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

[Aleksa Sarai]

On 2019-09-06, Jeff Layton <jlayton@xxxxxxxxxx> wrote:
> On Sat, 2019-09-07 at 03:13 +1000, Aleksa Sarai wrote:
> > On 2019-09-06, Jeff Layton <jlayton@xxxxxxxxxx> 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.

In that case, maybe openat2(2) should default to not allowing any
upgrades by default? The reason I pitched UPGRADE_NOEXEC is because
UPGRADE_NO{READ,WRITE} are the existing @how->upgrade_mask flags.

However, I just noticed something else about this series -- if you do
O_PATH|O_MAYEXEC the new flag gets ignored. Given that you can do
fexecve(2) on an O_PATH (and O_PATHs have some other benefits), is this
something that we'd want to have?

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

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