On Mon, Apr 22, 2019 at 05:10:45PM +0800, Baoquan He wrote:
> kernel_randomize_memory() hardcodes the size of vmemmap section as 1 TB,
> to support the maximum amount of system RAM in 4-level paging mode, 64 TB.
>
> However, 1 TB is not enough for vmemmap in 5-level paging mode. Assuming
> the size of struct page is 64 Bytes, to support 4 PB system RAM in 5-level,
> 64 TB of vmemmap area is needed. The wrong hardcoding may cause vmemmap
> stamping into the following cpu_entry_area section, if KASLR puts vmemmap
> very close to cpu_entry_area , and the actual area of vmemmap is much bigger
> than 1 TB.
>
> So here calculate the actual size of vmemmap region, then align up to 1 TB
> boundary. In 4-level it's always 1 TB. In 5-level it's adjusted on demand.
> The current code reserves 0.5 PB for vmemmap in 5-level. In this new method,
> the left space can be saved to join randomization to increase the entropy.
>
> Signed-off-by: Baoquan He <b...@redhat.com>
> ---
> v2->v3:
> Fix typo Boris pointed out.
>
> arch/x86/mm/kaslr.c | 11 ++++++++++-
> 1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/mm/kaslr.c b/arch/x86/mm/kaslr.c
> index 387d4ed25d7c..4679a0075048 100644
> --- a/arch/x86/mm/kaslr.c
> +++ b/arch/x86/mm/kaslr.c
> @@ -52,7 +52,7 @@ static __initdata struct kaslr_memory_region {
> } kaslr_regions[] = {
> { &page_offset_base, 0 },
> { &vmalloc_base, 0 },
> - { &vmemmap_base, 1 },
> + { &vmemmap_base, 0 },
> };
>
> /* Get size in bytes used by the memory region */
> @@ -78,6 +78,7 @@ void __init kernel_randomize_memory(void)
> unsigned long rand, memory_tb;
> struct rnd_state rand_state;
> unsigned long remain_entropy;
> + unsigned long vmemmap_size;
>
> vaddr_start = pgtable_l5_enabled() ? __PAGE_OFFSET_BASE_L5 :
> __PAGE_OFFSET_BASE_L4;
> vaddr = vaddr_start;
> @@ -109,6 +110,14 @@ void __init kernel_randomize_memory(void)
> if (memory_tb < kaslr_regions[0].size_tb)
> kaslr_regions[0].size_tb = memory_tb;
>
> + /**
Nit: that is weird style for inline comment.
> + * Calculate how many TB vmemmap region needs, and aligned to
> + * 1TB boundary.
> + */
> + vmemmap_size = (kaslr_regions[0].size_tb << (TB_SHIFT - PAGE_SHIFT)) *
> + sizeof(struct page);
Hm. Don't we need to take into account alignment requirements for struct
page here? I'm worried about some exotic debug kernel config where
sizeof(struct page) doesn't satify __alignof__(struct page).
> + kaslr_regions[2].size_tb = DIV_ROUND_UP(vmemmap_size, 1UL << TB_SHIFT);
> +
> /* Calculate entropy available between regions */
> remain_entropy = vaddr_end - vaddr_start;
> for (i = 0; i < ARRAY_SIZE(kaslr_regions); i++)
--
Kirill A. Shutemov
