linux-hardening - Re: [PATCH v4 08/26] arm64: kernel: drop unnecessary PoC cache clean+invalidate

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Date:   Wed, 15 Jun 2022 10:02:47 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Ard Biesheuvel <ardb@...nel.org>,
        linux-arm-kernel@...ts.infradead.org
Cc:     linux-hardening@...r.kernel.org, Marc Zyngier <maz@...nel.org>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Kees Cook <keescook@...omium.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Brown <broonie@...nel.org>
Subject: Re: [PATCH v4 08/26] arm64: kernel: drop unnecessary PoC cache
 clean+invalidate



On 6/13/22 20:15, Ard Biesheuvel wrote:
> Some early boot code runs before the virtual placement of the kernel is
> finalized, and we used to go back to the very start and recreate the ID
> map along with the page tables describing the virtual kernel mapping,
> and this involved setting some global variables with the caches off.
> 
> In order to ensure that global state created by the KASLR code is not
> corrupted by the cache invalidation that occurs in that case, we needed
> to clean those global variables to the PoC explicitly.
> 
> This is no longer needed now that the ID map is created only once (and
> the associated global variable updates are no longer repeated). So drop
> the cache maintenance that is no longer necessary.
> 
> Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
> ---
>  arch/arm64/kernel/kaslr.c | 11 -----------
>  1 file changed, 11 deletions(-)
> 
> diff --git a/arch/arm64/kernel/kaslr.c b/arch/arm64/kernel/kaslr.c
> index 418b2bba1521..d5542666182f 100644
> --- a/arch/arm64/kernel/kaslr.c
> +++ b/arch/arm64/kernel/kaslr.c
> @@ -13,7 +13,6 @@
>  #include <linux/pgtable.h>
>  #include <linux/random.h>
>  
> -#include <asm/cacheflush.h>
>  #include <asm/fixmap.h>
>  #include <asm/kernel-pgtable.h>
>  #include <asm/memory.h>
> @@ -72,9 +71,6 @@ u64 __init kaslr_early_init(void)
>  	 * we end up running with module randomization disabled.
>  	 */
>  	module_alloc_base = (u64)_etext - MODULES_VSIZE;
> -	dcache_clean_inval_poc((unsigned long)&module_alloc_base,
> -			    (unsigned long)&module_alloc_base +
> -				    sizeof(module_alloc_base));
>  
>  	/*
>  	 * Try to map the FDT early. If this fails, we simply bail,
> @@ -174,13 +170,6 @@ u64 __init kaslr_early_init(void)
>  	module_alloc_base += (module_range * (seed & ((1 << 21) - 1))) >> 21;
>  	module_alloc_base &= PAGE_MASK;
>  
> -	dcache_clean_inval_poc((unsigned long)&module_alloc_base,
> -			    (unsigned long)&module_alloc_base +
> -				    sizeof(module_alloc_base));
> -	dcache_clean_inval_poc((unsigned long)&memstart_offset_seed,
> -			    (unsigned long)&memstart_offset_seed +
> -				    sizeof(memstart_offset_seed));
> -
>  	return offset;
>  }
>  

Reviewed-by: Anshuman Khandual <anshuman.khandual@....com>