Subject: [PATCH v4 7/8] ARM: mm: allow non-text sections to be non-executable

From: [email-masked] (Will Deacon)
Date: Tue, 19 Aug 2014 13:33:42 +0100

On Wed, Aug 13, 2014 at 06:06:32PM +0100, Kees Cook wrote:
> Adds CONFIG_ARM_KERNMEM_PERMS to separate the kernel memory regions
> into section-sized areas that can have different permisions. Performs
> the NX permission changes during free_initmem, so that init memory can be
> reclaimed.
> 
> This uses section size instead of PMD size to reduce memory lost to
> padding on non-LPAE systems.
> 
> Based on work by Brad Spengler, Larry Bassel, and Laura Abbott.

[...]

> diff --git a/arch/arm/kernel/vmlinux.lds.S b/arch/arm/kernel/vmlinux.lds.S
> index 6f57cb94367f..a3d07ca2bbb4 100644
> --- a/arch/arm/kernel/vmlinux.lds.S
> +++ b/arch/arm/kernel/vmlinux.lds.S
> @@ -8,6 +8,9 @@
>  #include <asm/thread_info.h>
>  #include <asm/memory.h>
>  #include <asm/page.h>
> +#ifdef CONFIG_ARM_KERNMEM_PERMS
> +#include <asm/pgtable.h>
> +#endif
>  	
>  #define PROC_INFO							\
>  	. = ALIGN(4);							\
> @@ -90,6 +93,11 @@ SECTIONS
>  		_text = .;
>  		HEAD_TEXT
>  	}
> +
> +#ifdef CONFIG_ARM_KERNMEM_PERMS
> +	. = ALIGN(1<<SECTION_SHIFT);
> +#endif
> +
>  	.text : {			/* Real text segment		*/
>  		_stext = .;		/* Text and read-only data	*/
>  			__exception_text_start = .;
> @@ -145,7 +153,11 @@ SECTIONS
>  	_etext = .;			/* End of text and rodata section */
>  
>  #ifndef CONFIG_XIP_KERNEL
> +# ifdef CONFIG_ARM_KERNMEM_PERMS
> +	. = ALIGN(1<<SECTION_SHIFT);
> +# else
>  	. = ALIGN(PAGE_SIZE);
> +# endif

This might be cleaner if we had a single macro (ALIGN_MIN?) that expanded to
ALIGN(1 << SECTION_SHIFT) #ifdef  CONFIG_ARM_KERNMEM_PERMS.

> diff --git a/arch/arm/mm/init.c b/arch/arm/mm/init.c
> index ad82c05bfc3a..ccf392ef40d4 100644
> --- a/arch/arm/mm/init.c
> +++ b/arch/arm/mm/init.c
> @@ -32,6 +32,11 @@
>  #include <asm/tlb.h>
>  #include <asm/fixmap.h>
>  
> +#ifdef CONFIG_ARM_KERNMEM_PERMS
> +#include <asm/system_info.h>
> +#include <asm/cp15.h>
> +#endif

We already #include cp15.h in this file. If you need system_info.h, I don't
think you should bother making the include conditional.

> +/*
> + * Updates section permissions only for the current mm (sections are
> + * copied into each mm). During startup, this is the init_mm.
> + */
> +static inline void section_update(unsigned long addr, pmdval_t mask,
> +				  pmdval_t prot)
> +{
> +	struct mm_struct *mm;
> +	pmd_t *pmd;
> +
> +	mm = current->active_mm;
> +	pmd = pmd_offset(pud_offset(pgd_offset(mm, addr), addr), addr);
> +
> +#ifdef CONFIG_ARM_LPAE
> +	pmd[0] = __pmd((pmd_val(pmd[0]) & mask) | prot);
> +#else
> +	if (addr & SECTION_SIZE)
> +		pmd[1] = __pmd((pmd_val(pmd[1]) & mask) | prot);
> +	else
> +		pmd[0] = __pmd((pmd_val(pmd[0]) & mask) | prot);
> +#endif
> +	flush_pmd_entry(pmd);
> +	local_flush_tlb_kernel_range(addr, addr + SECTION_SIZE);

Why only a local flush? You're changing global mappings here, right?

> +}
> +
> +/* Make sure extended page tables are in use. */
> +static inline bool arch_has_strict_perms(void)
> +{
> +	unsigned int cr;
> +
> +	if (cpu_architecture() < CPU_ARCH_ARMv6)
> +		return false;
> +
> +	cr = get_cr();
> +	if (!(cr & CR_XP))
> +		return false;
> +
> +	return true;

return !!(get_cr() & CR_XP)?

> +}
> +
> +#define set_section_perms(perms, field)	{				\
> +	size_t i;							\
> +	unsigned long addr;						\
> +									\
> +	if (!arch_has_strict_perms())					\
> +		return;							\
> +									\
> +	for (i = 0; i < ARRAY_SIZE(perms); i++) {			\
> +		if (!IS_ALIGNED(perms[i].start, SECTION_SIZE) ||	\
> +		    !IS_ALIGNED(perms[i].end, SECTION_SIZE)) {		\
> +			pr_err("BUG: section %lx-%lx not aligned to %lx\n", \
> +				perms[i].start, perms[i].end,		\
> +				SECTION_SIZE);				\
> +			continue;					\
> +		}							\
> +									\
> +		for (addr = perms[i].start;				\
> +		     addr < perms[i].end;				\
> +		     addr += SECTION_SIZE)				\
> +			section_update(addr, perms[i].mask,		\
> +				       perms[i].field);			\
> +	}								\
> +}
> +
> +static inline void fix_kernmem_perms(void)
> +{
> +	set_section_perms(nx_perms, prot);
> +}
> +#else
> +static inline void fix_kernmem_perms(void) { }
> +#endif /* CONFIG_ARM_KERNMEM_PERMS */
> +
>  void free_initmem(void)
>  {
>  #ifdef CONFIG_HAVE_TCM
>  	extern char __tcm_start, __tcm_end;
> +#endif
> +
> +	fix_kernmem_perms();
>  
> +#ifdef CONFIG_HAVE_TCM

You could avoid the double #ifdef by moving the tcm stuff into another
function (free_tcmmem?)

Will