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

[Tushar Sugandhi]

On 2020-10-22 3:35 p.m., Mimi Zohar wrote:
> 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.
Sure. I will drop 2/6, and reverse the order of 4/6 and 5/6.
> > ---
> >   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.
Makes sense. Will do.
> > 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).
Yes. Precisely. For example, in “dm-crypt” case: the data source is
“dm-crypt” and possible events are “add_target”, “post_suspend”,
"resume" etc. I will add a more detailed hook description as you
suggested below, and explain this point in it.
> 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.
Will add. Thanks Mimi.
> thanks,
>
> Mimi