linux-kernel - Re: [PATCH 4/6] kexec: A new system call, kexec_file

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Date:	Tue, 26 Nov 2013 04:23:36 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Matthew Garrett <mjg59@...f.ucam.org>, Greg KH <greg@...ah.com>,
	linux-kernel@...r.kernel.org, kexec@...ts.infradead.org,
	hpa@...or.com, Peter Jones <pjones@...hat.com>
Subject: Re: [PATCH 4/6] kexec: A new system call, kexec_file_load, for in kernel kexec

Vivek Goyal <vgoyal@...hat.com> writes:

> On Fri, Nov 22, 2013 at 07:39:14PM -0800, Eric W. Biederman wrote:
>
> [..]
>> > Hmm..., I am running out of ideas here. This is what I understand.
>> >
>> > - If I sign the bzImage (using PKCS1.5 signature), and later it is signed
>> >   with authenticode format signatures, then PKCS1.5 signatures will not be
>> >   valid as PE/COFF signing will do some modification to PE/COFF header in
>> >   bzImage. And another problem is that then I don't have a way to find
>> >   PKCS1.5 signature.
>> >
>> > - If bzImage is first signed with authenticode format signature and then
>> >   signed using PKCS1.5 signature, then  authenticode format signature
>> >   will become invalid as it will also hash the data appened at the end
>> >   of file.
>> >
>> > So looks like both signatures can't co-exist on same file. That means
>> > one signature has to be detached.
>> >
>> > I am beginning to think that create a kernel option which allows to choose
>> > between attached and detached signatures. Extend kexec syscall to allow
>> > a parameter to pass in detached signatures. If detached signatures are
>> > not passed, then look for signatures at the end of file. That way, those
>> > who are signing kernels using platform specific format (authenticode) in 
>> > this case, they can generate detached signature while others can just
>> > use attached signatures.
>> >
>> > Any thoughts on how this should be handled?
>> 
>> Inside of a modern bzImage there is an embedded ELF image.  How about in
>> userspace we just strip out the embedded ELF image and write that to a
>> file.  Then we can use the same signature checking scheme as we do for
>> kernel modules.  And you only have to support one file format.
>
> I think there is a problem with that. And that we lose the additional
> metadata info present in bzImage which is important.
>
> For example, knowing how much memory kernel will consume before it
> sets up its own GDT and page tables (init_size) is very important. That
> gives image loader lot of flexibility in figuring out where to place rest
> of the components safely (initrd, GDT, page tables, ELF header segment, 
> backup region etc).

The init_size should be reflected in the .bss of the ELF segments.  If
not it is a bug when generating the kernel ELF headers and should be
fixed.

For use by kexec I don't see any issues with just signing the embedded
ELF image.

Eric
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