Re: [PATCH bpf-next v2 3/3] bpf: Update LSM selftests for bpf_ima_inode_hash

[Yonghong Song <yhs@fb.com>]

On 11/23/20 10:46 AM, KP Singh wrote:
> On Mon, Nov 23, 2020 at 7:36 PM Yonghong Song <yhs@fb.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?

Maybe you can create a bash script on your prog_test files and do
system("./<>.sh"). In the shell script, you can use all the bash magic
(sed, awk, etc) to parse and store the needed result in a temp file, and
after a successful system(""), you just read that temp file. Does this work?

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