Messages in this thread | | | Date | Fri, 4 Feb 2022 08:48:27 -0500 | Subject | Re: [PATCH] SELinux: Always allow FIOCLEX and FIONCLEX | From | Chris PeBenito <> |
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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
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