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

[Alexandru Elisei]

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@xxxxxxx>
> > ---
> >  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
> > 
> >