Re: Immutable Page Protections

Justin Handville <nanolith@gmail.com> wrote:

> I'm assuming that misc@ is probably the best place for this e-mail,
> although it gets a bit in the tech@ weeds. I upgraded to 7.3 not so
> long ago, and I noticed that a daemon I had written was no longer
> working properly. For reasons that are probably too much to get into
> here, I statically link the daemon. It's a single binary that makes use
> of pledge / unveil, and privilege separation. This all works fine. It
> also has another trick, which unfortunately no longer works in 7.3.
>
> To reduce the code footprint of this daemon as well as the potential
> gadget attack surface, I have it drop any code that it will no longer
> execute. This happens after fork / exec on a child, and also after
> initialization code executes before the child process enters its steady
> state. This is trivially done by grouping functions into custom page
> aligned sections in the ELF binary, and running mprotect on these
> sections with PROT_NONE. I considered munmap as well as other tricks,
> but so far, this seems to be the most portable way to handle this trick
> that I could think of between BSD and Linux. I'm sure others are more
> clever. It's a cheap defense in depth protection that simplifies my use
> case.
>
> As of OpenBSD 7.3, when the immutable flag entered mainstream, this
> trick no longer works. Given that my trick is a total hack, I'm not too
> broken up about it. Of course, this change led me to doing some poking
> around.

Sorry.

But I don't see a real security benefit of what you are trying to do.

> I noticed that in sys/uvm/uvm_map.c, an exception was granted to allow
> Chrome to drop the write flag for a region for userland compatibility.
> That makes sense as a temporary measure. I'm wondering, however, if it
> might not make sense to think about this functionality differently.
> Instead of immutable memory regions, why can't we consider a more
> pledge-like ratcheting for memory regions, where bits can be removed,
> but never added back? How does this impact the gadget attack surface
> that led to the immutable flag being considered to begin with?

Because then programs need active code to do so. Where will those calls
be? Today, the kernel does it automatically before the program runs.
You want to do it at c runtime startup? It would be the same. Or you
want programs to have to do it themselves? Like, lock_my_text_segment()
called from inside main? That really doesn't make sense.


> For the time being, I extended the exception in uvm_map.c on my own
> OpenBSD systems to allow immutable regions to be stripped of all
> protection flags with a call to mprotect. So, in addition to allowing RW
> to R, if the region is any combination of PROT_READ, PROT_WRITE, or
> PROT_EXEC, then it can be reduced to PROT_NONE. This seemed the safer
> option for patching for now.

The ratchet-down piece in uvm will eventually be deleted, when we return
to review the entire corpus of software behaviour. We left that small
piece of behaviour due to a v8/chrome requirement, and were a bit
fearfull that other software also had such requirements, but were short
on doing a complete ecosystem review at the time. So if anything, this
is going to become more strict, not less strict.

> Of course, this further breaks the
> definition of "immutable", but at least immutable regions can only have
> protection bits removed.

To me, it is unclear if permission lowering lacks a bad failure mode.
There are a scary number of programs with sigsegv handlers, trying to
cope with unexpected behaviours in "clever ways". If you demote permissions,
you can get into that fragile rarely tested code, so I'm going to claim
permission reduction might not be safe.

> My reason for mailing misc@ is just to bring up this data point from a
> single user. I'm certain that the OpenBSD developers have reasons for
> preferring a pure immutable flag, but having a mechanism for ratcheting
> down protections is useful at least for me, and is apparently useful
> enough in userland going from RW to R, that an exception was carved out
> for now. Of course, I'm more than happy to work with the developers to
> come up with a plan for upstreaming this feature if it's something
> useful. If not, I have no problem adding it to my personal list of
> patches I maintain that I doubt anyone else would want or need.

I'll think about it a bit. But I am very much not convinced that those
text segments you have lying around are a real risk.