linux-kernel - Re: [PATCH] x86/efi: Map EFI memmap entries in-order at runtime

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Date:	Wed, 9 Sep 2015 11:59:58 +0200
From:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:	Matt Fleming <matt@...eblueprint.co.uk>
Cc:	"linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>,
	Matt Fleming <matt.fleming@...el.com>,
	Borislav Petkov <bp@...e.de>,
	Leif Lindholm <leif.lindholm@...aro.org>,
	Peter Jones <pjones@...hat.com>,
	James Bottomley <JBottomley@...n.com>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	"H. Peter Anvin" <hpa@...or.com>, Dave Young <dyoung@...hat.com>,
	"stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH] x86/efi: Map EFI memmap entries in-order at runtime

On 9 September 2015 at 11:58, Matt Fleming <matt@...eblueprint.co.uk> wrote:
> On Wed, 09 Sep, at 09:37:21AM, Ard Biesheuvel wrote:
>> On 8 September 2015 at 22:37, Matt Fleming <matt@...eblueprint.co.uk> wrote:
>> >
>> > diff --git a/arch/x86/platform/efi/efi.c b/arch/x86/platform/efi/efi.c
>> > index 691b333e0038..a2af35f6093a 100644
>> > --- a/arch/x86/platform/efi/efi.c
>> > +++ b/arch/x86/platform/efi/efi.c
>> > @@ -704,6 +704,44 @@ out:
>> >         return ret;
>> >  }
>> >
>> > +static inline void *efi_map_next_entry_reverse(void *entry)
>> > +{
>> > +       if (!entry)
>> > +               return memmap.map_end - memmap.desc_size;
>> > +
>> > +       entry -= memmap.desc_size;
>> > +       if (entry < memmap.map)
>> > +               return NULL;
>> > +
>> > +       return entry;
>> > +}
>> > +
>> > +static void *efi_map_next_entry(void *entry)
>> > +{
>> > +       bool reverse = false;
>> > +
>> > +       if (!efi_enabled(EFI_OLD_MEMMAP) && efi_enabled(EFI_64BIT)) {
>>
>> Here, you could also test whether the
>> EFI_PROPERTIES_RUNTIME_MEMORY_PROTECTION_NON_EXECUTABLE_PE_DATA bit
>> (sigh) is set
>
> No, leaving this out was intentional because we're already suffering
> from the combinatoral explosion of config options. Introducing more
> code paths is very much the wrong thing to do unless absolutely
> necessary.
>
> If we can get away with using one mapping scheme here, we should.
>
> When trying to debug this code in the future I do not want to be
> thinking "Do you have EFI_PROPERTIES_RUNTIME_OMG_THIS_IS_SILLY bit
> set? because that means we're mapping the runtime regions in a
> different order".
>

OK, point taken. I suppose buggy firmware already has the option of
using EFI_OLD_MEMMAP as a fallback.

-- 
Ard.
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