Re: [PATCH v2 1/5] fs: Add support for an O_MAYEXEC flag on sys_open()

[Aleksa Sarai <cyphar@cyphar.com>]

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On 2019-09-06, Jeff Layton <jlayton@kernel.org> wrote:
> On Fri, 2019-09-06 at 13:06 -0700, Andy Lutomirski wrote:
> > > On Sep 6, 2019, at 12:43 PM, Jeff Layton <jlayton@kernel.org> wrote:
> > > 
> > > > On Sat, 2019-09-07 at 03:13 +1000, Aleksa Sarai wrote:
> > > > > On 2019-09-06, Jeff Layton <jlayton@kernel.org> wrote:
> > > > > > On Fri, 2019-09-06 at 18:06 +0200, Mickaël Salaün wrote:
> > > > > > > On 06/09/2019 17:56, Florian Weimer wrote:
> > > > > > > Let's assume I want to add support for this to the glibc dynamic loader,
> > > > > > > while still being able to run on older kernels.
> > > > > > > 
> > > > > > > Is it safe to try the open call first, with O_MAYEXEC, and if that fails
> > > > > > > with EINVAL, try again without O_MAYEXEC?
> > > > > > 
> > > > > > The kernel ignore unknown open(2) flags, so yes, it is safe even for
> > > > > > older kernel to use O_MAYEXEC.
> > > > > > 
> > > > > 
> > > > > Well...maybe. What about existing programs that are sending down bogus
> > > > > open flags? Once you turn this on, they may break...or provide a way to
> > > > > circumvent the protections this gives.
> > > > 
> > > > It should be noted that this has been a valid concern for every new O_*
> > > > flag introduced (and yet we still introduced new flags, despite the
> > > > concern) -- though to be fair, O_TMPFILE actually does have a
> > > > work-around with the O_DIRECTORY mask setup.
> > > > 
> > > > The openat2() set adds O_EMPTYPATH -- though in fairness it's also
> > > > backwards compatible because empty path strings have always given ENOENT
> > > > (or EINVAL?) while O_EMPTYPATH is a no-op non-empty strings.
> > > > 
> > > > > Maybe this should be a new flag that is only usable in the new openat2()
> > > > > syscall that's still under discussion? That syscall will enforce that
> > > > > all flags are recognized. You presumably wouldn't need the sysctl if you
> > > > > went that route too.
> > > > 
> > > > I'm also interested in whether we could add an UPGRADE_NOEXEC flag to
> > > > how->upgrade_mask for the openat2(2) patchset (I reserved a flag bit for
> > > > it, since I'd heard about this work through the grape-vine).
> > > > 
> > > 
> > > I rather like the idea of having openat2 fds be non-executable by
> > > default, and having userland request it specifically via O_MAYEXEC (or
> > > some similar openat2 flag) if it's needed. Then you could add an
> > > UPGRADE_EXEC flag instead?
> > > 
> > > That seems like something reasonable to do with a brand new API, and
> > > might be very helpful for preventing certain classes of attacks.
> > > 
> > > 
> > 
> > There are at least four concepts of executability here:
> > 
> > - Just check the file mode and any other relevant permissions. Return a normal fd.  Makes sense for script interpreters, perhaps.
> > 
> > - Make the fd fexecve-able.
> > 
> > - Make the resulting fd mappable PROT_EXEC.
> > 
> > - Make the resulting fd upgradable.
> > 
> > I’m not at all convinced that the kernel needs to distinguish all these, but at least upgradability should be its own thing IMO.
> 
> Good point. Upgradability is definitely orthogonal, though the idea
> there is to alter the default behavior. If the default is NOEXEC then
> UPGRADE_EXEC would make sense.
> 
> In any case, I was mostly thinking about the middle two in your list
> above. After more careful reading of the patches, I now get get that
> Mickaël is more interested in the first, and that's really a different
> sort of use-case.
> 
> Most opens never result in the fd being fed to fexecve or mmapped with
> PROT_EXEC, so having userland explicitly opt-in to allowing that during
> the open sounds like a reasonable thing to do.
> 
> But I get that preventing execution via script interpreters of files
> that are not executable might be something nice to have.

My first glance at the patch lead me to believe that this was about
blocking at fexecve()-time (which was what my first attempt at this
problem looked like) -- hence why I mentioned the upgrade_mask stuff
(because of the dances you can do with O_PATH, if blocking at
fexecve()-time was the goal then you seriously do need the upgrade_mask
and "O_PATH mask" in order for it to be even slightly secure).

But I also agree this is useful, and we can always add FMODE_EXEC,
FMODE_MAP_EXEC, and FMODE_UPGRADE_EXEC (and the related bits) at a later
date.

> Perhaps we need two flags for openat2?
> 
> OA2_MAYEXEC : test that permissions allow execution and that the file
> doesn't reside on a noexec mount before allowing the open
> 
> OA2_EXECABLE : only allow fexecve or mmapping with PROT_EXEC if the fd
> was opened with this

That seems reasonable to me. The only thing is that there currently
isn't any code to restrict fexecve() or PROT_EXEC in that fashion
(doubly so when you consider binfmt_script). So if we want to make
certain things default behaviour (such as disallowing exec by default)
we'd need to get the PROT_EXEC restriction work done first.

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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