Re: [PATCH v4 5/6] IMA: add hook to measure critical data from kernel components

[Mimi Zohar]

Hi Tushar,

On Wed, 2020-09-23 at 12:20 -0700, Tushar Sugandhi wrote:
> Currently, IMA does not provide a generic function for kernel components
> to measure their data. A generic function provided by IMA would
> enable various parts of the kernel with easier and faster on-boarding to
> use IMA infrastructure, would avoid code duplication, and consistent
> usage of IMA policy option "data_sources:=" across the kernel.
> 
> Add a new IMA func CRITICAL_DATA and a corresponding IMA hook
> ima_measure_critical_data() to support measuring various critical kernel
> components. Limit the measurement to the components that are specified
> in the IMA policy - CRITICAL_DATA+data_sources.
> 
> Signed-off-by: Tushar Sugandhi <tusharsu@xxxxxxxxxxxxxxxxxxx>

Normally the new LSM or IMA hook is defined before defining a method of
constraining that hook.  Please drop 2/6 (IMA: conditionally allow
empty rule data) and reverse the order of 4/6 and 5/6.   That will
allow each patch to update the Documentation appropriately, making the
change self contained.

> ---
>  Documentation/ABI/testing/ima_policy |  8 ++++++-
>  include/linux/ima.h                  |  8 +++++++
>  security/integrity/ima/ima.h         |  1 +
>  security/integrity/ima/ima_api.c     |  2 +-
>  security/integrity/ima/ima_main.c    | 26 +++++++++++++++++++++
>  security/integrity/ima/ima_policy.c  | 34 ++++++++++++++++++++++++----
>  6 files changed, 72 insertions(+), 7 deletions(-)
> 
> diff --git a/Documentation/ABI/testing/ima_policy b/Documentation/ABI/testing/ima_policy
> index a81cf79fb255..d33bb51309fc 100644
> --- a/Documentation/ABI/testing/ima_policy
> +++ b/Documentation/ABI/testing/ima_policy
> @@ -29,7 +29,7 @@ Description:
>  		base: 	func:= [BPRM_CHECK][MMAP_CHECK][CREDS_CHECK][FILE_CHECK][MODULE_CHECK]
>  				[FIRMWARE_CHECK]
>  				[KEXEC_KERNEL_CHECK] [KEXEC_INITRAMFS_CHECK]
> -				[KEXEC_CMDLINE] [KEY_CHECK]
> +				[KEXEC_CMDLINE] [KEY_CHECK] [CRITICAL_DATA]
>  			mask:= [[^]MAY_READ] [[^]MAY_WRITE] [[^]MAY_APPEND]
>  			       [[^]MAY_EXEC]
>  			fsmagic:= hex value
> @@ -51,6 +51,8 @@ Description:
>  			data_sources:= list of kernel components
>  			(eg, selinux|apparmor|dm-crypt) that contain data critical
>  			to the security of the kernel.
> +			Only valid when action is "measure" and func is
> +			CRITICAL_DATA.
>  
>  		default policy:
>  			# PROC_SUPER_MAGIC
> @@ -128,3 +130,7 @@ Description:
>  		keys added to .builtin_trusted_keys or .ima keyring:
>  
>  			measure func=KEY_CHECK keyrings=.builtin_trusted_keys|.ima
> +
> +		Example of measure rule using CRITICAL_DATA to measure critical data
> +
> +			measure func=CRITICAL_DATA data_sources=selinux|apparmor|dm-crypt


As data sources are added, the documentation example should be updated
to reflect the new source.  Please do not include examples that don't
yet exist.


> diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
> index 6888fc372abf..d55896f28790 100644
> --- a/security/integrity/ima/ima_main.c
> +++ b/security/integrity/ima/ima_main.c
> @@ -867,6 +867,32 @@ void ima_kexec_cmdline(int kernel_fd, const void *buf, int size)
>  	fdput(f);
>  }
>  
> +/**
> + * ima_measure_critical_data - measure critical data
> + * @event_name: name for the given data
> + * @event_data_source: name of the event data source
> + * @buf: pointer to buffer containing data to measure
> + * @buf_len: length of buffer(in bytes)
> + * @measure_buf_hash: if set to true - will measure hash of the buf,
> + *                    instead of buf
> + *
> + * Buffers can only be measured, not appraised.
> + */

Perhaps the reason for defining both the event_name and
event_data_source will become clearer with an example.  At this point I
can only guess as to why both are needed (e.g. perhaps a data source
defines multiple events).

While "Buffers can only be measured, not appraised" is true, it was cut
& pasted from ima_kexec_cmdline.  Measuring the kexec boot cmdline is
self describing.  Here, a larger, more detailed IMA hook description
would be appropriate.

thanks,

Mimi