Subject: Re: [v6 11/15] arm64/kasan: explicitly zero kasan shadow memory

From: Will Deacon <[email-masked]>
Date: Tue, 8 Aug 2017 10:07:44 +0100

On Mon, Aug 07, 2017 at 04:38:45PM -0400, Pavel Tatashin wrote:
> To optimize the performance of struct page initialization,
> vmemmap_populate() will no longer zero memory.
> 
> We must explicitly zero the memory that is allocated by vmemmap_populate()
> for kasan, as this memory does not go through struct page initialization
> path.
> 
> Signed-off-by: Pavel Tatashin <[email-masked]>
> Reviewed-by: Steven Sistare <[email-masked]>
> Reviewed-by: Daniel Jordan <[email-masked]>
> Reviewed-by: Bob Picco <[email-masked]>
> ---
>  arch/arm64/mm/kasan_init.c | 42 ++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 42 insertions(+)
> 
> diff --git a/arch/arm64/mm/kasan_init.c b/arch/arm64/mm/kasan_init.c
> index 81f03959a4ab..e78a9ecbb687 100644
> --- a/arch/arm64/mm/kasan_init.c
> +++ b/arch/arm64/mm/kasan_init.c
> @@ -135,6 +135,41 @@ static void __init clear_pgds(unsigned long start,
>  		set_pgd(pgd_offset_k(start), __pgd(0));
>  }
>  
> +/*
> + * Memory that was allocated by vmemmap_populate is not zeroed, so we must
> + * zero it here explicitly.
> + */
> +static void
> +zero_vmemmap_populated_memory(void)
> +{
> +	struct memblock_region *reg;
> +	u64 start, end;
> +
> +	for_each_memblock(memory, reg) {
> +		start = __phys_to_virt(reg->base);
> +		end = __phys_to_virt(reg->base + reg->size);
> +
> +		if (start >= end)
> +			break;
> +
> +		start = (u64)kasan_mem_to_shadow((void *)start);
> +		end = (u64)kasan_mem_to_shadow((void *)end);
> +
> +		/* Round to the start end of the mapped pages */
> +		start = round_down(start, SWAPPER_BLOCK_SIZE);
> +		end = round_up(end, SWAPPER_BLOCK_SIZE);
> +		memset((void *)start, 0, end - start);
> +	}
> +
> +	start = (u64)kasan_mem_to_shadow(_text);
> +	end = (u64)kasan_mem_to_shadow(_end);
> +
> +	/* Round to the start end of the mapped pages */
> +	start = round_down(start, SWAPPER_BLOCK_SIZE);
> +	end = round_up(end, SWAPPER_BLOCK_SIZE);
> +	memset((void *)start, 0, end - start);
> +}

I can't help but think this would be an awful lot nicer if you made
vmemmap_alloc_block take extra GFP flags as a parameter. That way, we could
implement a version of vmemmap_populate that does the zeroing when we need
it, without having to duplicate a bunch of the code like this. I think it
would also be less error-prone, because you wouldn't have to do the
allocation and the zeroing in two separate steps.

Will