[PATCH 1/1] arm: mmu: catch NULL pointer dereferences

[Sascha Hauer]

On Sat, Aug 31, 2013 at 05:54:22PM +0200, Jean-Christophe PLAGNIOL-VILLARD wrote:
> For high vectors if memory start at 0x0. We have to
> live without being able to catch NULL pointer dereferences.
> 
> Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj at jcrosoft.com>
> ---
>  arch/arm/cpu/mmu.c | 27 +++++++++++++++++++++------
>  1 file changed, 21 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/arm/cpu/mmu.c b/arch/arm/cpu/mmu.c
> index e3ea3b6..2e69b16 100644
> --- a/arch/arm/cpu/mmu.c
> +++ b/arch/arm/cpu/mmu.c
> @@ -218,9 +218,9 @@ static int arm_mmu_remap_sdram(struct memory_bank *bank)
>  /*
>   * Map vectors and zero page
>   */
> -static void vectors_init(void)
> +static void vectors_init(bool zero_valid_addr)
>  {
> -	u32 *exc, *zero = NULL;
> +	u32 *exc;
>  	void *vectors;
>  	u32 cr;
>  
> @@ -235,10 +235,19 @@ static void vectors_init(void)
>  		 * page table for the high vectors and zero page
>  		 */
>  		exc = arm_create_pte(0xfff00000);
> -		zero = arm_create_pte(0x0);
>  
> -		/* Set the zero page to faulting */
> -		zero[0] = 0;
> +		/*
> +		 * Memory start at 0x0. We have to
> +		 * live without being able to catch NULL pointer dereferences
> +		 */
> +		if (!zero_valid_addr) {
> +			u32 *zero = NULL;
> +
> +			zero = arm_create_pte(0x0);
> +
> +			/* Set the zero page to faulting */
> +			zero[0] = 0;
> +		}
>  	} else {
>  		/*
>  		 * Otherwise map the vectors to the zero page. We have to
> @@ -267,6 +276,7 @@ static int mmu_init(void)
>  {
>  	struct memory_bank *bank;
>  	int i;
> +	bool is_start_zero = 0;
>  
>  	arm_set_cache_functions();
>  
> @@ -303,7 +313,12 @@ static int mmu_init(void)
>  	create_sections(0, 0, PAGE_SIZE, PMD_SECT_AP_WRITE | PMD_SECT_AP_READ |
>  			PMD_TYPE_SECT);
>  
> -	vectors_init();
> +	for_each_memory_bank(bank) {
> +		if (bank->start == 0x0)
> +			is_start_zero = 1;
> +	}
> +
> +	vectors_init(is_start_zero);

Why not add the check to vectors_init() instead of passing this as
argument?

I must say I'm not entirely happy with this approach. Being able to
catch NULL pointer derefs is really a good thing. When exactly does the
faulting zero SDRAM page become a problem for you? obviously something
like

cp /somehing /dev/ram0

does not work. Some care must probably be taken when setting up the
kernel parameters. Are these the problems or is there something else?

Sascha

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