linux-kernel - Re: [PATCH v2 12/13] arm64: add support for relocatable kernel

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Date:	Fri, 8 Jan 2016 11:25:14 +0100
From:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:	James Morse <james.morse@....com>
Cc:	"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>,
	Mark Rutland <mark.rutland@....com>,
	Leif Lindholm <leif.lindholm@...aro.org>,
	Kees Cook <keescook@...omium.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Stuart Yoder <stuart.yoder@...escale.com>,
	Sharma Bhupesh <bhupesh.sharma@...escale.com>,
	Arnd Bergmann <arnd@...db.de>,
	Marc Zyngier <marc.zyngier@....com>,
	Christoffer Dall <christoffer.dall@...aro.org>
Subject: Re: [PATCH v2 12/13] arm64: add support for relocatable kernel

On 8 January 2016 at 11:17, James Morse <james.morse@....com> wrote:
> Hi Ard!
>
> On 30/12/15 15:26, Ard Biesheuvel wrote:
>> This adds support for runtime relocation of the kernel Image, by
>> building it as a PIE (ET_DYN) executable and applying the dynamic
>> relocations in the early boot code.
>>
>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
>> ---
>
>> diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
>> index 01a33e42ed70..ab582ee58b58 100644
>> --- a/arch/arm64/kernel/head.S
>> +++ b/arch/arm64/kernel/head.S
>> @@ -243,6 +253,16 @@ ENDPROC(stext)
>>  preserve_boot_args:
>>       mov     x21, x0                         // x21=FDT
>>
>> +#ifdef CONFIG_ARM64_RELOCATABLE_KERNEL
>> +     /*
>> +      * Mask off the bits of the random value supplied in x1 so it can serve
>> +      * as a KASLR displacement value which will move the kernel image to a
>> +      * random offset in the lower half of the VMALLOC area.
>> +      */
>> +     mov     x23, #(1 << (VA_BITS - 2)) - 1
>> +     and     x23, x23, x1, lsl #SWAPPER_BLOCK_SHIFT
>> +#endif
>
> I've managed to make this fail to boot by providing a seed that caused
> the kernel to overlap a 1G boundary on a 4K system.
>

Ah, yes. Thanks for spotting that.

> (It looks like your v3 may have the same issue - but I haven't tested it.)
>
>

Yes, it does. It probably makes sense to sacrifice some entropy bits
and simply round up the kaslr offset to a log2 upper bound of the
kernel Image size, rather than hacking up some logic in assembly to
test whether we are crossing a PMD/PUD boundary


>> +
>>       adr_l   x0, boot_args                   // record the contents of
>>       stp     x21, x1, [x0]                   // x0 .. x3 at kernel entry
>>       stp     x2, x3, [x0, #16]
>
>
> Thanks!
>
> James
>