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SubjectRe: [PATCH v10 13/14] arm64: kexec_file: add kernel signature verification support
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Hi Akashi,

On 23/06/18 03:20, AKASHI Takahiro wrote:
> With this patch, kernel verification can be done without IMA security
> subsystem enabled. Turn on CONFIG_KEXEC_VERIFY_SIG instead.
>
> On x86, a signature is embedded into a PE file (Microsoft's format) header
> of binary. Since arm64's "Image" can also be seen as a PE file as far as
> CONFIG_EFI is enabled, we adopt this format for kernel signing.
>
> You can create a signed kernel image with:
> $ sbsign --key ${KEY} --cert ${CERT} Image

> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index f68318f61c85..5133c22a01ab 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -845,6 +845,30 @@ config KEXEC_FILE
> for kernel and initramfs as opposed to list of segments as
> accepted by previous system call.
>
> +config KEXEC_VERIFY_SIG
> + bool "Verify kernel signature during kexec_file_load() syscall"
> + depends on KEXEC_FILE
> + help
> + Select this option to verify a signature with loaded kernel
> + image. If configured, any attempt of loading a image without
> + valid signature will fail.
> +
> + In addition to that option, you need to enable signature
> + verification for the corresponding kernel image type being
> + loaded in order for this to work.
> +
> +config KEXEC_IMAGE_VERIFY_SIG
> + bool "Enable Image signature verification support"
> + default y
> + depends on KEXEC_VERIFY_SIG
> + depends on EFI && SIGNED_PE_FILE_VERIFICATION
> + help
> + Enable Image signature verification support.
> +
> +comment "Image signature verification is missing yet"
> + depends on KEXEC_VERIFY_SIG
> + depends on !EFI || !SIGNED_PE_FILE_VERIFICATION


This comment thing is a good idea, but its also a bit confusing... it took me
quite a while to work out what was missing. Could we phrase it something like:
"Support for PE file signature verification disabled!"
This tells us its about PE files, and its probably a missing config option
somewhere, not some code that hasn't been written yet. (which was my first
assumption!).

KEXEC_VERIFY_SIG presumably turns on just the IMA verification, which verifies
the Image, but not in the same way as KEXEC_IMAGE_VERIFY_SIG.... (if I've
understood it properly) Is there any reason to have these as separate enables?
Couldn't we 'select SIGNED_PE_FILE_VERIFICATION if EFI' in KEXEC_VERIFY_SIG?
This would mean there is one option to verify signatures, instead of two...
(does it really depend on EFI?)


Thanks,

James

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