Re: Semantics of blktrace with lockdown (integrity) enabled kernel.

[Paul Moore]

On Mon, Apr 10, 2023 at 5:28 PM Junxiao Bi <junxiao.bi@xxxxxxxxxx> wrote:
> On 4/10/23 1:22 PM, Paul Moore wrote:
> > On Mon, Apr 10, 2023 at 3:20 PM Junxiao Bi <junxiao.bi@xxxxxxxxxx> wrote:
> >> On 4/6/23 2:43 PM, Paul Moore wrote:
> >>> On Thu, Apr 6, 2023 at 3:33 PM Konrad Rzeszutek Wilk
> >>> <konrad.wilk@xxxxxxxxxx> wrote:
> >>>> On Thu, Apr 06, 2023 at 02:39:57PM -0400, Paul Moore wrote:
> >>> ...
> >>>
> >>>>> Before we go any further, can you please verify that your issue is
> >>>>> reproducible on a supported, upstream tree (preferably Linus')?
> >>>> Yes. Very much so.
> >>> Okay, in that case I suspect the issue is due to the somewhat limited
> >>> granularity in the lockdown LSM.  While there are a number of
> >>> different lockdown "levels", the reality is that the admin has to
> >>> choose from either NONE, INTEGRITY, or CONFIDENTIALITY.  Without
> >>> digging to deep into the code path that you would be hitting, we can
> >>> see that TRACEFS is blocked by the CONFIDENTIALITY (and therefore
> >>> INTEGRITY too) setting and DEBUGFS is blocked by the INTEGRITY
> >>> setting.  With DEBUGFS blocked by INTEGRITY, the only lockdown option
> >>> that would allow DEBUGFS is NONE.
> >>>
> >>> Without knowing too much about blktrace beyond the manpage, it looks
> >>> like it has the ability to trace/snoop on the block device operations
> >>> so I don't think this is something we would want to allow in a
> >>> "locked" system.
> >> blktrace depends on tracepoint in block layer to trace io events of
> >> block devices,
> >>
> >> through the test with mainline, those tracepoints were not blocked by
> >> lockdown.
> >>
> >> If snoop block devices operations is a security concern in lock down, these
> >>
> >> tracepoints should be disabled?
> > Possibly, however, as I said earlier I'm not very familiar with
> > blktrace and the associated tracepoints.  If it is possible to snoop
> > on kernel/user data using blktrace then it probably should be
> > protected by a lockdown control point.
> >
> > Is this something you could verify and potentially submit a patch to resolve?
>
> blktrace can not snoop kernel/user data. The information it got from
> kernel is kind of "io metadata", basically include which process from
> which cpu, at what time, triggered what kind of IO events(issue, queue,
> complete etc.), to which disk, from which sector offset and how long.
> blktrace has no way to know what's inside that io. I am kind of think
> this is safe for lockdown mode.

Well, you could always submit a patch* and we would review it like any
other; that's usually a much better approach.

* Yes, there was a patch submitted, but it was against a distro kernel
that diverged significantly from the upstream kernel in the relevant
areas.

-- 
paul-moore.com