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

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Date:   Thu, 4 Oct 2018 10:14:38 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Joerg Roedel <joro@...tes.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Paul Menzel <pmenzel@...gen.mpg.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

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.

-- 
Regards/Gruss,
    Boris.

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