Re: [PATCH v10 1/6] arm64: mte: Sync tags for pages where PTE is untagged

[Catalin Marinas <catalin.marinas@arm.com>]

Hi Steven,

On Fri, Mar 12, 2021 at 03:18:57PM +0000, Steven Price wrote:
> A KVM guest could store tags in a page even if the VMM hasn't mapped
> the page with PROT_MTE. So when restoring pages from swap we will
> need to check to see if there are any saved tags even if !pte_tagged().
> 
> However don't check pages which are !pte_valid_user() as these will
> not have been swapped out.
> 
> Signed-off-by: Steven Price <steven.price@arm.com>
> ---
>  arch/arm64/include/asm/pgtable.h |  2 +-
>  arch/arm64/kernel/mte.c          | 16 ++++++++++++----
>  2 files changed, 13 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index e17b96d0e4b5..84166625c989 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -312,7 +312,7 @@ static inline void set_pte_at(struct mm_struct *mm, unsigned long addr,
>  		__sync_icache_dcache(pte);
>  
>  	if (system_supports_mte() &&
> -	    pte_present(pte) && pte_tagged(pte) && !pte_special(pte))
> +	    pte_present(pte) && pte_valid_user(pte) && !pte_special(pte))
>  		mte_sync_tags(ptep, pte);

With the EPAN patches queued in for-next/epan, pte_valid_user()
disappeared as its semantics weren't very clear.

So this relies on the set_pte_at() being done on the VMM address space.
I wonder, if the VMM did an mprotect(PROT_NONE), can the VM still access
it via stage 2? If yes, the pte_valid_user() test wouldn't work. We need
something like pte_present() && addr <= user_addr_max().

BTW, ignoring virtualisation, can we ever bring a page in from swap on a
PROT_NONE mapping (say fault-around)? It's not too bad if we keep the
metadata around for when the pte becomes accessible but I suspect we
remove it if the page is removed from swap.

-- 
Catalin