Re: [PATCH RFC 17/37] arm64: mte: Disable dynamic tag storage management if HW KASAN is enabled

Alexandru Elisei
2023-10-16T12:41:53Z

Hi,

On Thu, Oct 12, 2023 at 10:35:05AM +0900, Hyesoo Yu wrote:
> On Wed, Aug 23, 2023 at 02:13:30PM +0100, Alexandru Elisei wrote:
> > Reserving the tag storage associated with a tagged page requires the
> > ability to migrate existing data if the tag storage is in use for data.
> >
> > The kernel allocates pages, which are now tagged because of HW KASAN, in
> > non-preemptible contexts, which can make reserving the associate tag
> > storage impossible.
> >
> > Don't expose the tag storage pages to the memory allocator if HW KASAN is
> > enabled.
> >
> > Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
> > ---
> > arch/arm64/kernel/mte_tag_storage.c | 12 ++++++++++++
> > 1 file changed, 12 insertions(+)
> >
> > diff --git a/arch/arm64/kernel/mte_tag_storage.c b/arch/arm64/kernel/mte_tag_storage.c
> > index 4a6bfdf88458..f45128d0244e 100644
> > --- a/arch/arm64/kernel/mte_tag_storage.c
> > +++ b/arch/arm64/kernel/mte_tag_storage.c
> > @@ -314,6 +314,18 @@ static int __init mte_tag_storage_activate_regions(void)
> > return 0;
> > }
> >
> > + /*
> > + * The kernel allocates memory in non-preemptible contexts, which makes
> > + * migration impossible when reserving the associated tag storage.
> > + *
> > + * The check is safe to make because KASAN HW tags are enabled before
> > + * the rest of the init functions are called, in smp_prepare_boot_cpu().
> > + */
> > + if (kasan_hw_tags_enabled()) {
> > + pr_info("KASAN HW tags enabled, disabling tag storage");
> > + return 0;
> > + }
> > +
>
> Hi.
>
> Is there no plan to enable HW KASAN in the current design ?
> I wonder if dynamic MTE is only used for user ?

The tag storage pages are exposed to the page allocator if and only if HW KASAN
is disabled:

static int __init mte_tag_storage_activate_regions(void)
[..]
/*
* The kernel allocates memory in non-preemptible contexts, which makes
* migration impossible when reserving the associated tag storage.
*
* The check is safe to make because KASAN HW tags are enabled before
* the rest of the init functions are called, in smp_prepare_boot_cpu().
*/
if (kasan_hw_tags_enabled()) {
pr_info("KASAN HW tags enabled, disabling tag storage");
return 0;
}

No plans at the moment to have this series compatible with HW KASAN. I will
revisit this if/when the series gets merged.

Thanks,
Alex

>
> Thanks,
> Hyesoo Yu.
>
>
> > for (i = 0; i < num_tag_regions; i++) {
> > tag_range = &tag_regions[i].tag_range;
> > for (pfn = tag_range->start; pfn <= tag_range->end; pfn += pageblock_nr_pages) {
> > --
> > 2.41.0
> >
> >