Re: [PATCH 27/27] kexec: Allow kexec_file() with appropriate IMA policy when locked down

[Mimi Zohar <zohar@linux.ibm.com>]

On Mon, 2019-03-25 at 15:09 -0700, Matthew Garrett wrote:
> Systems in lockdown mode should block the kexec of untrusted kernels.
> For x86 and ARM we can ensure that a kernel is trustworthy by validating
> a PE signature, but this isn't possible on other architectures. On those
> platforms we can use IMA digital signatures instead. Add a function to
> determine whether IMA has or will verify signatures for a given event type,
> and if so permit kexec_file() even if the kernel is otherwise locked down.
> This is restricted to cases where CONFIG_INTEGRITY_TRUSTED_KEYRING is set
> in order to prevent an attacker from loading additional keys at runtime.
> 
> Signed-off-by: Matthew Garrett <mjg59@google.com>
> Cc: Mimi Zohar <zohar@linux.ibm.com>
> Cc: Dmitry Kasatkin <dmitry.kasatkin@gmail.com>
> Cc: linux-integrity@vger.kernel.org

Acked-by: Mimi Zohar <zohar@linux.ibm.com>

> ---
>  include/linux/ima.h                 |  9 ++++++
>  kernel/kexec_file.c                 |  7 +++-
>  security/integrity/ima/ima.h        |  2 ++
>  security/integrity/ima/ima_main.c   |  2 +-
>  security/integrity/ima/ima_policy.c | 50 +++++++++++++++++++++++++++++
>  5 files changed, 68 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/ima.h b/include/linux/ima.h
> index b5e16b8c50b7..05921227d700 100644
> --- a/include/linux/ima.h
> +++ b/include/linux/ima.h
> @@ -127,4 +127,13 @@ static inline int ima_inode_removexattr(struct dentry *dentry,
>  	return 0;
>  }
>  #endif /* CONFIG_IMA_APPRAISE */
> +
> +#if defined(CONFIG_IMA_APPRAISE) && defined(CONFIG_INTEGRITY_TRUSTED_KEYRING)
> +extern bool ima_appraise_signature(enum kernel_read_file_id func);
> +#else
> +static inline bool ima_appraise_kexec_signature(enum kernel_read_file_id func)
> +{
> +	return false;
> +}
> +#endif /* CONFIG_IMA_APPRAISE && CONFIG_INTEGRITY_TRUSTED_KEYRING */
>  #endif /* _LINUX_IMA_H */
> diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
> index 0cfe4f6f7f85..8ffa4b75c620 100644
> --- a/kernel/kexec_file.c
> +++ b/kernel/kexec_file.c
> @@ -240,7 +240,12 @@ kimage_file_prepare_segments(struct kimage *image, int kernel_fd, int initrd_fd,
>  
>  		ret = 0;
>  
> -		if (kernel_is_locked_down(reason)) {
> +		/* If IMA is guaranteed to appraise a signature on the kexec
> +		 * image, permit it even if the kernel is otherwise locked
> +		 * down.
> +		 */
> +		if (!ima_appraise_signature(READING_KEXEC_IMAGE) &&
> +		    kernel_is_locked_down(reason)) {
>  			ret = -EPERM;
>  			goto out;
>  		}
> diff --git a/security/integrity/ima/ima.h b/security/integrity/ima/ima.h
> index cc12f3449a72..fe03cc6f1ca4 100644
> --- a/security/integrity/ima/ima.h
> +++ b/security/integrity/ima/ima.h
> @@ -115,6 +115,8 @@ struct ima_kexec_hdr {
>  	u64 count;
>  };
>  
> +extern const int read_idmap[];
> +
>  #ifdef CONFIG_HAVE_IMA_KEXEC
>  void ima_load_kexec_buffer(void);
>  #else
> diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
> index 4ffac4f5c647..106f06dee9d1 100644
> --- a/security/integrity/ima/ima_main.c
> +++ b/security/integrity/ima/ima_main.c
> @@ -442,7 +442,7 @@ int ima_read_file(struct file *file, enum kernel_read_file_id read_id)
>  	return 0;
>  }
>  
> -static const int read_idmap[READING_MAX_ID] = {
> +const int read_idmap[READING_MAX_ID] = {
>  	[READING_FIRMWARE] = FIRMWARE_CHECK,
>  	[READING_FIRMWARE_PREALLOC_BUFFER] = FIRMWARE_CHECK,
>  	[READING_MODULE] = MODULE_CHECK,
> diff --git a/security/integrity/ima/ima_policy.c b/security/integrity/ima/ima_policy.c
> index 122797023bdb..f8f1cdb74a4f 100644
> --- a/security/integrity/ima/ima_policy.c
> +++ b/security/integrity/ima/ima_policy.c
> @@ -1341,3 +1341,53 @@ int ima_policy_show(struct seq_file *m, void *v)
>  	return 0;
>  }
>  #endif	/* CONFIG_IMA_READ_POLICY */
> +
> +#if defined(CONFIG_IMA_APPRAISE) && defined(CONFIG_INTEGRITY_TRUSTED_KEYRING)
> +/*
> + * ima_appraise_signature: whether IMA will appraise a given function using
> + * an IMA digital signature. This is restricted to cases where the kernel
> + * has a set of built-in trusted keys in order to avoid an attacker simply
> + * loading additional keys.
> + */
> +bool ima_appraise_signature(enum kernel_read_file_id id)
> +{
> +	struct ima_rule_entry *entry;
> +	bool found = false;
> +	enum ima_hooks func;
> +
> +	if (id >= READING_MAX_ID)
> +		return false;
> +
> +	func = read_idmap[id] ?: FILE_CHECK;
> +
> +	rcu_read_lock();
> +	list_for_each_entry_rcu(entry, ima_rules, list) {
> +		if (entry->action != APPRAISE)
> +			continue;
> +
> +		/*
> +		 * A generic entry will match, but otherwise require that it
> +		 * match the func we're looking for
> +		 */
> +		if (entry->func && entry->func != func)
> +			continue;
> +
> +		/*
> +		 * We require this to be a digital signature, not a raw IMA
> +		 * hash.
> +		 */
> +		if (entry->flags & IMA_DIGSIG_REQUIRED)
> +			found = true;
> +
> +		/*
> +		 * We've found a rule that matches, so break now even if it
> +		 * didn't require a digital signature - a later rule that does
> +		 * won't override it, so would be a false positive.
> +		 */
> +		break;
> +	}
> +
> +	rcu_read_unlock();
> +	return found;
> +}
> +#endif /* CONFIG_IMA_APPRAISE && CONFIG_INTEGRITY_TRUSTED_KEYRING */