Re: [PATCH v9 0/4] unify the keyrings of arm64 and s390 with x86 to verify kexec'ed kernel signature

[Mimi Zohar <zohar@linux.ibm.com>]

On Mon, 2022-07-04 at 09:51 +0800, Coiby Xu wrote:
> Currently when loading a kernel image via the kexec_file_load() system
> call, x86 can make use of three keyrings i.e. the .builtin_trusted_keys,
> .secondary_trusted_keys and .platform keyrings to verify a signature.
> However, arm64 and s390 can only use the .builtin_trusted_keys and
> .platform keyring respectively. For example, one resulting problem is
> kexec'ing a kernel image  would be rejected with the error "Lockdown:
> kexec: kexec of unsigned images is restricted; see man
> kernel_lockdown.7".
> 
> This patch set enables arm64 and s390 to make use of the same keyrings
> as x86 to verify the signature kexec'ed kernel image.
> 
> The recently introduced .machine keyring impacts the roots of trust by
> linking the .machine keyring to the .secondary keyring. The roots of
> trust for different keyrings are described as follows,
> 
> .builtin_trusted_keys:
> 
> Keys may be built into the kernel during build or inserted into memory
> reserved for keys post build. The root of trust is based on verification
> of the kernel image signature. For example, on a physical system in a
> secure boot environment, this trust is rooted in hardware.
> 
> .machine:
> 
> If the end-users choose to trust the keys provided by first-stage UEFI
> bootloader shim i.e. Machine Owner Keys (MOK keys), the keys will be
> added to this keyring which is linked to the .secondary_trusted_keys
> keyring as the same as the .builtin_trusted_keys keyring. Shim has
> built-in keys from a Linux distribution or the end-users-enrolled keys.
> So the root of trust of this keyring is either a Linux distribution
> vendor or the end-users.
> 
> .secondary_trusted_keys:
> 
> Certificates signed by keys on the .builtin_trusted_keys, .machine, or
> existing keys on the .secondary_trusted_keys keryings may be loaded
> onto the .secondary_trusted_keys keyring. This establishes a signature
> chain of trust based on keys loaded on either the .builtin_trusted_keys
> or .machine keyrings, if configured and enabled.
> 
> .platform:
> 
> The .platform keyring consist of UEFI db and MOK keys which are used by
> shim to verify the first boot kernel's image signature. If end-users
> choose to trust MOK keys and the kernel has the .machine keyring
> enabled, the .platform keyring only consists of UEFI db keys since the
> MOK keys are added to the .machine keyring instead. Because the
> end-users could also enroll their own MOK keys, the root of trust could
> be hardware and the end-users.
> 
> The 3rd arm64 patch depends on the first two patches. The 4th s390 patch
> can be applied independently.
> 
> I've tested the patch set on 5.19.0-rc2 and it works as expected. 
> 
> For arm64, the tests were done as follows,
>   1. build 5.19.0-rc2
>   2. generate keys and add them to .secondary_trusted_keys, MOK, UEFI
>      db;
>   3. sign different kernel images with different keys including keys
>      from .builtin_trusted_key, .secondary_trusted_keys keyring, a UEFI db
>      key and MOK key
>   4. Without lockdown, all kernel images can be kexec'ed; with lockdown
>      enabled, only the kernel image signed by the key from the
>      .builtin_trusted_key keyring can be kexec'ed

Just confirming, for arm64, this patch set allows verifying the
kexec'ed kernel image signature using keys on either the .platform or
.secondary_trusted_keys keyrings.

thanks,

Mimi

> 
> For 390, I commented out the code that skips signature verification when
> secure boot is not enabled since the testing machine doesn't supports
> secure boot and can confirm before applying the patch, kernel images
> signed by keys from .builtin_trusted_key, .secondary_trusted_keys
> couldn't be kexec'ed when lockdown is enabled; after applying the patch,
> those kernel images could be kexec'ed.



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