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: Tue, 7 Jun 2016 09:58:57 -0400 [thread overview] Message-ID: <20160607135857.GF7057@thunk.org> (raw) In-Reply-To: <5756BBC2.3735.D63200E@pageexec.freemail.hu> On Tue, Jun 07, 2016 at 02:19:14PM +0200, PaX Team wrote: > (i believe that) latent entropy is found in more than just interrupt timing, there're > also data dependent computations that can have entropy, either on a single system or > across a population of them. It's not clear how much data dependent computations you would have in kernel space that's not introduced by interrupts, but there would some, I'm sure. > > we're doing this already inside modern Linux kernels. On every single > > interrupt we are mixing into a per-CPU "fast mix" pool the IP from the > > interrupt registers. > > i agree that sampling the kernel register state can have entropy (the plugin > already extracts the current stack pointer) but i'm much less sure about > userland (at least i see no dependence on !user_mode(...)) since an attacker > could feed no entropy into the pool but still get it credited. 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. Of course, this would be rather noticeable, and if there were any other jobs running, the attacker would be degrading the amount of entropy that would be gathered, but not eliminating it. All of this goes into the question of how much entropy we can assume can be gathered per interrupt (or in the case of basic block instrumentation, per basic block). IIRC, in the latent_entropy patches, the assumption is that zero entropy should be credited, correct? In the case Linux's current get_interrupt_randomness(), there's a reason I'm using a very conservative 1/64th of a bit per interrupt. In practice, on most modern CPU where we have a cycle counter, even if the bad guy was doing a "0: jmp 0" spinning loop, we would still get entropy via the cycle counter interacting with what is hopefully a certain amount of entropy from the interrupt timing. On a crappy $50 Android phone/tablet from China, using an ancient ARM chip that doesn't have any cycle counting facilities, we're kind of screwed, but those devices have lousy batteries, so if you have an attacker that has disabled the wakelocks and is spinning in an infinite loop, the battery life won't last long, so the problem will mostly solve itself when the phone dies. :-) - Ted
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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: Tue, 7 Jun 2016 09:58:57 -0400 [thread overview] Message-ID: <20160607135857.GF7057@thunk.org> (raw) In-Reply-To: <5756BBC2.3735.D63200E@pageexec.freemail.hu> On Tue, Jun 07, 2016 at 02:19:14PM +0200, PaX Team wrote: > (i believe that) latent entropy is found in more than just interrupt timing, there're > also data dependent computations that can have entropy, either on a single system or > across a population of them. It's not clear how much data dependent computations you would have in kernel space that's not introduced by interrupts, but there would some, I'm sure. > > we're doing this already inside modern Linux kernels. On every single > > interrupt we are mixing into a per-CPU "fast mix" pool the IP from the > > interrupt registers. > > i agree that sampling the kernel register state can have entropy (the plugin > already extracts the current stack pointer) but i'm much less sure about > userland (at least i see no dependence on !user_mode(...)) since an attacker > could feed no entropy into the pool but still get it credited. 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. Of course, this would be rather noticeable, and if there were any other jobs running, the attacker would be degrading the amount of entropy that would be gathered, but not eliminating it. All of this goes into the question of how much entropy we can assume can be gathered per interrupt (or in the case of basic block instrumentation, per basic block). IIRC, in the latent_entropy patches, the assumption is that zero entropy should be credited, correct? In the case Linux's current get_interrupt_randomness(), there's a reason I'm using a very conservative 1/64th of a bit per interrupt. In practice, on most modern CPU where we have a cycle counter, even if the bad guy was doing a "0: jmp 0" spinning loop, we would still get entropy via the cycle counter interacting with what is hopefully a certain amount of entropy from the interrupt timing. On a crappy $50 Android phone/tablet from China, using an ancient ARM chip that doesn't have any cycle counting facilities, we're kind of screwed, but those devices have lousy batteries, so if you have an attacker that has disabled the wakelocks and is spinning in an infinite loop, the battery life won't last long, so the problem will mostly solve itself when the phone dies. :-) - 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:[~2016-06-07 13:59 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 [this message] 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 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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