Re: [PATCH v6 3/5] xen/arm64: mm: Introduce helpers to prepare/enable/disable the identity mapping

[Julien Grall <julien@xxxxxxx>]

Hi Bertrand,

On 03/03/2023 10:35, Bertrand Marquis wrote:
> Hi Julien,
>
> > On 2 Mar 2023, at 15:59, Julien Grall <julien@xxxxxxx> wrote:
> >
> > From: Julien Grall <jgrall@xxxxxxxxxx>
> >
> > In follow-up patches we will need to have part of Xen identity mapped in
> > order to safely switch the TTBR.
> >
> > On some platform, the identity mapping may have to start at 0. If we always
> > keep the identity region mapped, NULL pointer dereference would lead to
> > access to valid mapping.
> >
> > It would be possible to relocate Xen to avoid clashing with address 0.
> > However the identity mapping is only meant to be used in very limited
> > places. Therefore it would be better to keep the identity region invalid
> > for most of the time.
> >
> > Two new external helpers are introduced:
> >     - arch_setup_page_tables() will setup the page-tables so it is
> >       easy to create the mapping afterwards.
> >     - update_identity_mapping() will create/remove the identity mapping
> >
> > Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>
> In Arm internal CI this patch (or maybe an other in the serie) made one
> of our test crash on qemu-arm64.
Thanks for the report. I managed  to reproduce it by tweaking the QEMU 
command line option I was using:
42sh> qemu/build/qemu-system-aarch64 -machine virt,gic-version=3 
-machine virtualization=true -cpu cortex-a57 -smp 4 -m 2048 -serial 
mon:stdio -serial null -nographic -kernel xen/xen/xen
The problem is in patch #2 because I didn't adjust the address of the 
vmap/frametable areas. So they effectively are still right in the middle 
of the reserved region for identity mapping.
I will update patch #2. I am also thinking to add a check in 
xen_pt_update() to ensure no-one can create a non 1:1 mapping in the 
reserved area for identity mapping.
Cheers,

--
Julien Grall