Re: [PATCH v3 2/2] kexec, KEYS: Make use of platform keyring for signature verify

[Mimi Zohar]

On Wed, 2019-01-16 at 18:16 +0800, Kairui Song wrote:
> With KEXEC_BZIMAGE_VERIFY_SIG enabled, kexec_file_load will need to
> verify the kernel image. 

The kexec_file_load syscall can verify the PE signed kernel image
signature, the kernel image signature stored as an xattr, or both.

Anyone booting the system with the "appraise_tcb" policy or with a
similar appraise policy rule, wanting to only verifying the PE signed
kernel image, will need to include a "dont_appraise
func=KEXEC_KERNEL_CHECK" rule in their custom policy.

> The image might be signed with third part keys,
> and the keys could be stored in firmware, then got loaded into the
> .platform keyring. Now we have a symbol .platform_trusted_keyring as the
> reference to .platform keyring, this patch makes use if it and allow
> kexec_file_load to verify the image against keys in .platform keyring.

There's no need to introduce the concept of "third party" or "firmware
keys" here. ÂReferring to them as the "preboot" keys, can simplify the
above paragraph.

> 
> This commit adds a VERIFY_USE_PLATFORM_KEYRING similar to previous
> VERIFY_USE_SECONDARY_KEYRING indicating that verify_pkcs7_signature
> should verify the signature using platform keyring. Also, decrease
> the error message log level when verification failed with -ENOKEY,
> so that if called tried multiple time with different keyring it
> won't generate extra noises.
> 
> Signed-off-by: Kairui Song <kasong@xxxxxxxxxx>

Reviewed/Tested-by: Mimi Zohar <zohar@xxxxxxxxxxxxx>

> ---
>  arch/x86/kernel/kexec-bzimage64.c | 13 ++++++++++---
>  certs/system_keyring.c            | 13 ++++++++++++-
>  include/linux/verification.h      |  1 +
>  3 files changed, 23 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kernel/kexec-bzimage64.c b/arch/x86/kernel/kexec-bzimage64.c
> index 7d97e432cbbc..2c007abd3d40 100644
> --- a/arch/x86/kernel/kexec-bzimage64.c
> +++ b/arch/x86/kernel/kexec-bzimage64.c
> @@ -534,9 +534,16 @@ static int bzImage64_cleanup(void *loader_data)
>  #ifdef CONFIG_KEXEC_BZIMAGE_VERIFY_SIG
>  static int bzImage64_verify_sig(const char *kernel, unsigned long kernel_len)
>  {
> -	return verify_pefile_signature(kernel, kernel_len,
> -				       VERIFY_USE_SECONDARY_KEYRING,
> -				       VERIFYING_KEXEC_PE_SIGNATURE);
> +	int ret;
> +	ret = verify_pefile_signature(kernel, kernel_len,
> +				      VERIFY_USE_SECONDARY_KEYRING,
> +				      VERIFYING_KEXEC_PE_SIGNATURE);
> +	if (ret == -ENOKEY && IS_ENABLED(CONFIG_INTEGRITY_PLATFORM_KEYRING)) {
> +		ret = verify_pefile_signature(kernel, kernel_len,
> +					      VERIFY_USE_PLATFORM_KEYRING,
> +					      VERIFYING_KEXEC_PE_SIGNATURE);
> +	}
> +	return ret;
>  }
>  #endif
>  
> diff --git a/certs/system_keyring.c b/certs/system_keyring.c
> index 4690ef9cda8a..7085c286f4bd 100644
> --- a/certs/system_keyring.c
> +++ b/certs/system_keyring.c
> @@ -240,11 +240,22 @@ int verify_pkcs7_signature(const void *data, size_t len,
>  #else
>  		trusted_keys = builtin_trusted_keys;
>  #endif
> +	} else if (trusted_keys == VERIFY_USE_PLATFORM_KEYRING) {
> +#ifdef CONFIG_INTEGRITY_PLATFORM_KEYRING
> +		trusted_keys = platform_trusted_keys;
> +#else
> +		trusted_keys = NULL;
> +#endif
> +		if (!trusted_keys) {
> +			ret = -ENOKEY;
> +			pr_devel("PKCS#7 platform keyring is not available\n");
> +			goto error;
> +		}
>  	}
>  	ret = pkcs7_validate_trust(pkcs7, trusted_keys);
>  	if (ret < 0) {
>  		if (ret == -ENOKEY)
> -			pr_err("PKCS#7 signature not signed with a trusted key\n");
> +			pr_devel("PKCS#7 signature not signed with a trusted key\n");
>  		goto error;
>  	}
>  
> diff --git a/include/linux/verification.h b/include/linux/verification.h
> index cfa4730d607a..018fb5f13d44 100644
> --- a/include/linux/verification.h
> +++ b/include/linux/verification.h
> @@ -17,6 +17,7 @@
>   * should be used.
>   */
>  #define VERIFY_USE_SECONDARY_KEYRING ((struct key *)1UL)
> +#define VERIFY_USE_PLATFORM_KEYRING  ((struct key *)2UL)
>  
>  /*
>   * The use to which an asymmetric key is being put.