Re: [PATCH] SELinux: Always allow FIOCLEX and FIONCLEX

[Chris PeBenito <chpebeni@linux.microsoft.com>]

On 2/3/2022 18:44, Paul Moore wrote:
> On Wed, Feb 2, 2022 at 5:13 AM Demi Marie Obenour <demiobenour@gmail.com> wrote:
>> On 2/1/22 12:26, Paul Moore wrote:
>>> On Sat, Jan 29, 2022 at 10:40 PM Demi Marie Obenour
>>> <demiobenour@gmail.com> wrote:
>>>> On 1/26/22 17:41, Paul Moore wrote:
>>>>> On Tue, Jan 25, 2022 at 5:50 PM Demi Marie Obenour
>>>>> <demiobenour@gmail.com> wrote:
>>>>>> On 1/25/22 17:27, Paul Moore wrote:
>>>>>>> On Tue, Jan 25, 2022 at 4:34 PM Demi Marie Obenour
>>>>>>> <demiobenour@gmail.com> wrote:
>>>>>>>>
>>>>>>>> These ioctls are equivalent to fcntl(fd, F_SETFD, flags), which SELinux
>>>>>>>> always allows too.  Furthermore, a failed FIOCLEX could result in a file
>>>>>>>> descriptor being leaked to a process that should not have access to it.
>>>>>>>>
>>>>>>>> Signed-off-by: Demi Marie Obenour <demiobenour@gmail.com>
>>>>>>>> ---
>>>>>>>>   security/selinux/hooks.c | 5 +++++
>>>>>>>>   1 file changed, 5 insertions(+)
>>>>>>>
>>>>>>> I'm not convinced that these two ioctls should be exempt from SELinux
>>>>>>> policy control, can you explain why allowing these ioctls with the
>>>>>>> file:ioctl permission is not sufficient for your use case?  Is it a
>>>>>>> matter of granularity?
>>>>>>
>>>>>> FIOCLEX and FIONCLEX are applicable to *all* file descriptors, not just
>>>>>> files.  If I want to allow them with SELinux policy, I have to grant
>>>>>> *:ioctl to all processes and use xperm rules to determine what ioctls
>>>>>> are actually allowed.  That is incompatible with existing policies and
>>>>>> needs frequent maintenance when new ioctls are added.
>>>>>>
>>>>>> Furthermore, these ioctls do not allow one to do anything that cannot
>>>>>> already be done by fcntl(F_SETFD), and (unless I have missed something)
>>>>>> SELinux unconditionally allows that.  Therefore, blocking these ioctls
>>>>>> does not improve security, but does risk breaking userspace programs.
>>>>>> The risk is especially great because in the absence of SELinux, I
>>>>>> believe FIOCLEX and FIONCLEX *will* always succeed, and userspace
>>>>>> programs may rely on this.  Worse, if a failure of FIOCLEX is ignored,
>>>>>> a file descriptor can be leaked to a child process that should not have
>>>>>> access to it, but which SELinux allows access to.  Userspace
>>>>>> SELinux-naive sandboxes are one way this could happen.  Therefore,
>>>>>> blocking FIOCLEX may *create* a security issue, and it cannot solve one.
>>>>>
>>>>> I can see you are frustrated with my initial take on this, but please
>>>>> understand that excluding an operation from the security policy is not
>>>>> something to take lightly and needs discussion.  I've added the
>>>>> SELinux refpolicy list to this thread as I believe their input would
>>>>> be helpful here.
>>>>
>>>> Absolutely it is not something that should be taken lightly, though I
>>>> strongly believe it is correct in this case.  Is one of my assumptions
>>>> mistaken?
>>>
>>> My concern is that there is a distro/admin somewhere which is relying
>>> on their SELinux policy enforcing access controls on these ioctls and
>>> removing these controls would cause them a regression.
>>
>> I obviously do not have visibility into all systems, but I suspect that
>> nobody is actually relying on this.  Setting and clearing CLOEXEC via
>> fcntl is not subject to SELinux restrictions, so blocking FIOCLEX
>> and FIONCLEX can be trivially bypassed unless fcntl(F_SETFD) is
>> blocked by seccomp or another LSM.  Clearing close-on-exec can also be
>> implemented with dup2(), and setting it can be implemented with dup3()
>> and F_DUPFD_CLOEXEC (which SELinux also allows).  In short, I believe
>> that unconditionally allowing FIOCLEX and FIONCLEX may fix real-world
>> problems, and that it is highly unlikely that anyone is relying on the
>> current behavior.
> 
> I understand your point, but I remain concerned about making a kernel
> change for something that can be addressed via policy.  I'm also
> concerned that in the nine days this thread has been on both the mail
> SELinux developers and refpolicy lists no one other than you and I
> have commented on this patch.  In order to consider this patch
> further, I'm going to need to see comments from others, preferably
> those with a background in supporting SELinux policy.
> 
> Also, while I'm sure you are already well aware of this, I think it is
> worth mentioning that SELinux does apply access controls when file
> descriptors are inherited across an exec() boundary.


I don't have a strong opinion either way.  If one were to allow this 
using a policy rule, it would result in a major policy breakage.  The 
rule would turn on extended perm checks across the entire system, which 
the SELinux Reference Policy isn't written for.  I can't speak to the 
Android policy, but I would imagine it would be the similar problem 
there too.


--
Chris PeBenito