linux-kernel - Re: [PATCH 5/8] efi: Get the secure boot status [ver #6]

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Date:   Mon, 16 Jan 2017 14:49:54 +0000
From:   Matt Fleming <matt@...eblueprint.co.uk>
To:     David Howells <dhowells@...hat.com>
Cc:     ard.biesheuvel@...aro.org, linux-efi@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org, keyrings@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        "H. Peter Anvin" <hpa@...or.com>, Peter Jones <pjones@...hat.com>
Subject: Re: [PATCH 5/8] efi: Get the secure boot status [ver #6]

(Cc'ing Peter A. and Peter J. for boot params discussion)

On Wed, 11 Jan, at 03:27:23PM, David Howells wrote:
> Matt Fleming <matt@...eblueprint.co.uk> wrote:
> 
> > > +	movb	$0, BP_secure_boot(%rsi)
> > >  #ifdef CONFIG_EFI_STUB
> > >  	/*
> > >  	 * The entry point for the PE/COFF executable is efi_pe_entry, so
> > 
> > Is clearing ::secure_boot really necessary? Any code path that goes
> > via efi_main() will set it correctly and all other code paths should
> > get it cleared in sanitize_boot_params(), no?
> 
> No.
> 
> The boot_params->secure_boot parameter exists whether or not efi_main() is
> traversed (ie. if EFI isn't enabled or CONFIG_EFI_STUB=n) and, if not cleared,
> is of uncertain value.
>
> Further, sanitize_boot_params() has to be modified by this patch so as not to
> clobber the secure_boot flag.

Any new parameters that boot loaders do not know about should be
cleared to zero by default in the boot loader because boot_params
itself should be zero'd when allocated.

There are two cases to consider:

 1) boot_params is not zero'd
 2) boot_params is zero'd

1) This is a broken boot loader implementation that violates the x86
boot specification and I would never expect ->secure_boot to have a
valid value. It should not be special-cased in sanitize_boot_params(),
it should be zero'd.

2) In this case ->secure_boot should be zero unless modified inside of
efi_main().

Did you hit the scenario where ->secure_boot has a garbage value while
developing these patches? I wouldn't expect to see it in practice.