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

[Jeff Layton <jlayton@kernel.org>]

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.

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

-- 
Jeff Layton <jlayton@kernel.org>