[PATCH] arm64: kaslr: Adjust the offset to avoid Image across alignment boundary

[Ard Biesheuvel]

On 18 August 2017 at 16:22, Catalin Marinas <catalin.marinas at arm.com> wrote:
> On Fri, Aug 18, 2017 at 04:20:16PM +0100, Ard Biesheuvel wrote:
>> On 18 August 2017 at 16:19, Catalin Marinas <catalin.marinas at arm.com> wrote:
>> > On Fri, Aug 18, 2017 at 04:04:34PM +0100, Catalin Marinas wrote:
>> >> With 16KB pages and a kernel Image larger than 16MB, the current
>> >> kaslr_early_init() logic for avoiding mappings across swapper table
>> >> boundaries fails since increasing the offset by kimg_sz just moves the
>> >> problem to the next boundary.
>> >>
>> >> This patch decreases the offset by the boundary overflow amount, with
>> >> slight risk of reduced entropy as the kernel is more likely to be found
>> >> at kimg_sz below a swapper table boundary.
>> >>
>> >> Trying-to-fix: afd0e5a87670 ("arm64: kaslr: Fix up the kernel image alignment")
>> >> Cc: Ard Biesheuvel <ard.biesheuvel at linaro.org>
>> >> Cc: Mark Rutland <mark.rutland at arm.com>
>> >> Cc: Will Deacon <will.deacon at arm.com>
>> >> Cc: Neeraj Upadhyay <neeraju at codeaurora.org>
>> >> Signed-off-by: Catalin Marinas <catalin.marinas at arm.com>
>> >> ---
>> >>
>> >> While preparing this email, I noticed that the kernel eventually failed
>> >> to boot, though after a lot more reboot iterations. Mark Rutland also
>> >> managed to make the KASLR kernel fail to boot with 64K pages which
>> >> wouldn't be explained by this patch.
>> >>
>> >> So, any suggestions are welcome. My testing method, qemu starting a
>> >> guest in a loop with virtio-rng-pci.
>> >
>> > Apparently, the booting gets much more stable if I disable the physical
>> > relocation in arm64-stub.c (but keep the virtual one with the fix in
>> > this patch). So I guess we are chasing two different issues.
>>
>> So this is using QEMU with 16k pages support?
>
> Qemu running on a ThunderX, so native KVM support.
>

Ah ok. I did not realize QEMU supports 16 KB pages in that case. Nice!

However, that makes it rather difficult to reproduce on my side.