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

Will Deacon
2017-08-08T09:07:49Z

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 <pasha.tatashin@oracle.com>
> Reviewed-by: Steven Sistare <steven.sistare@oracle.com>
> Reviewed-by: Daniel Jordan <daniel.m.jordan@oracle.com>
> Reviewed-by: Bob Picco <bob.picco@oracle.com>
> ---
> 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