation.org>, spender@grsecurity.net, mmarek@suse.com, keescook@chromium.org, linux-kernel@vger.kernel.org, yamada.masahiro@socionext.com, linux-kbuild@vger.kernel.org, linux-mm@kvack.org, axboe@kernel.dk, viro@zeniv.linux.org.uk, paulmck@linux.vnet.ibm.com, mingo@redhat.com, tglx@linutronix.de, bart.vanassche@sandisk.com, davem@davemloft.net Subject: Re: [kernel-hardening] Re: [PATCH v2 1/3] Add the latent_entropy gcc plugin Date: Thu, 9 Jun 2016 15:55:33 -0400 [thread overview] Message-ID: <20160609195533.GE5421@thunk.org> (raw) In-Reply-To: <5759A5D5.7023.18C58969@pageexec.freemail.hu> On Thu, Jun 09, 2016 at 07:22:29PM +0200, PaX Team wrote: > > Well, the attacker can't control when the interrupts happen, but it > > could try to burn power by simply having a thread spin in an infinite > > loop ("0: jmp 0"), sure. > > yes, that's one obvious way to accomplish it but even normal applications can > behave in a similar way, think about spinning event loops, media decoding, etc > whose sampled insn ptrs may provide less entropy than they get credited for. Sure, as long as we're assuming less than one bit of entropy per interrupt, even for a loop which which is: 1: cmpl $1, -8(%rsp) jz 1b there would still be *some* uncertainty. And with an event loop there would be more instructions to sample. Granted, the number of cycles spent in each will be different, so there will be some biasing, but that's one of the reason why we've been using 1/64 bit per interrupt. > yes, no entropy is credited since i don't know how much there is and i tend to err > on the side of safety which means crediting 0 entropy for latent entropy. of course > the expectation is that it's not actually 0 but to prove any specific value or limit > is beyond my skills at least. Sure, that's fair. > i think it's not just per 64 interrupts but also after each elapsed second (i.e., > whichever condition occurs first), so on an idle system (which i believe is more > likely to occur on exactly those small systems that the referenced paper was concerned > about) the credited entropy could be overestimated. That's a fair concern. It might be that we should enforce some minimum (at least 8 interrupts in all cases), but this is where it's all about hueristics, especially on those systems that don't have random_get_entropy(). > > In practice, on most modern CPU where we have a cycle counter, > > a quick check for get_cycles shows that at least these archs seem to return 0: > arc, avr32, cris, frv, m32r, m68k, xtensa. now you may not think of them as modern, > but they're still used in real life devices. i think that latent entropy is still > an option on them. It's possible for a system not to have a cycle counter, but to have something that can be used instead for random_get_entropy. That's only being used for the m68k/amiga and mips/R6000[A] cases, but I keep hoping that the archiecture maintainers for osme of these other oddball platform (is that better than "non-modern"? :-) will come up with something, but yes, it is those platforms where I've always been the most worried. On the one hand, if the hardware is crap, there's very little you can do. Unfortnuately, very often these crap architectures have a very low BOM cost, so they are most likely to be used in IOT devices. :-( One could try to claim that these IOT devics won't have upgradeable firmware and, so they'll probably be security disasters even without a good random number generators, but oddly, that doesn't give me much solace... And in the end, that may be the strongest argment for the latent_entropy plugin. Even if it doesn't provide a lot of extra entropy, on those platforms we're going to be so starved of real entropy that almost anything will be better than what we have today. - Ted
WARNING: multiple messages have this Message-ID (diff)
From: Theodore Ts'o <tytso@mit.edu> To: PaX Team <pageexec@freemail.hu> Cc: kernel-hardening@lists.openwall.com, David Brown <david.brown@linaro.org>, emese Revfy <re.emese@gmail.com>, Andrew Morton <akpm@linux-foundation.org>, spender@grsecurity.net, mmarek@suse.com, keescook@chromium.org, linux-kernel@vger.kernel.org, yamada.masahiro@socionext.com, linux-kbuild@vger.kernel.org, linux-mm@kvack.org, axboe@kernel.dk, viro@zeniv.linux.org.uk, paulmck@linux.vnet.ibm.com, mingo@redhat.com, tglx@linutronix.de, bart.vanassche@sandisk.com, davem@davemloft.net Subject: Re: [kernel-hardening] Re: [PATCH v2 1/3] Add the latent_entropy gcc plugin Date: Thu, 9 Jun 2016 15:55:33 -0400 [thread overview] Message-ID: <20160609195533.GE5421@thunk.org> (raw) In-Reply-To: <5759A5D5.7023.18C58969@pageexec.freemail.hu> On Thu, Jun 09, 2016 at 07:22:29PM +0200, PaX Team wrote: > > Well, the attacker can't control when the interrupts happen, but it > > could try to burn power by simply having a thread spin in an infinite > > loop ("0: jmp 0"), sure. > > yes, that's one obvious way to accomplish it but even normal applications can > behave in a similar way, think about spinning event loops, media decoding, etc > whose sampled insn ptrs may provide less entropy than they get credited for. Sure, as long as we're assuming less than one bit of entropy per interrupt, even for a loop which which is: 1: cmpl $1, -8(%rsp) jz 1b