Re: [tip:x86/security] x86: Add NX protection for kernel data

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Fri, Jan 21, 2011 at 10:41:54PM +0100, matthieu castet wrote:
> Konrad Rzeszutek Wilk a écrit :
>>> -	 * .data and .bss should always be writable.
>>> +	 * .data and .bss should always be writable, but xen won't like
>>> +	 * if we make page table rw (that live in .data or .bss)
>>>  	 */
>>> +#ifdef CONFIG_X86_32
>>>  	if (within(address, (unsigned long)_sdata, (unsigned long)_edata) ||
>>> -	    within(address, (unsigned long)__bss_start, (unsigned long)__bss_stop))
>>> -		pgprot_val(required) |= _PAGE_RW;
>>> +	    within(address, (unsigned long)__bss_start, (unsigned long)__bss_stop)) {
>>> +		unsigned int level;
>>> +		if (lookup_address(address, &level) && (level != PG_LEVEL_4K))
>>> +			pgprot_val(forbidden) |= _PAGE_RW;
>>> +	}
>>> +#endif
>>>   #if defined(CONFIG_X86_64) && defined(CONFIG_DEBUG_RODATA)
>>>
>>> fyi, it does make it boot.
>>
>> Hold it.. ccache is a wonderful tool but I think I've just "rebuilt" the
>> binaries with the .bss HPAGE_ALIGN aligment by mistake, so this path got never
>> taken.
>>
>>
> Ok,
>
> ATM I saw the following solution to solve the problem :
> 1) remove the data/bss check in static_protections, it was introduced by NX patches (64edc8ed). But I am not sure it
> is really needed anymore.
> 2) add ". = ALIGN(HPAGE_SIZE)" somewhere after init section. But if we want not to be allocated in image we
> should put it before bss. And if we want to be freed after init, we should put before .init.end.
> This mean moving .smp_locks (and .data_nosave when x86 will be added) before init section. I have no idea of the impact.
> 3) add some logic in arch/x86/xen/mmu.c, that will ignore RW page setting for the page table marked RO.
> 4) make static_protections take and old_prot argument, and only apply RW .data/.bss requirement if page is already RW.
>
> If possible I will go for 1).

Sounds good. Just send me the patch and I will test it.