Date: Thu, 14 Apr 2016 15:42:48 -0700 From: Kees Cook <keescook@...omium.org> To: Pavel Machek <pavel@...x.de> Cc: Linus Torvalds <torvalds@...ux-foundation.org>, "Rafael J. Wysocki" <rafael@...nel.org>, Ingo Molnar <mingo@...nel.org>, James Morse <james.morse@....com>, Ard Biesheuvel <ard.biesheuvel@...aro.org>, Matt Redfearn <matt.redfearn@...tec.com>, Yves-Alexis Perez <corsac@...ian.org>, Emrah Demir <ed@...sec.com>, Jonathan Corbet <corbet@....net>, "x86@...nel.org" <x86@...nel.org>, Len Brown <len.brown@...el.com>, Borislav Petkov <bp@...e.de>, Andy Lutomirski <luto@...nel.org>, "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>, Linux PM list <linux-pm@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com> Subject: Re: [PATCH v2] kaslr: allow kASLR to be default over Hibernation On Thu, Apr 14, 2016 at 1:34 PM, Pavel Machek <pavel@...x.de> wrote: > On Thu 2016-04-14 13:14:07, Kees Cook wrote: >> On Thu, Apr 14, 2016 at 1:01 PM, Pavel Machek <pavel@...x.de> wrote: >> > Hi! >> > >> >> Since kASLR and Hibernation can not currently coexist at runtime >> >> on x86, the default behavior was to disable kASLR by default when >> >> CONFIG_HIBERNATION was present (to retain original behavior). >> >> >> >> The behavior of kASLR on arm64 (and soon MIPS) is to be enabled by >> >> default when selected at build time. Since arm64 Hibernation does not >> >> conflict with kASLR, this fixes the hibernation argument parsing to be >> >> x86-specific. Additionally, since end users want to be able to select >> >> kASLR on x86 by default at build time, create CONFIG_RANDOMIZE_BASE_ON >> >> that is present only on x86. >> >> >> >> Signed-off-by: Kees Cook <keescook@...omium.org> >> > >> > I believe this is bad idea. arm64 shows that kaslr and hibernation can >> > coexist, and hibernation is still useful when your battery runs out. >> >> What? I'm confused -- this patch leaves the x86 behavior as-is by >> default but allows hibernation to work with arm64. (For example, right >> now, if you boot arm64 with "kaslr" kernel argument, hibernation will >> get needlessly disabled.) > > So it is very different from the PATCH v1, still it shares the > subject? It was similar, but not the same: v1: Prefer kASLR over Hibernation v2: kaslr: allow kASLR to be default over Hibernation I'm happy to call it whatever you like, though! :) > This is the part I don't like: > >> >> Since kASLR and Hibernation can not currently coexist at runtime >> >> on x86, the default behavior was to disable kASLR by default when >> >> CONFIG_HIBERNATION was present (to retain original behavior). > > Now I notice that it is quite unclear if it actually changes > anything... Okay, right. So, there are a few problems that this patch is solving, and maybe it needs to be broken up into separate patches, but it didn't seem like it to me at the time. Specifically: 1) The x86 hibernation and KASLR code don't play well together currently. "1" was worked around so that both could be built in, but only one would be active at a time. This lead to: 2) The general hibernation code contains kernel arguments that should only affect x86. And we have the desire by folks to have KASLR enabled by default on x86, giving us: 3) There is no build-time way on x86 to switch the preference of KASLR vs hibernation. I think "2" should be solved for this release, since arm64 KASLR is landing, and mistakenly booting an arm64 system with "kaslr" on the command line will needlessly disable hibernation. 3 and 2 are a result of 1, and IIUC, you're saying you want to solve 1 to make everything else go away? My only concern with that idea is that I don't (yet) have the knowledge of x86 hibernation internals to fix this, and it'll take a while to get to having KASLR on by default if we have to wait on me to fix hibernation. (I'm not saying I won't/can't, it's just that it'll take me time to come up to speed on it.) That do you think? -Kees -- Kees Cook Chrome OS & Brillo Security
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