Matt Fleming <matt@xxxxxxxxxxxxxxxxxxx> wrote:
> On Wed, 11 Jan, at 03:27:23PM, David Howells wrote:
> > Matt Fleming <matt@xxxxxxxxxxxxxxxxxxx> 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.
Do you mean the boot loader or the boot wrapper? If the loader, that is
outside my control - and given the purpose of the value, I'm not sure I
want to rely on that.
> 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.
If there's a boot specification that must be complied with, why does
sanitize_boot_params() even exist? Why does the comment on it say:
* Deal with bootloaders which fail to initialize unknown fields in
* boot_params to zero. The list fields in this list are taken from
* analysis of kexec-tools; if other broken bootloaders initialize a
* different set of fields we will need to figure out how to disambiguate.
> It should not be special-cased in sanitize_boot_params(), it should be
> zero'd.
Sigh. sanitize_boot_params() is part of the problem. The startup sequence
goes something like this:
(0) We enter the boot wrapper.
(1) We clear the secure-boot status value [my patch adds this].
(2) The boot wrapper *may* invoke efi_main() - which will determine the
secure-boot status.
(3) The boot wrapper calls extract_kernel() to decompress the kernel.
(4) extract_kernel() calls sanitize_boot_params() which would otherwise clear
the secure-boot flag.
(5) The boot wrapper jumps into the main kernel image, which now does not see
the secure boot status value we calculated.
So, no, sanitize_boot_params() must *not* zero the value unless we change the
call point for s_b_p().
> 2) In this case ->secure_boot should be zero unless modified inside of
> efi_main().
I have no idea whether this is guaranteed or not.
> 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.
I haven't actually checked what the value was before I cleared it. But, I've
found that security people get seriously paranoid about assuming things to be
implicitly so;-).
David
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