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

[Konrad Rzeszutek Wilk]

On Sun, Jan 23, 2011 at 03:27:39PM +0100, matthieu castet wrote:
> 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. 

I would say get rid of the printk. It does not really help the users. Here is an excerpt of
2.6.38-rc2 with this patch:

   7.247448] NX-protecting the kernel data: 2412k
[    7.252489] RO page for 0xc15a0000 in bss/data.
[    7.253052] RO page for 0xc15a1000 in bss/data.
[    7.253052] RO page for 0xc15a3000 in bss/data.
[    7.365104] mv used greatest stack depth: 6616 bytes left

So Tested-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>

(I tested on baremetal x86,x86_64 and Xen x86 and x86_64)
> 
> 
> Matthieu


> 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;
>  }

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