Re: [PATCH v5 3/8] ima: based on policy require signed kexec kernel images

[J Freyensee]

On 7/2/18 7:37 AM, Mimi Zohar wrote:
> The original kexec_load syscall can not verify file signatures, nor can
> the kexec image be measured.  Based on policy, deny the kexec_load
> syscall.


Curiosity question: I thought kexec_load() syscall was used to load a 
crashdump? If this is true, how would this work if kexec_load() is 
being denied? I don't think I'd want to be hindered in cases where I'm 
trying to diagnose a crash.

Thanks,
Jay


> Signed-off-by: Mimi Zohar <zohar@xxxxxxxxxxxxxxxxxx>
> Cc: Eric Biederman <ebiederm@xxxxxxxxxxxx>
> Cc: Luis R. Rodriguez <mcgrof@xxxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: David Howells <dhowells@xxxxxxxxxx>
>
> ---
> Changelog v3:
> - use switch/case
>
>   include/linux/ima.h                 |  7 +++++++
>   security/integrity/ima/ima.h        |  1 +
>   security/integrity/ima/ima_main.c   | 27 +++++++++++++++++++++++++++
>   security/integrity/ima/ima_policy.c |  2 ++
>   security/security.c                 |  7 ++++++-
>   5 files changed, 43 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/ima.h b/include/linux/ima.h
> index 0e4647e0eb60..84806b54b50a 100644
> --- a/include/linux/ima.h
> +++ b/include/linux/ima.h
> @@ -11,6 +11,7 @@
>   #define _LINUX_IMA_H
>   
>   #include <linux/fs.h>
> +#include <linux/security.h>
>   #include <linux/kexec.h>
>   struct linux_binprm;
>   
> @@ -19,6 +20,7 @@ extern int ima_bprm_check(struct linux_binprm *bprm);
>   extern int ima_file_check(struct file *file, int mask, int opened);
>   extern void ima_file_free(struct file *file);
>   extern int ima_file_mmap(struct file *file, unsigned long prot);
> +extern int ima_load_data(enum kernel_load_data_id id);
>   extern int ima_read_file(struct file *file, enum kernel_read_file_id id);
>   extern int ima_post_read_file(struct file *file, void *buf, loff_t size,
>   			      enum kernel_read_file_id id);
> @@ -49,6 +51,11 @@ static inline int ima_file_mmap(struct file *file, unsigned long prot)
>   	return 0;
>   }
>   
> +static inline int ima_load_data(enum kernel_load_data_id id)
> +{
> +	return 0;
> +}
> +
>   static inline int ima_read_file(struct file *file, enum kernel_read_file_id id)
>   {
>   	return 0;
> diff --git a/security/integrity/ima/ima.h b/security/integrity/ima/ima.h
> index 2ab1affffa36..588e4813370c 100644
> --- a/security/integrity/ima/ima.h
> +++ b/security/integrity/ima/ima.h
> @@ -232,6 +232,7 @@ int ima_policy_show(struct seq_file *m, void *v);
>   #define IMA_APPRAISE_MODULES	0x08
>   #define IMA_APPRAISE_FIRMWARE	0x10
>   #define IMA_APPRAISE_POLICY	0x20
> +#define IMA_APPRAISE_KEXEC	0x40
>   
>   #ifdef CONFIG_IMA_APPRAISE
>   int ima_appraise_measurement(enum ima_hooks func,
> diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
> index dca44cf7838e..71fecfef0939 100644
> --- a/security/integrity/ima/ima_main.c
> +++ b/security/integrity/ima/ima_main.c
> @@ -496,6 +496,33 @@ int ima_post_read_file(struct file *file, void *buf, loff_t size,
>   				   MAY_READ, func, 0);
>   }
>   
> +/**
> + * ima_load_data - appraise decision based on policy
> + * @id: kernel load data caller identifier
> + *
> + * Callers of this LSM hook can not measure, appraise, or audit the
> + * data provided by userspace.  Enforce policy rules requring a file
> + * signature (eg. kexec'ed kernel image).
> + *
> + * For permission return 0, otherwise return -EACCES.
> + */
> +int ima_load_data(enum kernel_load_data_id id)
> +{
> +	if ((ima_appraise & IMA_APPRAISE_ENFORCE) != IMA_APPRAISE_ENFORCE)
> +		return 0;
> +
> +	switch (id) {
> +	case LOADING_KEXEC_IMAGE:
> +		if (ima_appraise & IMA_APPRAISE_KEXEC) {
> +			pr_err("impossible to appraise a kernel image without a file descriptor; try using kexec_file_load syscall.\n");
> +			return -EACCES;	/* INTEGRITY_UNKNOWN */
> +		}
> +	default:
> +		break;
> +	}
> +	return 0;
> +}
> +
>   static int __init init_ima(void)
>   {
>   	int error;
> diff --git a/security/integrity/ima/ima_policy.c b/security/integrity/ima/ima_policy.c
> index 7f4a4de7e831..ebfb389b79df 100644
> --- a/security/integrity/ima/ima_policy.c
> +++ b/security/integrity/ima/ima_policy.c
> @@ -448,6 +448,8 @@ static int ima_appraise_flag(enum ima_hooks func)
>   		return IMA_APPRAISE_FIRMWARE;
>   	else if (func == POLICY_CHECK)
>   		return IMA_APPRAISE_POLICY;
> +	else if (func == KEXEC_KERNEL_CHECK)
> +		return IMA_APPRAISE_KEXEC;
>   	return 0;
>   }
>   
> diff --git a/security/security.c b/security/security.c
> index 05fe5b1932d7..7b870df0a335 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -1063,7 +1063,12 @@ EXPORT_SYMBOL_GPL(security_kernel_post_read_file);
>   
>   int security_kernel_load_data(enum kernel_load_data_id id)
>   {
> -	return call_int_hook(kernel_load_data, 0, id);
> +	int ret;
> +
> +	ret = call_int_hook(kernel_load_data, 0, id);
> +	if (ret)
> +		return ret;
> +	return ima_load_data(id);
>   }
>   
>   int security_task_fix_setuid(struct cred *new, const struct cred *old,