linux-kernel - Re: [PATCH bpf-next v2 3/3] bpf: Update LSM selftests for bpf_ima_inode

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Date:   Mon, 23 Nov 2020 19:46:33 +0100
From:   KP Singh <kpsingh@...omium.org>
To:     Yonghong Song <yhs@...com>
Cc:     Mimi Zohar <zohar@...ux.ibm.com>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        James Morris <jmorris@...ei.org>,
        open list <linux-kernel@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>,
        Linux Security Module list 
        <linux-security-module@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Florent Revest <revest@...omium.org>,
        Brendan Jackman <jackmanb@...omium.org>,
        Petr Vorel <pvorel@...e.cz>
Subject: Re: [PATCH bpf-next v2 3/3] bpf: Update LSM selftests for bpf_ima_inode_hash

On Mon, Nov 23, 2020 at 7:36 PM Yonghong Song <yhs@...com> wrote:
>
>
>
> On 11/23/20 10:27 AM, KP Singh wrote:
> > [...]
> >
> >>>>
> >>>> Even if a custom policy has been loaded, potentially additional
> >>>> measurements unrelated to this test would be included the measurement
> >>>> list.  One way of limiting a rule to a specific test is by loopback
> >>>> mounting a file system and defining a policy rule based on the loopback
> >>>> mount unique uuid.
> >>>
> >>> Thanks Mimi!
> >>>
> >>> I wonder if we simply limit this to policy to /tmp and run an executable
> >>> from /tmp (like test_local_storage.c does).
> >>>
> >>> The only side effect would be of extra hashes being calculated on
> >>> binaries run from /tmp which is not too bad I guess?
> >>
> >> The builtin measurement policy (ima_policy=tcb") explicitly defines a
> >> rule to not measure /tmp files.  Measuring /tmp results in a lot of
> >> measurements.
> >>
> >> {.action = DONT_MEASURE, .fsmagic = TMPFS_MAGIC, .flags = IMA_FSMAGIC},
> >>
> >>>
> >>> We could do the loop mount too, but I am guessing the most clean way
> >>> would be to shell out to mount from the test? Are there some other examples
> >>> of IMA we could look at?
> >>
> >> LTP loopback mounts a filesystem, since /tmp is not being measured with
> >> the builtin "tcb" policy.  Defining new policy rules should be limited
> >> to the loopback mount.  This would pave the way for defining IMA-
> >> appraisal signature verification policy rules, without impacting the
> >> running system.
> >
> > +Andrii
> >
> > Do you think we can split the IMA test out,
> > have a little shell script that does the loopback mount, gets the
> > FS UUID, updates the IMA policy and then runs a C program?
> >
> > This would also allow "test_progs" to be independent of CONFIG_IMA.
> >
> > I am guessing the structure would be something similar
> > to test_xdp_redirect.sh
>
> Look at sk_assign test.
>
> sk_assign.c:    if (CHECK_FAIL(system("ip link set dev lo up")))
> sk_assign.c:    if (CHECK_FAIL(system("ip route add local default dev lo")))
> sk_assign.c:    if (CHECK_FAIL(system("ip -6 route add local default dev
> lo")))
> sk_assign.c:    if (CHECK_FAIL(system("tc qdisc add dev lo clsact")))
> sk_assign.c:    if (CHECK(system(tc_cmd), "BPF load failed;"
>
> You can use "system" to invoke some bash commands to simulate a script
> in the tests.

Heh, that's what I was trying to avoid, I need to parse the output to the get
the name of which loop device was assigned and then call a command like:

# blkid /dev/loop0
/dev/loop0: UUID="607ed7ce-3fad-4236-8faf-8ab744f23e01" TYPE="ext3"

Running simple commands with "system" seems okay but parsing output
is a bit too much :)

I read about:

https://man7.org/linux/man-pages/man4/loop.4.html

But I still need to create a backing file, format it and then get the UUID.

Any simple trick that I may be missing?

- KP

>
> >
> > - KP
> >
> >>
> >> Mimi
> >>