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

[Paul Moore <paul@paul-moore.com>]

On Mon, Feb 14, 2022 at 2:11 AM Jeffrey Vander Stoep <jeffv@google.com> wrote:
> On Tue, Feb 8, 2022 at 3:18 PM William Roberts <bill.c.roberts@gmail.com> wrote:
> >
> > <snip>
> >
> > This is getting too long for me.
> >
> > > >
> > > > 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.
> > >
> > > Excuse me if I am wrong but AFAIK adding a xperm rule does not turn on
> > > xperm checks across the entire system.
> >
> > It doesn't as you state below its target + class.
> >
> > >
> > > If i am not mistaken it will turn on xperm checks only for the
> > > operations that have the same source and target/target class.
> >
> > That's correct.
> >
> > >
> > > This is also why i don't (with the exception TIOSCTI for termdev
> > > chr_file) use xperms by default.
> > >
> > > 1. it is really easy to selectively filter ioctls by adding xperm rules
> > > for end users (and since ioctls are often device/driver specific they
> > > know best what is needed and what not)
> >
> > > >>> and FIONCLEX can be trivially bypassed unless fcntl(F_SETFD)
> > >
> > > 2. if you filter ioctls in upstream policy for example like i do with
> > > TIOSCTI using for example (allowx foo bar (ioctl chr_file (not
> > > (0xXXXX)))) then you cannot easily exclude additional ioctls later where source is
> > > foo and target/tclass is bar/chr_file because there is already a rule in
> > > place allowing the ioctl (and you cannot add rules)
> >
> > Currently, fcntl flag F_SETFD is never checked, it's silently allowed, but
> > the equivalent FIONCLEX and FIOCLEX are checked. So if you wrote policy
> > to block the FIO*CLEX flags, it would be bypassable through F_SETFD and
> > FD_CLOEXEC. So the patch proposed makes the FIO flags behave like
> > F_SETFD. Which means upstream policy users could drop this allow, which
> > could then remove the target/class rule and allow all icotls. Which is easy
> > to prevent and fix you could be a rule in to allowx 0 as documented in the
> > wiki: https://selinuxproject.org/page/XpermRules
> >
> > The questions I think we have here are:
> > 1. Do we agree that the behavior between SETFD and the FIO flags are equivalent?
> >   I think they are.
> > 2. Do we want the interfaces to behave the same?
> >   I think they should.
> > 3. Do upstream users of the policy construct care?
> >   The patch is backwards compat, but I don't want their to be cruft
> > floating around with extra allowxperm rules.
>
> I think this proposed change is fine from Android's perspective. It
> implements in the kernel what we've already already put in place in
> our policy - that all domains are allowed to use these IOCLTs.
> https://cs.android.com/android/platform/superproject/+/master:system/sepolicy/public/domain.te;l=312
>
> It'll be a few years before we can clean up our policy since we need
> to support older kernels, but that's fine.

Thanks for the discussion everyone, it sounds like everybody is okay
with the change - that's good.  However, as I said earlier in this
thread I think we need to put this behind a policy capability, how
does POLICYDB_CAPABILITY_IOCTL_CLOEXEC/"ioctl_skip_cloexec" sound to
everyone?

Demi, are you able to respin this patch with policy capability changes?

-- 
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