Re: [PATCH v2 04/19] arm64: mte: Use Normal Tagged attributes for the linear map

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On 26/02/2020 18:05, Catalin Marinas wrote:
> Once user space is given access to tagged memory, the kernel must be
> able to clear/save/restore tags visible to the user. This is done via
> the linear mapping, therefore map it as such. The new MT_NORMAL_TAGGED
> index for MAIR_EL1 is initially mapped as Normal memory and later
> changed to Normal Tagged via the cpufeature infrastructure. From a
> mismatched attribute aliases perspective, the Tagged memory is
> considered a permission and it won't lead to undefined behaviour.
> 
> The empty_zero_page is cleared to ensure that the tags it contains are
> already zeroed. The actual tags-aware clear_page() implementation is
> part of a subsequent patch.
> 
> Signed-off-by: Catalin Marinas <catalin.marinas@xxxxxxx>
> ---
>  arch/arm64/include/asm/memory.h       |  1 +
>  arch/arm64/include/asm/pgtable-prot.h |  2 ++
>  arch/arm64/kernel/cpufeature.c        | 30 +++++++++++++++++++++++++++
>  arch/arm64/mm/dump.c                  |  4 ++++
>  arch/arm64/mm/mmu.c                   | 22 ++++++++++++++++++--
>  arch/arm64/mm/proc.S                  |  8 +++++--
>  6 files changed, 63 insertions(+), 4 deletions(-)
> 
[...]
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 128f70852bf3..a2c206444e47 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -120,7 +120,7 @@ static bool pgattr_change_is_safe(u64 old, u64 new)
>  	 * The following mapping attributes may be updated in live
>  	 * kernel mappings without the need for break-before-make.
>  	 */
> -	static const pteval_t mask = PTE_PXN | PTE_RDONLY | PTE_WRITE | PTE_NG;
> +	pteval_t mask = PTE_PXN | PTE_RDONLY | PTE_WRITE | PTE_NG;
>  
>  	/* creating or taking down mappings is always safe */
>  	if (old == 0 || new == 0)
> @@ -134,6 +134,19 @@ static bool pgattr_change_is_safe(u64 old, u64 new)
>  	if (old & ~new & PTE_NG)
>  		return false;
>  
> +	if (system_supports_mte()) {
> +		/*
> +		 * Changing the memory type between Normal and Normal-Tagged
> +		 * is safe since Tagged is considered a permission attribute
> +		 * from the mismatched attribute aliases perspective.
> +		 */
> +		if ((old & PTE_ATTRINDX_MASK) == PTE_ATTRINDX(MT_NORMAL) ||
> +		    (old & PTE_ATTRINDX_MASK) == PTE_ATTRINDX(MT_NORMAL_TAGGED) ||
> +		    (new & PTE_ATTRINDX_MASK) == PTE_ATTRINDX(MT_NORMAL) ||
> +		    (new & PTE_ATTRINDX_MASK) == PTE_ATTRINDX(MT_NORMAL_TAGGED))
> +			mask |= PTE_ATTRINDX_MASK;
> +	}
> +
>  	return ((old ^ new) & ~mask) == 0;
>  }

This is much more permissive than I would expect. If either the old or
new memory type is NORMAL (or NORMAL_TAGGED) then the memory type is
ignored altogether.

Should this check be:

if (((old & PTE_ATTRINDX_MASK) == PTE_ATTRINDX(MT_NORMAL) ||
     (old & PTE_ATTRINDX_MASK) == PTE_ATTRINDX(MT_NORMAL_TAGGED)) &&
    ((new & PTE_ATTRINDX_MASK) == PTE_ATTRINDX(MT_NORMAL) ||
     (new & PTE_ATTRINDX_MASK) == PTE_ATTRINDX(MT_NORMAL_TAGGED)))

Steve




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