linux-kernel - Re: [PATCH V3 3/5] x86/efi: Permanently save the EFI_MEMORY

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Date:   Wed, 5 Sep 2018 14:56:45 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>,
        linux-efi <linux-efi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        "Neri, Ricardo" <ricardo.neri@...el.com>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        Al Stone <astone@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Ingo Molnar <mingo@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Bhupesh Sharma <bhsharma@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH V3 3/5] x86/efi: Permanently save the EFI_MEMORY_MAP
 passed by the firmware

On Wed, Sep 05, 2018 at 02:27:49PM +0200, Ard Biesheuvel wrote:
> On 5 September 2018 at 00:12, Sai Praneeth Prakhya
> <sai.praneeth.prakhya@...el.com> wrote:
> > From: Sai Praneeth <sai.praneeth.prakhya@...el.com>
> >
> > The efi page fault handler that recovers from page faults caused by the
> > firmware needs the original memory map passed by the firmware. It looks
> > up this memory map to find the type of the memory region at which the
> > page fault occurred. Presently, EFI subsystem discards the original
> > memory map passed by the firmware and replaces it with a new memory map
> > that has only EFI_RUNTIME_SERVICES_<CODE/DATA> regions. But illegal
> > accesses by firmware can occur at any region. Hence, _only_ if
> > CONFIG_EFI_WARN_ON_ILLEGAL_ACCESS is defined, create a backup of the
> > original memory map passed by the firmware, so that efi page fault
> > handler could detect/recover from illegal accesses to *any* efi region.
> >
> 
> Why do we care about the memory map at all when a fault occurs during
> the invocation of a EFI runtime service?
> 
> I think reasoning about what went wrong and why, and distinguishing
> between allowable and non-allowable faults is a slippery slope, so
> [taking Thomas's feedback into account], I think we can simplify this
> series further and just block all subsequent EFI runtime services
> calls if any permission or page fault occurs while executing them.
> 
> Would we still need to preserve the old memory map in that case?

I thought the reason for having this was being able to know the fault is
in an EFI area. But of course, I'm not wel versed in this whole EFI
crapola.