2022-02-14 18:45:02

by Jeffrey Vander Stoep

[permalink] [raw]
Subject: Re: [PATCH] SELinux: Always allow FIOCLEX and FIONCLEX

On Tue, Feb 8, 2022 at 3:18 PM William Roberts <[email protected]> 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.

It'll be a few years before we can clean up our policy since we need
to support older kernels, but that's fine.