linux-kernel - Re: [PATCH v10 13/14] arm64: kexec_file: add kernel signature verification support

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Message-ID: <ff77f2d9-82c6-8fbb-3696-e1fcfd61c333@arm.com>
Date:   Tue, 3 Jul 2018 18:47:38 +0100
From:   James Morse <james.morse@....com>
To:     AKASHI Takahiro <takahiro.akashi@...aro.org>
Cc:     catalin.marinas@....com, will.deacon@....com, dhowells@...hat.com,
        vgoyal@...hat.com, herbert@...dor.apana.org.au,
        davem@...emloft.net, dyoung@...hat.com, bhe@...hat.com,
        arnd@...db.de, ard.biesheuvel@...aro.org, bhsharma@...hat.com,
        kexec@...ts.infradead.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v10 13/14] arm64: kexec_file: add kernel signature
 verification support

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