Re: [PATCH] arm64: mm: reset address tag set by kasan sw tagging

[Shyam Thombre]

Hi Catalin,


On 6/10/2020 5:06 PM, Catalin Marinas wrote:
> On Wed, Jun 10, 2020 at 04:39:44PM +0530, Shyam Thombre wrote:
> > KASAN sw tagging sets a random tag of 8 bits in the top byte of the pointer
> > returned by the memory allocating functions. So for the functions unaware
> > of this change, the top 8 bits of the address must be reset which is done
> > by the function arch_kasan_reset_tag().
> >
> > Signed-off-by: Shyam Thombre <sthombre@xxxxxxxxxxxxxx>
> > ---
> >   arch/arm64/mm/mmu.c | 1 +
> >   1 file changed, 1 insertion(+)
> >
> > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> > index e7fbc62..eae7655 100644
> > --- a/arch/arm64/mm/mmu.c
> > +++ b/arch/arm64/mm/mmu.c
> > @@ -723,6 +723,7 @@ int kern_addr_valid(unsigned long addr)
> >   	pmd_t *pmdp, pmd;
> >   	pte_t *ptep, pte;
> >   
> > +	addr = arch_kasan_reset_tag(addr);
> >   	if ((((long)addr) >> VA_BITS) != -1UL)
> >   		return 0;
>
> It would be interesting to know what fails without this patch. The only
> user seems to be read_kcore() and, at a quick look, I don't see how it
> can generate tagged addresses.

This issue is seen in downstream GPU drivers. It currently doesn't look 
to impact any upstream users.


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