linux-kernel - Re: [PATCH v16 15/16] arm64: kexec_file: add kernel signature verification support

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Date:   Fri, 30 Nov 2018 13:21:14 +0000
From:   Will Deacon <will.deacon@....com>
To:     AKASHI Takahiro <takahiro.akashi@...aro.org>
Cc:     catalin.marinas@....com, dhowells@...hat.com, vgoyal@...hat.com,
        herbert@...dor.apana.org.au, davem@...emloft.net,
        dyoung@...hat.com, bhe@...hat.com, arnd@...db.de,
        schwidefsky@...ibm.com, heiko.carstens@...ibm.com,
        prudo@...ux.ibm.com, ard.biesheuvel@...aro.org,
        james.morse@....com, bhsharma@...hat.com,
        kexec@...ts.infradead.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v16 15/16] arm64: kexec_file: add kernel signature
 verification support

On Thu, Nov 15, 2018 at 02:52:54PM +0900, 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
> 
> Signed-off-by: AKASHI Takahiro <takahiro.akashi@...aro.org>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Will Deacon <will.deacon@....com>
> Reviewed-by: James Morse <james.morse@....com>
> ---
>  arch/arm64/Kconfig              | 24 ++++++++++++++++++++++++
>  arch/arm64/kernel/kexec_image.c | 17 +++++++++++++++++
>  2 files changed, 41 insertions(+)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 93dc4d36d6db..11f3e1a00588 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -867,6 +867,30 @@ 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 "Support for PE file signature verification disabled"
> +	depends on KEXEC_VERIFY_SIG
> +	depends on !EFI || !SIGNED_PE_FILE_VERIFICATION
> +
>  config CRASH_DUMP
>  	bool "Build kdump crash kernel"
>  	help
> diff --git a/arch/arm64/kernel/kexec_image.c b/arch/arm64/kernel/kexec_image.c
> index 9f0d8b5d62d3..d1c6c54c22e3 100644
> --- a/arch/arm64/kernel/kexec_image.c
> +++ b/arch/arm64/kernel/kexec_image.c
> @@ -12,7 +12,9 @@
>  #include <linux/errno.h>
>  #include <linux/kernel.h>
>  #include <linux/kexec.h>
> +#include <linux/pe.h>
>  #include <linux/string.h>
> +#include <linux/verification.h>
>  #include <asm/byteorder.h>
>  #include <asm/cpufeature.h>
>  #include <asm/image.h>
> @@ -29,6 +31,10 @@ static int image_probe(const char *kernel_buf, unsigned long kernel_len)
>  				sizeof(h->magic)))
>  		return -EINVAL;
>  
> +	pr_debug("PE format: %s\n",
> +			memcmp(&((struct mz_hdr *)h)->magic, "MZ", 2) ?
> +							"no" : "yes");
> 

Is this hunk really necessary? I'd prefer not to commit pr_debug()
invocations.

Will