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

[Tycho Andersen <tycho@tycho.ws>]

On Fri, Sep 06, 2019 at 08:27:31PM +0200, Florian Weimer wrote:
> * Tycho Andersen:
> 
> > On Fri, Sep 06, 2019 at 07:20:51PM +0200, Christian Brauner wrote:
> >> On Sat, Sep 07, 2019 at 03:07:39AM +1000, Aleksa Sarai wrote:
> >> > On 2019-09-06, Mickaël Salaün <mickael.salaun@ssi.gouv.fr> 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.
> >> > 
> >> > Depends on your definition of "safe" -- a security feature that you will
> >> > silently not enable on older kernels doesn't sound super safe to me.
> >> > Unfortunately this is a limitation of open(2) that we cannot change --
> >> > which is why the openat2(2) proposal I've been posting gives -EINVAL for
> >> > unknown O_* flags.
> >> > 
> >> > There is a way to probe for support (though unpleasant), by creating a
> >> > test O_MAYEXEC fd and then checking if the flag is present in
> >> > /proc/self/fdinfo/$n.
> >> 
> >> Which Florian said they can't do for various reasons.
> >> 
> >> It is a major painpoint if there's no easy way for userspace to probe
> >> for support. Especially if it's security related which usually means
> >> that you want to know whether this feature works or not.
> >
> > What about just trying to violate the policy via fexecve() instead of
> > looking around in /proc? Still ugly, though.
> 
> How would we do this?  This is about opening the main executable as part
> of an explicit loader invocation.  Typically, an fexecve will succeed
> and try to run the program, but with the wrong dynamic loader.

Yeah, fexecve() was a think-o, sorry, you don't need to go that far. I
was thinking do what the tests in this series do: create a tmpfs with
MS_NOEXEC, put an executable file in it, and try and open it with
O_MAYEXEC. If that works, the kernel doesn't support the flag, and it
should give you -EACCES if the kernel does support the flag.

Still a lot of work, though. Seems better to just use openat2.

Tycho