Re: dm verity: log audit events for dm-verity target

[Mike Snitzer <snitzer@kernel.org>]

On Fri, Mar 17 2023 at  4:00P -0400,
Michael Weiß <michael.weiss@aisec.fraunhofer.de> wrote:

> On 02.03.23 03:25, Paul Moore wrote:
> > On Wed, Mar 1, 2023 at 6:34 AM Michael Weiß
> > <michael.weiss@aisec.fraunhofer.de> wrote:
> >>
> >> dm-verity signals integrity violations by returning I/O errors
> >> to user space. To identify integrity violations by a controlling
> >> instance, the kernel audit subsystem can be used to emit audit
> >> events to user space. Analogous to dm-integrity, we also use the
> >> dm-audit submodule allowing to emit audit events on verification
> >> failures of metadata and data blocks as well as if max corrupted
> >> errors are reached.
> >>
> >> The construction and destruction of verity device mappings are
> >> also relevant for auditing a system. Thus, those events are also
> >> logged as audit events.
> >>
> >> We tested this by starting a container with the container manager
> >> (cmld) of GyroidOS which uses a dm-verity protected rootfs image
> >> root.img mapped to /dev/mapper/<uuid>-root. We than manipulated
> >> one block in the underlying image file and reading it from the
> >> protected mapper device again and again until we reach the max
> >> corrupted errors like this:
> >>
> >>   dd if=/dev/urandom of=root.img bs=512 count=1 seek=1000
> >>   for i in range {1..101}; do \
> >>     dd if=/dev/mapper/<uuid>-root of=/dev/null bs=4096 \
> >>        count=1 skip=1000 \
> >>   done
> >>
> >> The resulting audit log looks as follows:
> >>
> >>   type=DM_CTRL msg=audit(1677618791.876:962):
> >>     module=verity op=ctr ppid=4876 pid=29102 auid=0 uid=0 gid=0
> >>     euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=(none) ses=44
> >>     comm="cmld" exe="/usr/sbin/cml/cmld" subj=unconfined
> >>     dev=254:3 error_msg='success' res=1
> >>
> >>   type=DM_EVENT msg=audit(1677619463.786:1074): module=verity
> >>     op=verify-data dev=7:0 sector=1000 res=0
> >>   ...
> >>   type=DM_EVENT msg=audit(1677619596.727:1162): module=verity
> >>     op=verify-data dev=7:0 sector=1000 res=0
> >>
> >>   type=DM_EVENT msg=audit(1677619596.731:1163): module=verity
> >>     op=max-corrupted-errors dev=254:3 sector=? res=0
> >>
> >> Signed-off-by: Michael Weiß <michael.weiss@aisec.fraunhofer.de>
> >> ---
> >>  drivers/md/dm-verity-target.c | 20 ++++++++++++++++++--
> >>  1 file changed, 18 insertions(+), 2 deletions(-)
> > 
> > This looks reasonable to me from an audit perspective.
> > 
> > Acked-by: Paul Moore <paul@paul-moore.com>
> 
> Hi Mike, since Paul already gave his ack from audit perspective,
> do you pick this up? Or is there anything which I can improve from device
> mapper side?

Looks fine, but I haven't started a 6.4 branch yet. I'll pick this up
from dm-devel's patchwork when I do.

Mike