[RFC] riscv: Enable KFENCE for riscv64

[Marco Elver]

On Sat, 8 May 2021 at 04:56, Liu Shixin <liushixin2 at huawei.com> wrote:
>
> Add architecture specific implementation details for KFENCE and enable
> KFENCE for the riscv64 architecture. In particular, this implements the
> required interface in <asm/kfence.h>.

Nice to see KFENCE on more architectures.

> KFENCE requires that attributes for pages from its memory pool can
> individually be set. Therefore, force the kfence pool to be mapped at
> page granularity.
>
> I tested this patch using the testcases in kfence_test.c and all passed.
>
> Signed-off-by: Liu Shixin <liushixin2 at huawei.com>
[...]
> diff --git a/arch/riscv/include/asm/kfence.h b/arch/riscv/include/asm/kfence.h
> new file mode 100644
> index 000000000000..590c5b7e3514
> --- /dev/null
> +++ b/arch/riscv/include/asm/kfence.h
> @@ -0,0 +1,51 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#ifndef _ASM_RISCV_KFENCE_H
> +#define _ASM_RISCV_KFENCE_H
> +
> +#include <linux/pfn.h>
> +#include <linux/slab.h>
> +#include <linux/kfence.h>
> +#include <asm/pgtable.h>
> +
> +static inline bool arch_kfence_init_pool(void)
> +{
> +       int i;
> +       unsigned long addr;
> +       pte_t *pte;
> +       pmd_t *pmd;
> +
> +       for (addr = (unsigned long)__kfence_pool; is_kfence_address((void *)addr);
> +            addr += PAGE_SIZE) {
> +               pte = virt_to_kpte(addr);
> +               pmd = pmd_off_k(addr);
> +
> +               if (!pmd_leaf(*pmd) && pte_present(*pte))
> +                       continue;
> +
> +               pte = kmalloc(PAGE_SIZE, GFP_ATOMIC);

Using kmalloc() to allocate pte looks weird. Does riscv have helpers
for allocating pte? Otherwise, __get_free_page() perhaps?

> +               for (i = 0; i < PTRS_PER_PTE; i++)
> +                       set_pte(pte + i, pfn_pte(PFN_DOWN(__pa((addr & PMD_MASK) + i * PAGE_SIZE)), PAGE_KERNEL));
> +
> +               set_pmd(pmd, pfn_pmd(PFN_DOWN(__pa(pte)), PAGE_TABLE));
> +               flush_tlb_kernel_range(addr, addr + PMD_SIZE);
> +       }
> +
> +       return true;
> +}
> +
> +static inline bool kfence_protect_page(unsigned long addr, bool protect)
> +{
> +       pte_t *pte = virt_to_kpte(addr);
> +
> +       if (protect)
> +               set_pte(pte, __pte(pte_val(*pte) & ~_PAGE_PRESENT));
> +       else
> +               set_pte(pte, __pte(pte_val(*pte) | _PAGE_PRESENT));
> +
> +       flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
> +
> +       return true;
> +}
[...]