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

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Date:   Mon, 10 Apr 2023 18:00:58 -0400
From:   Paul Moore <paul@...l-moore.com>
To:     Junxiao Bi <junxiao.bi@...cle.com>
Cc:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org, jmorris@...ei.org,
        serge@...lyn.com, nathanl@...ux.ibm.com, joe.jin@...cle.com,
        Eric <eric.snowberg@...cle.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>, axboe@...nel.dk
Subject: Re: Semantics of blktrace with lockdown (integrity) enabled kernel.

On Mon, Apr 10, 2023 at 5:52 PM Junxiao Bi <junxiao.bi@...cle.com> wrote:
> On 4/10/23 2:44 PM, Paul Moore wrote:
> > On Mon, Apr 10, 2023 at 5:28 PM Junxiao Bi <junxiao.bi@...cle.com> wrote:
> >> On 4/10/23 1:22 PM, Paul Moore wrote:
> >>> On Mon, Apr 10, 2023 at 3:20 PM Junxiao Bi <junxiao.bi@...cle.com> 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@...cle.com> 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.
>
> Sure, i will submit a new one.
>
> Before that, may i ask this question? It may affect the approach of the
> patch.
>
> Lockdown blocked files with mmap operation even that files are
> read-only, may i know what's the security concern there?
>
> static int debugfs_locked_down(struct inode *inode,
>                     struct file *filp,
>                     const struct file_operations *real_fops)
> {
>      if ((inode->i_mode & 07777 & ~0444) == 0 &&
>          !(filp->f_mode & FMODE_WRITE) &&
>          !real_fops->unlocked_ioctl &&
>          !real_fops->compat_ioctl &&
>          !real_fops->mmap)
>          return 0;
>
>      if (security_locked_down(LOCKDOWN_DEBUGFS))
>          return -EPERM;
>
>      return 0;
> }

I think the comment block at the top of that function describes it well:

/*
 * Only permit access to world-readable files when the kernel is locked down.
 * We also need to exclude any file that has ways to write or alter it as root
 * can bypass the permissions check.
 */

--
paul-moore.com