Re: [PATCH v3] powerpc: mm: support ARCH_MMAP_RND_BITS

[Aneesh Kumar K.V]

On Thursday 13 April 2017 12:22 PM, Bhupesh Sharma wrote:
> Hi Aneesh,
>
> On Thu, Apr 13, 2017 at 12:06 PM, Aneesh Kumar K.V
> <aneesh.kumar@xxxxxxxxxxxxxxxxxx> wrote:
> > Bhupesh Sharma <bhsharma@xxxxxxxxxx> writes:
> >
> > > powerpc arch_mmap_rnd() currently uses hard-coded values - (23-PAGE_SHIFT) for
> > > 32-bit and (30-PAGE_SHIFT) for 64-bit, to generate the random offset
> > > for the mmap base address for a ASLR ELF.
> > >
> > > This patch makes sure that powerpc mmap arch_mmap_rnd() implementation
> > > is similar to other ARCHs (like x86, arm64) and uses mmap_rnd_bits
> > > and helpers to generate the mmap address randomization.
> > >
> > > The maximum and minimum randomization range values represent
> > > a compromise between increased ASLR effectiveness and avoiding
> > > address-space fragmentation.
> > >
> > > Using the Kconfig option and suitable /proc tunable, platform
> > > developers may choose where to place this compromise.
> > >
> > > Also this patch keeps the default values as new minimums.
> > >
> > > Signed-off-by: Bhupesh Sharma <bhsharma@xxxxxxxxxx>
> > > Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > > ---
> > > * Changes since v2:
> > > v2 can be seen here (https://patchwork.kernel.org/patch/9551509/)
> > >     - Changed a few minimum and maximum randomization ranges as per Michael's suggestion.
> > >     - Corrected Kees's email address in the Reviewed-by line.
> > >     - Added further comments in kconfig to explain how the address ranges were worked out.
> > >
> > > * Changes since v1:
> > > v1 can be seen here (https://lists.ozlabs.org/pipermail/linuxppc-dev/2017-February/153594.html)
> > >     - No functional change in this patch.
> > >     - Dropped PATCH 2/2 from v1 as recommended by Kees Cook.
> > >
> > >  arch/powerpc/Kconfig   | 44 ++++++++++++++++++++++++++++++++++++++++++++
> > >  arch/powerpc/mm/mmap.c |  7 ++++---
> > >  2 files changed, 48 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> > > index 97a8bc8..84aae67 100644
> > > --- a/arch/powerpc/Kconfig
> > > +++ b/arch/powerpc/Kconfig
> > > @@ -22,6 +22,48 @@ config MMU
> > >       bool
> > >       default y
> > >
> > > +# min bits determined by the following formula:
> > > +# VA_BITS - PAGE_SHIFT - CONSTANT
> > > +# where,
> > > +#    VA_BITS = 46 bits for 64BIT and 4GB - 1 Page = 31 bits for 32BIT
> >
> >
> > Where did we derive that 46 bits from ? is that based on TASK_SIZE ?
>
> Yes. It was derived from TASK_SIZE :
> http://lxr.free-electrons.com/source/arch/powerpc/include/asm/processor.h#L105

That is getting update to 128TB by default and conditionally to 512TB

-aneesh