Date: Thu, 17 Aug 2017 00:23:22 -0400 From: Daniel Micay <danielmicay@...il.com> To: Theodore Ts'o <tytso@....edu>, Laura Abbott <labbott@...hat.com> Cc: Kees Cook <keescook@...omium.org>, kernel-hardening@...ts.openwall.com, linux-kernel@...r.kernel.org, linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org> Subject: Re: [PATCHv3 2/2] extract early boot entropy from the passed cmdline On Wed, 2017-08-16 at 23:31 -0400, Theodore Ts'o wrote: > On Wed, Aug 16, 2017 at 04:14:58PM -0700, Laura Abbott wrote: > > From: Daniel Micay <danielmicay@...il.com> > > > > Existing Android bootloaders usually pass data useful as early > > entropy > > on the kernel command-line. It may also be the case on other > > embedded > > systems..... > > May I suggest a slight adjustment to the beginning commit description? > > Feed the boot command-line as to the /dev/random entropy pool > > Existing Android bootloaders usually pass data which may not be > known by an external attacker on the kernel command-line. It may > also be the case on other embedded systems. Sample command-line > from a Google Pixel running CopperheadOS.... > > The idea here is to if anything, err on the side of under-promising > the amount of security we can guarantee that this technique will > provide. For example, how hard is it really for an attacker who has > an APK installed locally to get the device serial number? Or the OS > version? And how much variability is there in the bootloader stages > in milliseconds? The serial number is currently accessible to local apps up until Android 7.x so it doesn't have value if the adversary has local access. Access to it without the READ_PHONE_STATE permission is being removed for apps targeting Android 8.0 and will presumably be restructed for all apps at some point in the future: https://android-developers.googleblog.com/2017/04/changes-to-device-identifiers-in.html Some bootloader stages vary a bit in time each boot. There's not much variance or measurement precision so there's only a small amount of entropy from this. The ones that consistently vary in timing do so independently from each other so that helps a bit. Also worth noting that before Android 8.0+, local apps can access the boot times since it's written to a system property. After Android 8.0+, all that stuff is inaccessible to them (no permission to get them) since there's a whitelisting model for system property access. > I think we should definitely do this. So this is more of a request to > be very careful what we promise in the commit description, not an > objection to the change itself. I did say 'external attacker' but it could be made clearer. It's primarily aimed at getting a tiny bit of extra entropy for the kernel stack canary and other probabilistic exploit mitigations set up in early boot. On non-x86 archs, i.e. 99.9% of Android devices, the kernel stack canary remains the same after it's set up in that early boot code. Android devices almost all have a hardware RNG and Android init blocks until a fair bit of data is read from it along with restoring entropy that's regularly saved while running, but unfortunately that's not available at this point in the boot process. The kernel could save / restore entropy using pstore (which at least Nexus / Pixel devices have - not sure about others). I don't know how early that could feasibly be done. Ideally it would do that combined with early usage of the hwrng.
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