On 1/26/22 17:41, Paul Moore wrote: > On Tue, Jan 25, 2022 at 5:50 PM Demi Marie Obenour > <demiobenour@xxxxxxxxx> wrote: >> On 1/25/22 17:27, Paul Moore wrote: >>> On Tue, Jan 25, 2022 at 4:34 PM Demi Marie Obenour >>> <demiobenour@xxxxxxxxx> 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@xxxxxxxxx> >>>> --- >>>> 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? -- Sincerely, Demi Marie Obenour (she/her/hers)
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