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SubjectRe: [PATCH 4/6] kexec: A new system call, kexec_file_load, for in kernel kexec
Vivek Goyal <vgoyal@redhat.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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