[PATCH] arm64: kernel: avoid literal load of virtual address with MMU off

Catalin Marinas catalin.marinas at arm.com
Thu Aug 18 01:34:48 PDT 2016

On Wed, Aug 17, 2016 at 07:26:32PM +0200, Ard Biesheuvel wrote:
> On 17 August 2016 at 19:11, Catalin Marinas <catalin.marinas at arm.com> wrote:
> > On Wed, Aug 17, 2016 at 05:54:41PM +0200, Ard Biesheuvel wrote:
> >> Literal loads of virtual addresses are subject to runtime relocation when
> >> CONFIG_RELOCATABLE=y, and given that the relocation routines run with the
> >> MMU and caches enabled, literal loads of relocated values performed with
> >> the MMU off are not guaranteed to return the latest value unless the
> >> memory covering the literal is cleaned to the PoC explicitly.
> >>
> >> So defer the literal load until after the MMU has been enabled, just like
> >> we do for primary_switch() and secondary_switch() in head.S.
> >
> > I think for consistency with head.S the patch is fine but do we actually
> > expect a literal pool value to be inconsistent between cache and PoC?
> Yes. Any literals covered by R_AARCH64_ABS64 relocations at link time
> will be covered by R_AARCH64_RELATIVE at runtime (when
> CONFIG_RELOCATABLE=y). Since AArch64 uses the RELA format, the targets
> of the relocations will contain zeroes before the relocation routine
> runs, and no cache maintenance is performed. This means a relocatable
> image requires runtime processing even if it is loaded unrandomised.

Ah, I forgot about this. Patch queued for 4.8-rc3. Thanks.


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