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

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Thu, 08 Jun 2017 12:30:04 +0530

Michael Ellerman <[email-masked]> writes:

> 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
>

Reviewed-by: Aneesh Kumar K.V <[email-masked]>

> Fixes: 2bfd65e45e87 ("powerpc/mm/radix: Add radix callbacks for early init routines")
> Cc: [email-masked] # v4.7+
> Signed-off-by: Michael Ellerman <[email-masked]>
> ---
>  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;
> +
> +		rc = radix__map_kernel_page(vaddr, addr, prot, mapping_size);
>  		if (rc)
>  			return rc;
>  	}
> -- 
> 2.7.4