[RFC PATCH v2 07/11] ARM64: mm: Make PAGE_NONE pages read only and no-execute.

[Will Deacon]

On Wed, May 08, 2013 at 10:52:39AM +0100, Steve Capper wrote:
> If we consider the following code sequence:
> 
> 	my_pte = pte_modify(entry, myprot);
> 	x = pte_write(my_pte);
> 	y = pte_exec(my_pte);
> 
> If myprot comes from a PROT_NONE page, then x and y will both be
> true which is undesireable behaviour.
> 
> This patch sets the no-execute and read-only bits for PAGE_NONE
> such that the code above will return false for both x and y.
> 
> Signed-off-by: Steve Capper <steve.capper at linaro.org>
> ---
>  arch/arm64/include/asm/pgtable.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index e333a24..b1a1b59 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -66,7 +66,7 @@ extern pgprot_t pgprot_default;
>  
>  #define _MOD_PROT(p, b)		__pgprot_modify(p, 0, b)
>  
> -#define PAGE_NONE		__pgprot_modify(pgprot_default, PTE_TYPE_MASK, PTE_PROT_NONE)
> +#define PAGE_NONE		__pgprot_modify(pgprot_default, PTE_TYPE_MASK, PTE_PROT_NONE | PTE_RDONLY | PTE_UXN)
>  #define PAGE_SHARED		_MOD_PROT(pgprot_default, PTE_USER | PTE_NG | PTE_PXN | PTE_UXN)
>  #define PAGE_SHARED_EXEC	_MOD_PROT(pgprot_default, PTE_USER | PTE_NG | PTE_PXN)
>  #define PAGE_COPY		_MOD_PROT(pgprot_default, PTE_USER | PTE_NG | PTE_PXN | PTE_UXN | PTE_RDONLY)
> @@ -76,7 +76,7 @@ extern pgprot_t pgprot_default;
>  #define PAGE_KERNEL		_MOD_PROT(pgprot_default, PTE_PXN | PTE_UXN | PTE_DIRTY)
>  #define PAGE_KERNEL_EXEC	_MOD_PROT(pgprot_default, PTE_UXN | PTE_DIRTY)
>  
> -#define __PAGE_NONE		__pgprot(((_PAGE_DEFAULT) & ~PTE_TYPE_MASK) | PTE_PROT_NONE)
> +#define __PAGE_NONE		__pgprot(((_PAGE_DEFAULT) & ~PTE_TYPE_MASK) | PTE_PROT_NONE | PTE_RDONLY | PTE_UXN)
>  #define __PAGE_SHARED		__pgprot(_PAGE_DEFAULT | PTE_USER | PTE_NG | PTE_PXN | PTE_UXN)
>  #define __PAGE_SHARED_EXEC	__pgprot(_PAGE_DEFAULT | PTE_USER | PTE_NG | PTE_PXN)
>  #define __PAGE_COPY		__pgprot(_PAGE_DEFAULT | PTE_USER | PTE_NG | PTE_PXN | PTE_UXN | PTE_RDONLY)

Whilst it's not strictly needed for pte_exec to work, I think you should
include PTE_PXN in the PAGE_NONE definitions as well.

Will