linux-kernel - Re: [PATCH] arm64: mm: reset address tag set by kasan sw tagging

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Date:   Wed, 10 Jun 2020 12:36:27 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Shyam Thombre <sthombre@...eaurora.org>
Cc:     will.deacon@....com, ard.biesheuvel@....com, mark.rutland@....com,
        anshuman.khandual@....com, sashal@...nel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: mm: reset address tag set by kasan sw tagging

On Wed, Jun 10, 2020 at 04:39:44PM +0530, Shyam Thombre wrote:
> KASAN sw tagging sets a random tag of 8 bits in the top byte of the pointer
> returned by the memory allocating functions. So for the functions unaware
> of this change, the top 8 bits of the address must be reset which is done
> by the function arch_kasan_reset_tag().
> 
> Signed-off-by: Shyam Thombre <sthombre@...eaurora.org>
> ---
>  arch/arm64/mm/mmu.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index e7fbc62..eae7655 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -723,6 +723,7 @@ int kern_addr_valid(unsigned long addr)
>  	pmd_t *pmdp, pmd;
>  	pte_t *ptep, pte;
>  
> +	addr = arch_kasan_reset_tag(addr);
>  	if ((((long)addr) >> VA_BITS) != -1UL)
>  		return 0;

It would be interesting to know what fails without this patch. The only
user seems to be read_kcore() and, at a quick look, I don't see how it
can generate tagged addresses.

-- 
Catalin