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

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Message-ID: <20181004080321.GA3630@8bytes.org>
Date:   Thu, 4 Oct 2018 10:03:21 +0200
From:   Joerg Roedel <joro@...tes.org>
To:     Borislav Petkov <bp@...en8.de>
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 Wed, Oct 03, 2018 at 11:22:55PM +0200, Borislav Petkov wrote:
> On Fri, Sep 28, 2018 at 04:55:19PM +0200, Thomas Gleixner wrote:
> > Sorry for the delay and thanks for the data. A quick diff did not reveal
> > anything obvious. I'll have a closer look and we probably need more (other)
> > information to nail that down.

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.


Regards,

	Joerg