linux-kernel - Re: [PATCH] arm64/mm: fold check for KFENCE into can_set_direct

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Date:   Wed, 21 Sep 2022 17:09:19 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Mike Rapoport <rppt@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>
Cc:     Mike Rapoport <rppt@...ux.ibm.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64/mm: fold check for KFENCE into can_set_direct_map()



On 9/21/22 13:18, Mike Rapoport wrote:
> From: Mike Rapoport <rppt@...ux.ibm.com>
> 
> KFENCE requires linear map to be mapped at page granularity, so that it
> is possible to protect/unprotect single pages, just like with
> rodata_full and DEBUG_PAGEALLOC.
> 
> Instead of repating
> 
> 	can_set_direct_map() || IS_ENABLED(CONFIG_KFENCE)
> 
> make can_set_direct_map() handle the KFENCE case.
> 
> This also prevents potential false positives in kernel_page_present()
> that may return true for non-present page if CONFIG_KFENCE is enabled.
> 
> Signed-off-by: Mike Rapoport <rppt@...ux.ibm.com>
> ---
>  arch/arm64/mm/mmu.c      | 8 ++------
>  arch/arm64/mm/pageattr.c | 8 +++++++-
>  2 files changed, 9 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index e7ad44585f40..c5065abec55a 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -535,7 +535,7 @@ static void __init map_mem(pgd_t *pgdp)
>  	 */
>  	BUILD_BUG_ON(pgd_index(direct_map_end - 1) == pgd_index(direct_map_end));
>  
> -	if (can_set_direct_map() || IS_ENABLED(CONFIG_KFENCE))
> +	if (can_set_direct_map())
>  		flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
>  
>  	/*
> @@ -1547,11 +1547,7 @@ int arch_add_memory(int nid, u64 start, u64 size,
>  
>  	VM_BUG_ON(!mhp_range_allowed(start, size, true));
>  
> -	/*
> -	 * KFENCE requires linear map to be mapped at page granularity, so that
> -	 * it is possible to protect/unprotect single pages in the KFENCE pool.
> -	 */
> -	if (can_set_direct_map() || IS_ENABLED(CONFIG_KFENCE))
> +	if (can_set_direct_map())
>  		flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
>  
>  	__create_pgd_mapping(swapper_pg_dir, start, __phys_to_virt(start),
> diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
> index 64e985eaa52d..d107c3d434e2 100644
> --- a/arch/arm64/mm/pageattr.c
> +++ b/arch/arm64/mm/pageattr.c
> @@ -21,7 +21,13 @@ bool rodata_full __ro_after_init = IS_ENABLED(CONFIG_RODATA_FULL_DEFAULT_ENABLED
>  
>  bool can_set_direct_map(void)
>  {
> -	return rodata_full || debug_pagealloc_enabled();
> +	/*
> +	 * rodata_full, DEBUG_PAGEALLOC and KFENCE require linear map to be
> +	 * mapped at page granularity, so that it is possible to
> +	 * protect/unprotect single pages.
> +	 */
> +	return rodata_full || debug_pagealloc_enabled() ||
> +		IS_ENABLED(CONFIG_KFENCE);
>  }

Changing can_set_direct_map() also changes behaviour for other functions such as 

set_direct_map_default_noflush()
set_direct_map_invalid_noflush()
__kernel_map_pages()

Is that okay ?

>  
>  static int change_page_range(pte_t *ptep, unsigned long addr, void *data)