Re: [PATCH] arm64: Allow vmalloc regions to be set with set_memory_*

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On 12 January 2016 at 22:46, Laura Abbott <labbott@fedoraproject.org> wrote:
>
> The range of set_memory_* is currently restricted to the module address
> range because of difficulties in breaking down larger block sizes.
> vmalloc maps PAGE_SIZE pages so it is safe to use as well. Update the
> function ranges and add a comment explaining why the range is restricted
> the way it is.
>
> Signed-off-by: Laura Abbott <labbott@fedoraproject.org>
> ---
> This should let the protections for the eBPF work as expected, I don't
> know if there is some sort of self test for thatL.


This is going to conflict with my KASLR implementation, since it puts
the kernel image right in the middle of the vmalloc area, and the
kernel is obviously mapped with block mappings. In fact, I am
proposing enabling huge-vmap for arm64 as well, since it seems an
improvement generally, but also specifically allows me to unmap the
__init section using the generic vunmap code (remove_vm_area). But in
general, I think the assumption that the whole vmalloc area is mapped
using pages is not tenable.

AFAICT, vmalloc still use pages exclusively even with huge-vmap (but
ioremap does not). So perhaps it would make sense to check for the
VM_ALLOC bit in the VMA flags (which I will not set for the kernel
regions either)


> ---
>  arch/arm64/mm/pageattr.c | 25 +++++++++++++++++++++----
>  1 file changed, 21 insertions(+), 4 deletions(-)
>
> diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
> index 3571c73..274208e 100644
> --- a/arch/arm64/mm/pageattr.c
> +++ b/arch/arm64/mm/pageattr.c
> @@ -36,6 +36,26 @@ static int change_page_range(pte_t *ptep, pgtable_t token, unsigned long addr,
>         return 0;
>  }
>
> +static bool validate_addr(unsigned long start, unsigned long end)
> +{
> +       /*
> +        * This check explicitly excludes most kernel memory. Most kernel
> +        * memory is mapped with a larger page size and breaking down the
> +        * larger page size without causing TLB conflicts is very difficult.
> +        *
> +        * If you need to call set_memory_* on a range, the recommendation is
> +        * to use vmalloc since that range is mapped with pages.
> +        */
> +       if (start >= MODULES_VADDR && start < MODULES_END &&
> +           end >= MODULES_VADDR && end < MODULES_END)
> +               return true;
> +
> +       if (is_vmalloc_addr(start) && is_vmalloc_addr(end))
> +               return true;
> +
> +       return false;
> +}
> +
>  static int change_memory_common(unsigned long addr, int numpages,
>                                 pgprot_t set_mask, pgprot_t clear_mask)
>  {
> @@ -51,10 +71,7 @@ static int change_memory_common(unsigned long addr, int numpages,
>                 WARN_ON_ONCE(1);
>         }
>
> -       if (start < MODULES_VADDR || start >= MODULES_END)
> -               return -EINVAL;
> -
> -       if (end < MODULES_VADDR || end >= MODULES_END)
> +       if (!validate_addr(start, end))
>                 return -EINVAL;
>
>         data.set_mask = set_mask;
> --
> 2.5.0
>
>
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