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

[matthieu castet <castet.matthieu@free.fr>]

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Konrad Rzeszutek Wilk a écrit :
> 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.

Ok, what give you the attached patch.

I don't know if I should give the printk or not.


Matthieu

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diff --git a/arch/x86/mm/pageattr.c b/arch/x86/mm/pageattr.c
index 8b830ca..eec93c5 100644
--- a/arch/x86/mm/pageattr.c
+++ b/arch/x86/mm/pageattr.c
@@ -256,7 +256,6 @@ static inline pgprot_t static_protections(pgprot_t prot, unsigned long address,
 				   unsigned long pfn)
 {
 	pgprot_t forbidden = __pgprot(0);
-	pgprot_t required = __pgprot(0);
 
 	/*
 	 * The BIOS area between 640k and 1Mb needs to be executable for
@@ -283,11 +282,13 @@ static inline pgprot_t static_protections(pgprot_t prot, unsigned long address,
 		   __pa((unsigned long)__end_rodata) >> PAGE_SHIFT))
 		pgprot_val(forbidden) |= _PAGE_RW;
 	/*
-	 * .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)
 	 */
 	if (within(address, (unsigned long)_sdata, (unsigned long)_edata) ||
 	    within(address, (unsigned long)__bss_start, (unsigned long)__bss_stop))
-		pgprot_val(required) |= _PAGE_RW;
+		if ((pgprot_val(prot) & _PAGE_RW) == 0)
+			printk(KERN_INFO "RO page for 0x%lx in bss/data.\n", address);
 
 #if defined(CONFIG_X86_64) && defined(CONFIG_DEBUG_RODATA)
 	/*
@@ -327,7 +328,6 @@ static inline pgprot_t static_protections(pgprot_t prot, unsigned long address,
 #endif
 
 	prot = __pgprot(pgprot_val(prot) & ~pgprot_val(forbidden));
-	prot = __pgprot(pgprot_val(prot) | pgprot_val(required));
 
 	return prot;
 }