[PATCH] powerpc/mm/radix: Only add X for pages overlapping kernel text

[Balbir Singh]

On Tue, Jun 6, 2017 at 3:48 PM, Michael Ellerman <mpe at ellerman.id.au> wrote:
> Currently we map the whole linear mapping with PAGE_KERNEL_X. Instead we
> should check if the page overlaps the kernel text and only then add
> PAGE_KERNEL_X.
>
> Note that we still use 1G pages if they're available, so this will
> typically still result in a 1G executable page at KERNELBASE. So this fix is
> primarily useful for catching stray branches to high linear mapping addresses.
>
> Without this patch, we can execute at 1G in xmon using:
>
>   0:mon> m c000000040000000
>   c000000040000000  00 l
>   c000000040000000  00000000 01006038
>   c000000040000004  00000000 2000804e
>   c000000040000008  00000000 x
>   0:mon> di c000000040000000
>   c000000040000000  38600001      li      r3,1
>   c000000040000004  4e800020      blr
>   0:mon> p c000000040000000
>   return value is 0x1
>
> After we get a 400 as expected:
>
>   0:mon> p c000000040000000
>   *** 400 exception occurred
>
> Fixes: 2bfd65e45e87 ("powerpc/mm/radix: Add radix callbacks for early init routines")
> Cc: stable at vger.kernel.org # v4.7+
> Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>
> ---
>  arch/powerpc/mm/pgtable-radix.c | 14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 deletions(-)
>
> diff --git a/arch/powerpc/mm/pgtable-radix.c b/arch/powerpc/mm/pgtable-radix.c
> index c28165d8970b..6c062f92b9e4 100644
> --- a/arch/powerpc/mm/pgtable-radix.c
> +++ b/arch/powerpc/mm/pgtable-radix.c
> @@ -19,6 +19,7 @@
>  #include <asm/mmu.h>
>  #include <asm/firmware.h>
>  #include <asm/powernv.h>
> +#include <asm/sections.h>
>
>  #include <trace/events/thp.h>
>
> @@ -121,7 +122,8 @@ static inline void __meminit print_mapping(unsigned long start,
>  static int __meminit create_physical_mapping(unsigned long start,
>                                              unsigned long end)
>  {
> -       unsigned long addr, mapping_size = 0;
> +       unsigned long vaddr, addr, mapping_size = 0;
> +       pgprot_t prot;
>
>         start = _ALIGN_UP(start, PAGE_SIZE);
>         for (addr = start; addr < end; addr += mapping_size) {
> @@ -145,8 +147,14 @@ static int __meminit create_physical_mapping(unsigned long start,
>                         start = addr;
>                 }
>
> -               rc = radix__map_kernel_page((unsigned long)__va(addr), addr,
> -                                           PAGE_KERNEL_X, mapping_size);
> +               vaddr = (unsigned long)__va(addr);
> +
> +               if (overlaps_kernel_text(vaddr, vaddr + mapping_size))
> +                       prot = PAGE_KERNEL_X;
> +               else
> +                       prot = PAGE_KERNEL;

Do we need the kvm tmp/trampoline bits like hash?

Balbir Singh.