[PATCH] arm64: remove page granularity limitation from KFENCE

[Alexander Potapenko]

On Sat, Sep 18, 2021 at 10:10 AM Liu Shixin <liushixin2 at huawei.com> wrote:
>
> Currently if KFENCE is enabled in arm64, the entire linear map will be
> mapped at page granularity which seems overkilled. Actually only the
> kfence pool requires to be mapped at page granularity. We can remove the
> restriction from KFENCE and force the linear mapping of the kfence pool
> at page granularity later in arch_kfence_init_pool().

There was a previous patch by Jisheng Zhang intended to remove this
requirement: https://lore.kernel.org/linux-arm-kernel/20210524180656.395e45f6@xhacker.debian/
Which of the two is more preferable?

> Signed-off-by: Liu Shixin <liushixin2 at huawei.com>
> ---
>  arch/arm64/include/asm/kfence.h | 69 ++++++++++++++++++++++++++++++++-
>  arch/arm64/mm/mmu.c             |  4 +-
>  2 files changed, 70 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm64/include/asm/kfence.h b/arch/arm64/include/asm/kfence.h
> index aa855c6a0ae6..bee101eced0b 100644
> --- a/arch/arm64/include/asm/kfence.h
> +++ b/arch/arm64/include/asm/kfence.h
> @@ -8,9 +8,76 @@
>  #ifndef __ASM_KFENCE_H
>  #define __ASM_KFENCE_H
>
> +#include <linux/kfence.h>
>  #include <asm/set_memory.h>
> +#include <asm/pgalloc.h>
>
> -static inline bool arch_kfence_init_pool(void) { return true; }
> +static inline int split_pud_page(pud_t *pud, unsigned long addr)
> +{
> +       int i;
> +       pmd_t *pmd = pmd_alloc_one(&init_mm, addr);
> +       unsigned long pfn = PFN_DOWN(__pa(addr));
> +
> +       if (!pmd)
> +               return -ENOMEM;
> +
> +       for (i = 0; i < PTRS_PER_PMD; i++)
> +               set_pmd(pmd + i, pmd_mkhuge(pfn_pmd(pfn + i * PTRS_PER_PTE, PAGE_KERNEL)));
> +
> +       smp_wmb(); /* See comment in __pte_alloc */
> +       pud_populate(&init_mm, pud, pmd);
> +       flush_tlb_kernel_range(addr, addr + PUD_SIZE);
> +       return 0;
> +}
> +
> +static inline int split_pmd_page(pmd_t *pmd, unsigned long addr)
> +{
> +       int i;
> +       pte_t *pte = pte_alloc_one_kernel(&init_mm);
> +       unsigned long pfn = PFN_DOWN(__pa(addr));
> +
> +       if (!pte)
> +               return -ENOMEM;
> +
> +       for (i = 0; i < PTRS_PER_PTE; i++)
> +               set_pte(pte + i, pfn_pte(pfn + i, PAGE_KERNEL));
> +
> +       smp_wmb(); /* See comment in __pte_alloc */
> +       pmd_populate_kernel(&init_mm, pmd, pte);
> +
> +       flush_tlb_kernel_range(addr, addr + PMD_SIZE);
> +       return 0;
> +}
> +
> +static inline bool arch_kfence_init_pool(void)
> +{
> +       unsigned long addr;
> +       pgd_t *pgd;
> +       p4d_t *p4d;
> +       pud_t *pud;
> +       pmd_t *pmd;
> +
> +       for (addr = (unsigned long)__kfence_pool; is_kfence_address((void *)addr);
> +            addr += PAGE_SIZE) {
> +               pgd = pgd_offset(&init_mm, addr);
> +               if (pgd_leaf(*pgd))
> +                       return false;
> +               p4d = p4d_offset(pgd, addr);
> +               if (p4d_leaf(*p4d))
> +                       return false;
> +               pud = pud_offset(p4d, addr);
> +               if (pud_leaf(*pud)) {
> +                       if (split_pud_page(pud, addr & PUD_MASK))
> +                               return false;
> +               }
> +               pmd = pmd_offset(pud, addr);
> +               if (pmd_leaf(*pmd)) {
> +                       if (split_pmd_page(pmd, addr & PMD_MASK))
> +                               return false;
> +               }
> +       }
> +       return true;
> +}
>
>  static inline bool kfence_protect_page(unsigned long addr, bool protect)
>  {
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index cfd9deb347c3..b2c79ccfb1c5 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -516,7 +516,7 @@ static void __init map_mem(pgd_t *pgdp)
>          */
>         BUILD_BUG_ON(pgd_index(direct_map_end - 1) == pgd_index(direct_map_end));
>
> -       if (can_set_direct_map() || crash_mem_map || IS_ENABLED(CONFIG_KFENCE))
> +       if (can_set_direct_map() || crash_mem_map)
>                 flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
>
>         /*
> @@ -1485,7 +1485,7 @@ int arch_add_memory(int nid, u64 start, u64 size,
>          * KFENCE requires linear map to be mapped at page granularity, so that
>          * it is possible to protect/unprotect single pages in the KFENCE pool.
>          */
> -       if (can_set_direct_map() || IS_ENABLED(CONFIG_KFENCE))
> +       if (can_set_direct_map())
>                 flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
>
>         __create_pgd_mapping(swapper_pg_dir, start, __phys_to_virt(start),
> --
> 2.18.0.huawei.25
>
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-- 
Alexander Potapenko
Software Engineer

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