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Re: [PATCH v2 1/7] x86/kexec: add digest checks
- To: Jan Beulich <jbeulich@xxxxxxxx>, Kevin Lampis <kevin.lampis@xxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Tue, 23 Jun 2026 17:08:38 +0100
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, roger.pau@xxxxxxxxxx, ross.lagerwall@xxxxxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Tue, 23 Jun 2026 16:08:51 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 23/06/2026 3:44 pm, Jan Beulich wrote:
> On 22.06.2026 17:18, Kevin Lampis wrote:
>> From: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
>>
>> To support UEFI Secure Boot we must check that the kexec data has not
>> changed between signature verification and actual execution.
>> However, this is also a good check to perform generally.
>>
>> During kexec load, calculate a digest over all the kexec segments. This
>> digest is stored and verified again later prior to entering the image.
>>
>> For now, only kexec crash images are supported.
>>
>> Signed-off-by: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
>> Signed-off-by: Kevin Lampis <kevin.lampis@xxxxxxxxxx>
> I guess I'm not quite following here. For secure boot purposes, shouldn't
> the new kernel already come with a digest (or really with a signature),
> which we could check in both kexec_load() and kexec_crash()? If we check
> against a digest we calculated ourselves, we'd apply more trust than we
> should.
There are two problems.
One is plain TOCTOU. We load the crash kernel at boot, and we jump to
it in the case that something has fatally-but-not-catastrophically gone
wrong. Really, the digest here is an integrity check.
One is that Xen cannot perform the signature check on the passed
kernel. At least, not without gaining a full X.509 stack and
authenticode algorithm, or a PGP implementation or equivalent.
ExitBootServices() nukes SHIM_LOCK/LOADER protocols so they cannot be
used later in runtime.
Because we are defining the TCB of the system as "Xen + Dom0 kernel",
it's fine to let Dom0 do the certificate check and say "trust me, I
checked this" to Xen (albeit requiring that dom0 userspace can't issue
the hypercall). In the current implementation the dom0 checks the
signature of the binary that userspace proposes, and either rejects it
or passes it forward to Xen.
All Xen needs to do is ensure that the bytes the dom0 kernel said were
good are still the same bytes we're about to jump into. Hence the
integrity check, which is a good move even in the non UEFI-SB case.
~Andrew
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