[RFC PATCH 02/12] riscv: Protect the kernel linear mapping

[Anup Patel]

On Tue, Jan 5, 2021 at 1:31 AM Alexandre Ghiti <alex at ghiti.fr> wrote:
>
> The kernel is now mapped at the end of the address space and it should
> be accessed through this mapping only: so map the whole kernel in the
> linear mapping as read only.
>
> Signed-off-by: Alexandre Ghiti <alex at ghiti.fr>
> ---
>  arch/riscv/include/asm/page.h |  9 ++++++++-
>  arch/riscv/mm/init.c          | 29 +++++++++++++++++++++--------
>  2 files changed, 29 insertions(+), 9 deletions(-)
>
> diff --git a/arch/riscv/include/asm/page.h b/arch/riscv/include/asm/page.h
> index 98188e315e8d..a93e35aaa717 100644
> --- a/arch/riscv/include/asm/page.h
> +++ b/arch/riscv/include/asm/page.h
> @@ -102,8 +102,15 @@ extern unsigned long pfn_base;
>  extern unsigned long max_low_pfn;
>  extern unsigned long min_low_pfn;
>  extern unsigned long kernel_virt_addr;
> +extern uintptr_t load_pa, load_sz;
> +
> +#define linear_mapping_pa_to_va(x)     ((void *)((unsigned long)(x) + va_pa_offset))
> +#define kernel_mapping_pa_to_va(x)     \
> +       ((void *)((unsigned long) (x) + va_kernel_pa_offset))
> +#define __pa_to_va_nodebug(x)                          \
> +       ((x >= load_pa && x < load_pa + load_sz) ?      \
> +               kernel_mapping_pa_to_va(x): linear_mapping_pa_to_va(x))

This change should be part of PATCH1

>
> -#define __pa_to_va_nodebug(x)  ((void *)((unsigned long) (x) + va_pa_offset))
>  #define linear_mapping_va_to_pa(x)     ((unsigned long)(x) - va_pa_offset)
>  #define kernel_mapping_va_to_pa(x)     \
>         ((unsigned long)(x) - va_kernel_pa_offset)
> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> index 9d06ff0e015a..7b87c14f1d24 100644
> --- a/arch/riscv/mm/init.c
> +++ b/arch/riscv/mm/init.c
> @@ -159,8 +159,6 @@ void __init setup_bootmem(void)
>  {
>         phys_addr_t mem_start = 0;
>         phys_addr_t start, end = 0;
> -       phys_addr_t vmlinux_end = __pa_symbol(&_end);
> -       phys_addr_t vmlinux_start = __pa_symbol(&_start);

This as well.

>         u64 i;
>
>         /* Find the memory region containing the kernel */
> @@ -168,7 +166,7 @@ void __init setup_bootmem(void)
>                 phys_addr_t size = end - start;
>                 if (!mem_start)
>                         mem_start = start;
> -               if (start <= vmlinux_start && vmlinux_end <= end)
> +               if (start <= load_pa && (load_pa + load_sz) <= end)
>                         BUG_ON(size == 0);
>         }
>
> @@ -179,8 +177,13 @@ void __init setup_bootmem(void)
>          */
>         memblock_enforce_memory_limit(mem_start - PAGE_OFFSET);
>
> -       /* Reserve from the start of the kernel to the end of the kernel */
> -       memblock_reserve(vmlinux_start, vmlinux_end - vmlinux_start);
> +       /*
> +        * Reserve from the start of the kernel to the end of the kernel
> +        * and make sure we align the reservation on PMD_SIZE since we will
> +        * map the kernel in the linear mapping as read-only: we do not want
> +        * any allocation to happen between _end and the next pmd aligned page.
> +        */
> +       memblock_reserve(load_pa, (load_sz + PMD_SIZE - 1) & ~(PMD_SIZE - 1));
>
>         max_pfn = PFN_DOWN(memblock_end_of_DRAM());
>         max_low_pfn = max_pfn;
> @@ -438,7 +441,9 @@ static uintptr_t __init best_map_size(phys_addr_t base, phys_addr_t size)
>  #error "setup_vm() is called from head.S before relocate so it should not use absolute addressing."
>  #endif
>
> -static uintptr_t load_pa, load_sz;
> +uintptr_t load_pa, load_sz;
> +EXPORT_SYMBOL(load_pa);
> +EXPORT_SYMBOL(load_sz);

I think all changes till here should be in PATCH1.

Only the changes here onwards seems to be as-per PATCH description.

>
>  static void __init create_kernel_page_table(pgd_t *pgdir, uintptr_t map_size)
>  {
> @@ -596,9 +601,17 @@ static void __init setup_vm_final(void)
>
>                 map_size = best_map_size(start, end - start);
>                 for (pa = start; pa < end; pa += map_size) {
> -                       va = (uintptr_t)__va(pa);
> +                       pgprot_t prot = PAGE_KERNEL;
> +
> +                       /* Protect the kernel mapping that lies in the linear mapping */
> +                       if (pa >= __pa(_start) && pa < __pa(_end))
> +                               prot = PAGE_KERNEL_READ;
> +
> +                       /* Make sure we get virtual addresses in the linear mapping */
> +                       va = (uintptr_t)linear_mapping_pa_to_va(pa);
> +
>                         create_pgd_mapping(swapper_pg_dir, va, pa,
> -                                          map_size, PAGE_KERNEL);
> +                                          map_size, prot);
>                 }
>         }
>
> --
> 2.20.1
>

Regards,
Anup