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

[Mark Rutland <mark.rutland@arm.com>]

On Wed, Jan 13, 2016 at 05:10:31PM +0100, Ard Biesheuvel wrote:
> On 13 January 2016 at 15:03, Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote:
> > 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)
> >
> 
> Something along these lines, perhaps?
> 
> diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
> index 3571c7309c5e..bda0a776c58e 100644
> --- a/arch/arm64/mm/pageattr.c
> +++ b/arch/arm64/mm/pageattr.c
> @@ -13,6 +13,7 @@
>  #include <linux/kernel.h>
>  #include <linux/mm.h>
>  #include <linux/module.h>
> +#include <linux/vmalloc.h>
>  #include <linux/sched.h>
> 
>  #include <asm/pgtable.h>
> @@ -44,6 +45,7 @@ static int change_memory_common(unsigned long addr
>         unsigned long end = start + size;
>         int ret;
>         struct page_change_data data;
> +       struct vm_struct *area;
> 
>         if (!PAGE_ALIGNED(addr)) {
>                 start &= PAGE_MASK;
> @@ -51,10 +53,14 @@ static int change_memory_common(unsigned long addr,
>                 WARN_ON_ONCE(1);
>         }
> 
> -       if (start < MODULES_VADDR || start >= MODULES_END)
> -               return -EINVAL;
> -
> -       if (end < MODULES_VADDR || end >= MODULES_END)
> +       /*
> +        * Check whether the [addr, addr + size) interval is entirely
> +        * covered by precisely one VM area that has the VM_ALLOC flag set
> +        */
> +       area = find_vm_area((void *)addr);
> +       if (!area ||
> +           end > (unsigned long)area->addr + area->size ||
> +           !(area->flags & VM_ALLOC))
>                 return -EINVAL;
> 
>         data.set_mask = set_mask;

Neat. That fixes the fencepost bug too.

Looks good to me, though as Laura suggested we should have a comment as
to why we limit changes to such regions. Fancy taking her wording below
and spinning this as a patch?

> >> +       /*
> >> +        * 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.
> >> +        */

Thanks,
Mark.