linux-kernel - RE: [PATCH V1 6/6] x86/efi: Introduce EFI_WARN_ON_ILLEGAL

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Date:   Fri, 10 Aug 2018 17:34:04 +0000
From:   "Prakhya, Sai Praneeth" <sai.praneeth.prakhya@...el.com>
To:     Bhupesh Sharma <bhsharma@...hat.com>
CC:     "linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        "ricardo.ner@...el.com" <ricardo.ner@...el.com>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        Lee Chun-Yi <jlee@...e.com>, Al Stone <astone@...hat.com>,
        Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>
Subject: RE: [PATCH V1 6/6] x86/efi: Introduce EFI_WARN_ON_ILLEGAL_ACCESSES

> > +config EFI_WARN_ON_ILLEGAL_ACCESSES
> 
> How about shortening the name to just 'EFI_WARN_ON_ILLEGAL_ACCESS'?
> 

Makes sense.. I will shorten it.

> > +       bool "Warn about illegal memory accesses by firmware"
> > +       depends on EFI
> 
> From a distribution p-o-v, I would suggest that we set this CONFIG option only if
> we are in the EXPERT mode, as this need more thrashing with the behaviour we
> see on old, buggy EFI firmwares available on very old x86 machines. So
> something like:
>            bool "Warn about illegal memory accesses by firmware" if EXPERT
> 

Agreed. Although the feature is intended to warn about all these buggy firmware's
instead of silently working around (as we are doing presently), it needs more testing.
Hence, as you said, I think it's safe to have it as an EXPERT config option.

> > +       help
> > +         Enable this debug feature so that the kernel can detect illegal
> > +         memory accesses by firmware and issue a warning. Also,
> > +         1. If the illegally accessed region is EFI_BOOT_SERVICES_<CODE/DATA>,
> > +            the kernel fixes it up by mapping the requested region.

[....]

> > diff --git a/init/main.c b/init/main.c index
> > 3b4ada11ed52..dce0520861a1 100644
> > --- a/init/main.c
> > +++ b/init/main.c
> > @@ -730,7 +730,8 @@ asmlinkage __visible void __init start_kernel(void)
> >         arch_post_acpi_subsys_init();
> >         sfi_init_late();
> >
> > -       if (efi_enabled(EFI_RUNTIME_SERVICES)) {
> > +       if (efi_enabled(EFI_RUNTIME_SERVICES) &&
> > +           !IS_ENABLED(CONFIG_EFI_WARN_ON_ILLEGAL_ACCESSES)) {
> 
> Since this is an arch agnostic code leg, do we really want to introduce a generic
> check for CONFIG_EFI_WARN_ON_ILLEGAL_ACCESSES
> here, without checking for whether we are actually running this on a
> x86 hardware, or alternatively we can consider moving
> CONFIG_EFI_WARN_ON_ILLEGAL_ACCESSES out of 'arch/x86/Kconfig' to
> 'arch/Kconfig' so that later it can be used on other archs like ARM64 as well.
> 
> I think the later would be more useful..

Thanks for bringing this up. I see your point. I will refrain from polluting architecture
agnostic code. As we don't need efi_free_boot_services() at all, if EFI_WARN_ON_ILLEGAL_ACCESSES
is enabled, probably making changes to include/linux/efi.h would be better, I guess..

Moving this to arch/Kconfig could be too futuristic.. I guess.. because I am not sure
if this problem exists on ARM machines, if so, probably Ard could suggest something here.

Regards,
Sai