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

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Message-ID: <CAKv+Gu_DC=+7_YmDGGJ9fYJkGSNbUK-eXOAGMuZ2B7XXPWvZ9g@mail.gmail.com>
Date:   Wed, 5 Sep 2018 15:03:52 +0200
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Peter Zijlstra <peterz@...radead.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 5 September 2018 at 14:56, Peter Zijlstra <peterz@...radead.org> wrote:
> 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.

I'm not entirely sure whether that really matters. The EFI services
access the stack and can access byref/pointer arguments which are not
covered by the EFI memory map as runtime services code/data, and so
they can trigger page faults by running off the vmapped stack or
writing to const byref arguments.

EFI runtime services using boot services regions after they are no
longer available are a known source of headaches, but I don't see why
we should restrict ourselves to such cases if we bother to wire up
fault handling specifically for EFI services calls.

So any page or permission fault occurring in the context of a EFI
runtime services invocation should be treated the same, I think.