linux-kernel - Re: [PATCH] x86/acpi: check rsdp address received via bootparams to be valid

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Date:   Tue, 16 Jan 2018 17:35:29 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Juergen Gross <jgross@...e.com>
Cc:     linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org,
        x86@...nel.org, lenb@...nel.org, rafael.j.wysocki@...el.com,
        hpa@...or.com, tglx@...utronix.de, mingo@...hat.com
Subject: Re: [PATCH] x86/acpi: check rsdp address received via bootparams to
 be valid


* Juergen Gross <jgross@...e.com> wrote:

> On 16/01/18 16:46, Ingo Molnar wrote:
> > 
> > * Juergen Gross <jgross@...e.com> wrote:
> > 
> >> There seem to exist several grub2 versions trashing
> >> boot_params.hdr.acpi_rsdp_addr.
> >>
> >> So don't just believe this address to be valid, but verify it pointing
> >> to a valid RSDP table.
> > 
> > Exactly what kind of values do those Grub2 versions put into that field? Pointer 
> > to something, or random noise?
> 
> Looks like random noise. On Mike's system it was 0x000000000151.
> 
> > Also, what exactly does 'validation' mean, how robustly does it filter out of spec 
> > uses of the boot protocol?
> 
> It validates the RSDP has the correct 8 byte eye catcher in it and
> the checksum of the structure is correct. Searching the RSDP by
> scanning memory is using the same checks, so I guess this ought to
> be okay. Odds are about 1 : 2^80 for false positives.

Ok, this should work - but only because the RSDP is defined in such a robust 
fashion.

The boot protocol extension is still fragile: what I worry about is that if we 
start relying on the extended boot protocol with widespread installed base of out 
of spec Grub2 loaders, other extensions (which cannot be sanity checked) would be 
less robust.

Is there a way to detect the broken Grub2 versions somehow and just limit the boot 
protocol for them?

The other solution would be to just discontinue this boot protocol extension and 
define a new one.

Thanks,

	Ingo