Re: [RFC] arm64/mm: Fix idmap on [16K|36VA|48PA]

[Anshuman Khandual]

On 7/23/21 11:28 AM, Anshuman Khandual wrote:
> When creating the idmap, the kernel may add one extra level to idmap memory
> outside the VA range. But for [16K|36VA|48PA], we need two levels to reach
> 48 bits. If the bootloader places the kernel in memory above (1 << 46), the
> kernel will fail to enable the MMU. Although we are not aware of a platform
> where this happens, it is worth to accommodate such scenarios and prevent a
> possible kernel crash.
> 
> Lets fix the problem on the above configuration by creating two additional
> idmap page table levels when 'idmap_text_end' is outside the VA range. This
> reduces 'idmap_t0sz' to cover the entire PA range which would prevent table
> misconfiguration (fault) when a given 'idmap_t0sz' value requires a single
> additional page table level where as two have been built.
> 
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: James Morse <james.morse@xxxxxxx>
> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Fixes: 215399392fe4 ("arm64: 36 bit VA")
> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> ---
> This applies on v5.14-rc2

Hello,

This should ideally be backported to stable releases even though it
is only applicable with CONFIG_EXPERT (which lets the 36 bit VA get
selected on 16K page size). Tried to keep this as clean and minimal.
But being part of the very early code, wondering if there would be
any concern in getting this backported to stable ?

- Anshuman

> 
>  arch/arm64/kernel/head.S | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
> 
> diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
> index c5c994a..da33bbc 100644
> --- a/arch/arm64/kernel/head.S
> +++ b/arch/arm64/kernel/head.S
> @@ -329,7 +329,9 @@ SYM_FUNC_START_LOCAL(__create_page_tables)
>  
>  #if (VA_BITS < 48)
>  #define EXTRA_SHIFT	(PGDIR_SHIFT + PAGE_SHIFT - 3)
> +#define EXTRA_SHIFT_1	(EXTRA_SHIFT + PAGE_SHIFT - 3)
>  #define EXTRA_PTRS	(1 << (PHYS_MASK_SHIFT - EXTRA_SHIFT))
> +#define EXTRA_PTRS_1	(1 << (PHYS_MASK_SHIFT - EXTRA_SHIFT_1))
>  
>  	/*
>  	 * If VA_BITS < 48, we have to configure an additional table level.
> @@ -342,8 +344,30 @@ SYM_FUNC_START_LOCAL(__create_page_tables)
>  #error "Mismatch between VA_BITS and page size/number of translation levels"
>  #endif
>  
> +/*
> + * In this particular CONFIG_ARM64_16K_PAGES config, there might be a
> + * scenario where 'idmap_text_end' ends up high enough in the PA range
> + * requiring two additional idmap page table levels. Reduce idmap_t0sz
> + * to cover the entire PA range. This prevents table misconfiguration
> + * when a given idmap_t0sz value just requires single additional level
> + * where as two levels have been built.
> + */
> +#if defined(CONFIG_ARM64_VA_BITS_36) && defined(CONFIG_ARM64_PA_BITS_48)
> +	mov	x4, EXTRA_PTRS_1
> +	create_table_entry x0, x3, EXTRA_SHIFT_1, x4, x5, x6
> +
> +	mov	x4, PTRS_PER_PTE
> +	create_table_entry x0, x3, EXTRA_SHIFT, x4, x5, x6
> +
> +	mov	x5, #64 - PHYS_MASK_SHIFT
> +	adr_l	x6, idmap_t0sz
> +	str	x5, [x6]
> +	dmb	sy
> +	dc	ivac, x6
> +#else
>  	mov	x4, EXTRA_PTRS
>  	create_table_entry x0, x3, EXTRA_SHIFT, x4, x5, x6
> +#endif
>  #else
>  	/*
>  	 * If VA_BITS == 48, we don't have to configure an additional
>