linux-kernel - Re: [RFC PATCH 01/10] arm64: introduce KIMAGE_VADDR as the virtual base of the kernel region

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Message-ID: <CAKv+Gu8eUc4zF6T5Tf5ui2ZBxYuytJam0Wa9fAmsvSSNzQFwNg@mail.gmail.com>
Date:	Sun, 3 Jan 2016 16:23:18 +0100
From:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:	Mark Rutland <mark.rutland@....com>
Cc:	Arnd Bergmann <arnd@...db.de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	kernel-hardening@...ts.openwall.com,
	Will Deacon <will.deacon@....com>,
	Catalin Marinas <catalin.marinas@....com>,
	Leif Lindholm <leif.lindholm@...aro.org>,
	Kees Cook <keescook@...omium.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Sharma Bhupesh <bhupesh.sharma@...escale.com>,
	Stuart Yoder <stuart.yoder@...escale.com>
Subject: Re: [RFC PATCH 01/10] arm64: introduce KIMAGE_VADDR as the virtual
 base of the kernel region

On 3 January 2016 at 15:50, Mark Rutland <mark.rutland@....com> wrote:
> On Mon, Dec 28, 2015 at 03:11:25PM +0100, Arnd Bergmann wrote:
>> On Monday 28 December 2015 13:07:44 Ard Biesheuvel wrote:
>> > On 28 December 2015 at 12:50, Arnd Bergmann <arnd@...db.de> wrote:
>> > > On Monday 28 December 2015 12:20:45 Ard Biesheuvel wrote:
>> > > How about a different approach that keeps the relocatable kernel, but moves it in
>> > > physical memory with the same random offset as the virtual address? That way, both
>> > > would be random, and you can keep the simple virt_to_phys() function.
>> > >
>> > > I suppose the downside of that is that the number of random bits is then limited
>> > > by the size of the first memblock, which is smaller than the vmalloc area.
>> > >
>> >
>> > I don't see a reason to use the same virtual and physical offset
>> > (other than the conditional). On arm64, it would be up to the
>> > bootloader to decide where to put the Image in physical memory, and it
>> > would be up to the kernel to decide whether or not to virtually remap
>> > itself.
>>
>> I see. If we pull in the bootloader to the discussion, there are a couple
>> of related points that are not directly required for your series but that
>> we should keep in mind anyway:
>>
>> - We need to implement the randomization for each boot loader separately.
>>   This is probably easy enough for grub, as it can tap the same random
>>   number source that you use here, but a little harder for u-boot (requiring
>>   to implement access to hardware rng separately on each platform) and
>>   much harder to get done consistently in UEFI for direct kernel loading
>>   since there is no common upstream.
>
> In the GRUB case the kernel is loaded as an EFI application -- as far as I am
> aware, GRUB for arm64 doesn't know anything about the Linux kernel Image
> binary.
>

No, it doesn't. Alexander Graf is even proposing a EFI compatible
runtime in U-boot so it can run EFI-GRUB as well, so it is unlikely
that something like that will get added soon. If he includes a
EFI_RNG_PROTOCOL implementation, we can run these patches on U-Boot as
well.

> When loaded as an EFI application the EFI stub can perform the relocation,
> which it already does if the kernel was laoded at an address it cannot execute
> from. It looks like Ard's implemented that for v2.
>

Indeed.

> Being (cold) booted from EFI is likely to be the most consistent case as we
> have complete control over where the kernel is placed, bar some limitations
> imposed by prior EFI applications or EFI itself.
>
>> - once we have a random number in the bootloader, we should also pass that
>>   through a DT property. This has been discussed multiple times in the past
>>   and I think we had reached consensus already but don't know if we had
>>   agreed on a specific DT property that contains the random number seed.
>
> Any links for this? I don't recall spotting this discussion.
>
> Thanks,
> Mark.
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