On 3/2/24 10:58, Amir Goldstein wrote:
On Fri, Mar 1, 2024 at 3:16 PM Mete Durlu <meted@xxxxxxxxxxxxx> wrote:
On 3/1/24 10:52, Amir Goldstein wrote:
On Thu, Feb 29, 2024 at 7:53 PM Mete Durlu <meted@xxxxxxxxxxxxx> wrote:
In current state do_fanotify_mark() does path permission and security
checking before doing the event configuration checks. In the case
where user configures mount and sb marks with kernel internal pseudo
fs, security_path_notify() yields an EACESS and causes an earlier
exit. Instead, this particular case should have been handled by
fanotify_events_supported() and exited with an EINVAL.
What makes you say that this is the expected outcome?
I'd say that the expected outcome is undefined and we have no reason
to commit to either EACCESS or EINVAL outcome.
TLDR; I saw the failing ltp test(fanotify14) started investigating, read
the comments on the related commits and noticed that the fanotify
documentation does not mention any EACESS as an errno. For these reasons
I made an attempt to provide a fix. The placement of the checks aim
minimal change, I just tried not to alter the logic more than needed.
Thanks for the feedback, will apply suggestions.
Generally speaking, the reasons above themselves are good enough
reasons for fixing the documentation and fixing the test code, but not
enough reasons to change the code.
There may be other good reasons for changing the code, but I am not
sure they apply here.
I understand the concerns and the reasoning. My findings and suggestions
are below.
The main reason is the following commit;
* linux: 69562eb0bd3e ("fanotify: disallow mount/sb marks on kernel
internal pseudo fs")
fanotify_user: fanotify_events_supported()
/*
* mount and sb marks are not allowed on kernel internal pseudo
* fs, like pipe_mnt, because that would subscribe to events on
* all the anonynous pipes in the system.
*/
if (mark_type != FAN_MARK_INODE &&
path->mnt->mnt_sb->s_flags & SB_NOUSER)
return -EINVAL;
It looks to me as, when configuring fanotify_mark with pipes and
FAN_MARK_MOUNT or FAN_MARK_FILESYSTEM, the path above should be taken
instead of an early return with EACCES.
It is a subjective opinion. I do not agree with it, but it does not matter if
I agree with this statement or not, what matters it that there is no clear
definition across system calls of what SHOULD happen in this case
and IMO there is no reason for us to commit to this behavior or the other.
Also the following commit on linux test project(ltp) expects EINVAL as
expected errno.
* ltp: 8e897008c ("fanotify14: Test disallow sb/mount mark on anonymous
pipe")
To be honest, the test added on above commit is the main reason why I
started investigating this.
This is something that I don't understand.
If you are running LTP in a setup that rejects fanotify_mark() due to
security policy, how do the rest of the fanotify tests pass?
I feel like I am missing information about the test regression report.
I never test with a security policy applied so I have no idea what
might be expected.
Ah, I always run with defconfig which has SELINUX enabled and by default
SELINUX is configured to `enforcing` (so far I tested with x86 and s390x
but a quick grep shows most other architectures also have it enabled on
their defconfigs). With SELINUX enabled LTP's fanotify14 shows failures
on
fanotify14.c:284: TINFO: Testing FAN_MARK_MOUNT with anonymous pipe
fanotify14.c:284: TINFO: Testing FAN_MARK_FILESYSTEM with anonymous pipe
since they return -EACCES instead of -EINVAL.Other test cases pass.
Once I disable SELINUX, ALL test cases pass.
If we do accept your argument that security_path_notify() should be
after fanotify_events_supported(). Why not also after fanotify_test_fsid()
and fanotify_test_fid()?
I tried to place the checks as close as possible to their original
position, that is why I placed them right after
fanotify_events_supported(). I wanted to keep the ordering as close as
possible to original to not break any other check. I am open to
suggestions regarding this.
It is a matter of principle IMO.
If you argue that access permission errors have priority over validity
of API arguments, then fanotify_test_{fsid,fid}() are not that much
different (priority-wise) from fanotify_events_supported().
My preference is to not change the code, but maybe Jan will
have a different opinion.
I understand the argument, then I propose patching the LTP and appending
the documentation. My first idea is to send a patch for LTP so that,
fanotify14 could check if SELINUX is enforcing and change the testcases
expected errno accordingly. How does that sound?
Thank you.
-Mete Durlu
