linux-kernel - Re: x86/mm: Found insecure W+X mapping at address (ptrval)/0xc00a0000

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Date:   Thu, 4 Oct 2018 10:40:49 +0200
From:   Paul Menzel <pmenzel@...gen.mpg.de>
To:     Borislav Petkov <bp@...en8.de>,
        Jörg Rödel <joro@...tes.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>, linux-mm@...ck.org,
        x86@...nel.org, lkml <linux-kernel@...r.kernel.org>
Subject: Re: x86/mm: Found insecure W+X mapping at address (ptrval)/0xc00a0000

Dear Borislav,


On 10/04/18 10:14, Borislav Petkov wrote:
> On Thu, Oct 04, 2018 at 10:03:21AM +0200, Joerg Roedel wrote:
>> I also triggered this when working in the PTI-x32 code. It always
>> happens on a 32-bit PAE kernel for me.
>>
>> Tracking it down I ended up in (iirc) arch/x86/mm/pageattr.c
>> 	function static_protections():
>>
>> 		/*
>> 		 * The BIOS area between 640k and 1Mb needs to be executable for
>> 		 * PCI BIOS based config access (CONFIG_PCI_GOBIOS) support.
>> 		 */
>> 	#ifdef CONFIG_PCI_BIOS
>> 		if (pcibios_enabled && within(pfn, BIOS_BEGIN >> PAGE_SHIFT, BIOS_END >> PAGE_SHIFT))
>> 			pgprot_val(forbidden) |= _PAGE_NX;
>> 	#endif
>>
>> I think that is the reason we are seeing this in that configuration.
> 
> So looking at this, BIOS_BEGIN and BIOS_END is the same range as the ISA
> range:
> 
> #define ISA_START_ADDRESS       0x000a0000
> #define ISA_END_ADDRESS         0x00100000
> 
> #define BIOS_BEGIN              0x000a0000
> #define BIOS_END                0x00100000
> 
> and I did try marking the ISA range RO in mark_rodata_ro() but the 
> machine wouldn't boot after. So I'm guessing BIOS needs to write
> there some crap.

Do you have a commit, I could test. My board is supported by coreboot,
and maybe it boots, so I can check, if the warning is gone.


Kind regards,

Paul


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