Re: [PATCH v5 2/6] powerpc/kexec_file: Add KEXEC_SIG support.

[Paul Menzel <pmenzel@xxxxxxxxxxxxx>]

Dear Michal,


Thank you for the patch.


Am 11.01.22 um 12:37 schrieb Michal Suchanek:

Could you please remove the dot/period at the end of the git commit 
message summary?

> Copy the code from s390x
>
> Both powerpc and s390x use appended signature format (as opposed to EFI
> based patforms using PE format).

patforms → platforms

How can this be tested?

> Signed-off-by: Michal Suchanek <msuchanek@xxxxxxx>
> ---
> v3: - Philipp Rudo <prudo@xxxxxxxxxx>: Update the comit message with
>        explanation why the s390 code is usable on powerpc.
>      - Include correct header for mod_check_sig
>      - Nayna <nayna@xxxxxxxxxxxxxxxxxx>: Mention additional IMA features
>        in kconfig text
> ---
>   arch/powerpc/Kconfig        | 16 ++++++++++++++++
>   arch/powerpc/kexec/elf_64.c | 36 ++++++++++++++++++++++++++++++++++++
>   2 files changed, 52 insertions(+)
>
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index dea74d7717c0..1cde9b6c5987 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -560,6 +560,22 @@ config KEXEC_FILE
>   config ARCH_HAS_KEXEC_PURGATORY
>   	def_bool KEXEC_FILE
>   
> +config KEXEC_SIG
> +	bool "Verify kernel signature during kexec_file_load() syscall"
> +	depends on KEXEC_FILE && MODULE_SIG_FORMAT
> +	help
> +	  This option makes kernel signature verification mandatory for
> +	  the kexec_file_load() syscall.
> +
> +	  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.
> +
> +	  Note: on powerpc IMA_ARCH_POLICY also implements kexec'ed kernel
> +	  verification. In addition IMA adds kernel hashes to the measurement
> +	  list, extends IMA PCR in the TPM, and implements kernel image
> +	  blacklist by hash.

So, what is the takeaway for the user? IMA_ARCH_POLICY is preferred? 
What is the disadvantage, and two implementations(?) needed then? More 
overhead?

> +
>   config RELOCATABLE
>   	bool "Build a relocatable kernel"
>   	depends on PPC64 || (FLATMEM && (44x || FSL_BOOKE))
> diff --git a/arch/powerpc/kexec/elf_64.c b/arch/powerpc/kexec/elf_64.c
> index eeb258002d1e..98d1cb5135b4 100644
> --- a/arch/powerpc/kexec/elf_64.c
> +++ b/arch/powerpc/kexec/elf_64.c
> @@ -23,6 +23,7 @@
>   #include <linux/of_fdt.h>
>   #include <linux/slab.h>
>   #include <linux/types.h>
> +#include <linux/module_signature.h>
>   
>   static void *elf64_load(struct kimage *image, char *kernel_buf,
>   			unsigned long kernel_len, char *initrd,
> @@ -151,7 +152,42 @@ static void *elf64_load(struct kimage *image, char *kernel_buf,
>   	return ret ? ERR_PTR(ret) : NULL;
>   }
>   
> +#ifdef CONFIG_KEXEC_SIG
> +int elf64_verify_sig(const char *kernel, unsigned long kernel_len)
> +{
> +	const unsigned long marker_len = sizeof(MODULE_SIG_STRING) - 1;
> +	struct module_signature *ms;
> +	unsigned long sig_len;

Use size_t to match the signature of `verify_pkcs7_signature()`?

> +	int ret;
> +
> +	if (marker_len > kernel_len)
> +		return -EKEYREJECTED;
> +
> +	if (memcmp(kernel + kernel_len - marker_len, MODULE_SIG_STRING,
> +		   marker_len))
> +		return -EKEYREJECTED;
> +	kernel_len -= marker_len;
> +
> +	ms = (void *)kernel + kernel_len - sizeof(*ms);
> +	ret = mod_check_sig(ms, kernel_len, "kexec");
> +	if (ret)
> +		return ret;
> +
> +	sig_len = be32_to_cpu(ms->sig_len);
> +	kernel_len -= sizeof(*ms) + sig_len;
> +
> +	return verify_pkcs7_signature(kernel, kernel_len,
> +				      kernel + kernel_len, sig_len,
> +				      VERIFY_USE_PLATFORM_KEYRING,
> +				      VERIFYING_MODULE_SIGNATURE,
> +				      NULL, NULL);
> +}
> +#endif /* CONFIG_KEXEC_SIG */
> +
>   const struct kexec_file_ops kexec_elf64_ops = {
>   	.probe = kexec_elf_probe,
>   	.load = elf64_load,
> +#ifdef CONFIG_KEXEC_SIG
> +	.verify_sig = elf64_verify_sig,
> +#endif
>   };


Kind regards,

Paul