there would still be *some* uncertainty. And with an event loop there would be more instructions to sample. Granted, the number of cycles spent in each will be different, so there will be some biasing, but that's one of the reason why we've been using 1/64 bit per interrupt. > yes, no entropy is credited since i don't know how much there is and i tend to err > on the side of safety which means crediting 0 entropy for latent entropy. of course > the expectation is that it's not actually 0 but to prove any specific value or limit > is beyond my skills at least. Sure, that's fair. > i think it's not just per 64 interrupts but also after each elapsed second (i.e., > whichever condition occurs first), so on an idle system (which i believe is more > likely to occur on exactly those small systems that the referenced paper was concerned > about) the credited entropy could be overestimated. That's a fair concern. It might be that we should enforce some minimum (at least 8 interrupts in all cases), but this is where it's all about hueristics, especially on those systems that don't have random_get_entropy(). > > In practice, on most modern CPU where we have a cycle counter, > > a quick check for get_cycles shows that at least these archs seem to return 0: > arc, avr32, cris, frv, m32r, m68k, xtensa. now you may not think of them as modern, > but they're still used in real life devices. i think that latent entropy is still > an option on them. It's possible for a system not to have a cycle counter, but to have something that can be used instead for random_get_entropy. That's only being used for the m68k/amiga and mips/R6000[A] cases, but I keep hoping that the archiecture maintainers for osme of these other oddball platform (is that better than "non-modern"? :-) will come up with something, but yes, it is those platforms where I've always been the most worried. On the one hand, if the hardware is crap, there's very little you can do. Unfortnuately, very often these crap architectures have a very low BOM cost, so they are most likely to be used in IOT devices. :-( One could try to claim that these IOT devics won't have upgradeable firmware and, so they'll probably be security disasters even without a good random number generators, but oddly, that doesn't give me much solace... And in the end, that may be the strongest argment for the latent_entropy plugin. Even if it doesn't provide a lot of extra entropy, on those platforms we're going to be so starved of real entropy that almost anything will be better than what we have today. - Ted -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
next prev parent reply other threads:[~2016-06-09 19:56 UTC|newest] Thread overview: 62+ messages / expand[flat|nested] mbox.gz Atom feed top 2016-05-30 23:30 [PATCH v2 0/3] Introduce the latent_entropy gcc plugin Emese Revfy 2016-05-30 23:30 ` [kernel-hardening] " Emese Revfy 2016-05-30 23:30 ` Emese Revfy 2016-05-30 23:31 ` [PATCH v2 1/3] Add " Emese Revfy 2016-05-30 23:31 ` [kernel-hardening] " Emese Revfy 2016-05-30 23:31 ` Emese Revfy 2016-06-01 19:42 ` Andrew Morton 2016-06-01 19:42 ` [kernel-hardening] " Andrew Morton 2016-06-01 19:42 ` Andrew Morton 2016-06-03 17:42 ` Emese Revfy 2016-06-03 17:42 ` [kernel-hardening] " Emese Revfy 2016-06-03 17:42 ` Emese Revfy 2016-06-06 13:38 ` [kernel-hardening] " David Brown 2016-06-06 13:38 ` David Brown 2016-06-06 15:50 ` Kees Cook 2016-06-06 15:50 ` Kees Cook 2016-06-06 15:50 ` Kees Cook 2016-06-06 19:30 ` PaX Team 2016-06-06 19:30 ` PaX Team 2016-06-06 23:13 ` Theodore Ts'o 2016-06-06 23:13 ` Theodore Ts'o 2016-06-07 12:19 ` PaX Team 2016-06-07 12:19 ` PaX Team 2016-06-07 13:58 ` Theodore Ts'o 2016-06-07 13:58 ` Theodore Ts'o 2016-06-09 17:22 ` PaX Team 2016-06-09 17:22 ` PaX Team 2016-06-09 19:55 ` Theodore Ts'o [this message] 2016-06-09 19:55 ` Theodore Ts'o 2016-06-09 20:08 ` Kees Cook 2016-06-09 20:08 ` Kees Cook 2016-06-09 20:08 ` Kees Cook 2016-06-09 21:51 ` Kees Cook 2016-06-09 21:51 ` [kernel-hardening] " Kees Cook 2016-06-09 21:51 ` Kees Cook 2016-06-09 21:51 ` Kees Cook 2016-06-13 21:49 ` Emese Revfy 2016-06-13 21:49 ` [kernel-hardening] " Emese Revfy 2016-06-13 21:49 ` Emese Revfy 2016-06-13 21:49 ` Emese Revfy 2016-06-14 18:27 ` Kees Cook 2016-06-14 18:27 ` [kernel-hardening] " Kees Cook 2016-06-14 18:27 ` Kees Cook 2016-06-14 18:27 ` Kees Cook 2016-06-14 22:31 ` Emese Revfy 2016-06-14 22:31 ` [kernel-hardening] " Emese Revfy 2016-06-14 22:31 ` Emese Revfy 2016-06-14 22:31 ` Emese Revfy 2016-05-30 23:32 ` [PATCH v2 2/3] Mark functions with the latent_entropy attribute Emese Revfy 2016-05-30 23:32 ` [kernel-hardening] " Emese Revfy 2016-05-30 23:32 ` Emese Revfy 2016-05-30 23:34 ` [PATCH v2 3/3] Add the extra_latent_entropy kernel parameter Emese Revfy 2016-05-30 23:34 ` [kernel-hardening] " Emese Revfy 2016-05-30 23:34 ` Emese Revfy 2016-06-09 21:18 ` [PATCH v2 0/3] Introduce the latent_entropy gcc plugin Kees Cook 2016-06-09 21:18 ` [kernel-hardening] " Kees Cook 2016-06-09 21:18 ` Kees Cook 2016-06-09 21:18 ` Kees Cook 2016-06-09 23:33 ` Emese Revfy 2016-06-09 23:33 ` [kernel-hardening] " Emese Revfy 2016-06-09 23:33 ` Emese Revfy 2016-06-09 23:33 ` Emese Revfy
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