[PATCH] Revert "arm64: mm: set the contiguous bit for kernel mappings where appropriate"

[Mark Rutland]

On Thu, Feb 23, 2017 at 04:25:26PM +0000, Ard Biesheuvel wrote:
> On 23 February 2017 at 16:22, Mark Rutland <mark.rutland at arm.com> wrote:
> > This reverts commit 0bfc445dec9dd8130d22c9f4476eed7598524129.
> >
> > When we change the permissions of regions mapped using contiguous
> > entries, the architecture requires us to follow a Break-Before-Make
> > strategy, breaking *all* associated entries before we can change any of
> > the following properties from the entries:
> >
> >  - presence of the contiguous bit
> >  - output address
> >  - attributes
> >  - permissiones
> >
> > Failure to do so can result in a number of problems (e.g. TLB conflict
> > aborts and/or erroneous results from TLB lookups).
> >
> > See ARM DDI 0487A.k_iss10775, "Misprogramming of the Contiguous bit",
> > page D4-1762.
> >
> > We do not take this into account when altering the permissions of kernel
> > segments in mark_rodata_ro(), where we change the permissions of live
> > contiguous entires one-by-one, leaving them transiently inconsistent.
> > This has been observed to result in failures on some fast model
> > configurations.
> >
> > Unfortunately, we cannot follow Break-Before-Make here as we'd have to
> > unmap kernel text and data used to perform the sequence.
> >
> > For the timebeing, revert commit 0bfc445dec9dd813 so as to avoid issues
> > resulting from this misuse of the contiguous bit.
> >
> > Signed-off-by: Mark Rutland <mark.rutland at arm.com>
> > Reported-by: Jean-Philippe Brucker <jean-philippe.brucker at arm.com>
> > Cc: Ard Biesheuvel <ard.biesheuvel at linaro.org>
> > Cc: Catalin Marinas <catalin.marinas at arm.com>
> > Cc: Will Deacon <Will.Deacon at arm.com>
> > Cc: stable at vger.kernel.org # v4.10
> 
> Acked-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>

Cheers.

> > ---
> >  arch/arm64/mm/mmu.c | 34 ++++------------------------------
> >  1 file changed, 4 insertions(+), 30 deletions(-)
> >
> > I'm aware that our hugtlbpage code has a similar issue. I'm looking into that
> > now, and will address that with separate patches. It should be possible to use
> > BBM there as it's a userspace mapping.
> 
> Are you looking into this issue as well?

I'm looking into it now.

Thanks,
Mark.