linux-kernel - Re: [PATCH] x86/efi: always map first physical page into EFI pagetables

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Date:   Fri, 27 Jan 2017 15:12:48 +0000
From:   Matt Fleming <matt@...eblueprint.co.uk>
To:     Jiri Kosina <jkosina@...e.cz>
Cc:     Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Waiman Long <waiman.long@....com>,
        Borislav Petkov <bp@...e.de>,
        Laura Abbott <labbott@...hat.com>,
        Vojtech Pavlik <vojtech@....cz>,
        Hanka Pavlikova <hanka@....cz>, linux-kernel@...r.kernel.org,
        linux-efi@...r.kernel.org
Subject: Re: [PATCH] x86/efi: always map first physical page into EFI
 pagetables

On Wed, 25 Jan, at 09:31:53PM, Jiri Kosina wrote:
> 
> [ CCing mailinglists that got eaten by my newly configured mail setup, 
>   sorry for that ]
> 
> On Wed, 25 Jan 2017, Jiri Kosina wrote:
> 
> > From: Jiri Kosina <jkosina@...e.cz>
> > 
> > Commit 129766708 ("x86/efi: Only map RAM into EFI page tables if in 
> > mixed-mode") stopped creating 1:1 mapping for all RAM in case of running 
> > in native 64bit mode.
> > 
> > It turns out though that there are 64bit EFI implementations in the wild 
> > (this particular problem has been reported on Lenovo Yoga 710-11IKB) which 
> > still make use of first physical page for their own private use (which is 
> > what legacy BIOS used to do, but EFI specification doesn't grant any such 
> > right to EFI BIOS ... oh well).
> > 
> > In case there is no mapping for this particular frame in EFI pagetables, 
> > as soon as firmware tries to make use of it, triple fault occurs and the 
> > system reboots (in case of Yoga 710-11IKB this is very early during boot).
> > 

The thing missing from this paragraph is that the EFI memmap entry
type for this page is EFI_CONVENTIONAL_MEMORY on these Lenovo Yoga's,
i.e. the firmware is telling the kernel that the first page is "free
memory" but will write to it anyway.

> > Fix that by always mapping the first page of physical memory into EFI 
> > pagetables.
> > 
> > Note: just reverting 129766708 is not enough on v4.9-rc1+ to fix the 
> > regression on affected hardware, as commit ab72a27da ("x86/efi: 
> > Consolidate region mapping logic") later made the first physical frame not 
> > to be mapped anyway.
> > 
> > Fixes: 129766708 ("x86/efi: Only map RAM into EFI page tables if in mixed-mode")
> > Cc: stable@...nel.org # v4.8+
> > Cc: Waiman Long <waiman.long@....com>
> > Cc: Borislav Petkov <bp@...e.de>
> > Cc: Laura Abbott <labbott@...hat.com>
> > Cc: Vojtech Pavlik <vojtech@....cz>
> > Reported-by: Hanka Pavlikova <hanka@....cz>
> > Signed-off-by: Jiri Kosina <jkosina@...e.cz>
> > ---
> > 
> > Thanks a lot to Matt for excellent hint how to debug EFI failures
> > 
> >  arch/x86/platform/efi/efi_64.c | 11 +++++++++++
> >  1 file changed, 11 insertions(+)
> > 
> > diff --git a/arch/x86/platform/efi/efi_64.c b/arch/x86/platform/efi/efi_64.c
> > index 319148b..02ae2ab 100644
> > --- a/arch/x86/platform/efi/efi_64.c
> > +++ b/arch/x86/platform/efi/efi_64.c
> > @@ -269,6 +269,17 @@ int __init efi_setup_page_tables(unsigned long pa_memmap, unsigned num_pages)
> >  	efi_scratch.use_pgd = true;
> >  
> >  	/*
> > +	 * Certain firmware versions are way too sentimental and still believe
> > +	 * they are exclusive and unquestionable owners of first physical page.
> > +	 * Create 1:1 mapping for this page to avoid triple faults during early
> > +	 * boot with such firmware.
> > +	 */
> > +	if (kernel_map_pages_in_pgd(pgd, 0x0, 0x0, 1, _PAGE_RW)) {
> > +		pr_err("Failed to create 1:1 mapping of first page\n");
> > +		return 1;
> > +	}
> > +
> > +	/*
> >  	 * When making calls to the firmware everything needs to be 1:1
> >  	 * mapped and addressable with 32-bit pointers. Map the kernel
> >  	 * text and allocate a new stack because we can't rely on the

Could you update the comment above to include two additional points:

  1) We've seen machines that mark the first page as
     EFI_CONVENTIONAL_MEMORY but the firmware will write to it
     during SetVirtualAddressMap() nevertheless.

  2) trim_bios_range() takes care of actually reserving the first page
     and making it unavailable to the memory allocators.