From mboxrd@z Thu Jan 1 00:00:00 1970 From: Tycho Andersen Subject: Re: [PATCH v2 1/5] fs: Add support for an O_MAYEXEC flag on sys_open() Date: Fri, 6 Sep 2019 12:46:20 -0600 Message-ID: <20190906184620.GI7627@cisco> References: <20190906152455.22757-1-mic@digikod.net> <20190906152455.22757-2-mic@digikod.net> <87ef0te7v3.fsf@oldenburg2.str.redhat.com> <75442f3b-a3d8-12db-579a-2c5983426b4d@ssi.gouv.fr> <20190906170739.kk3opr2phidb7ilb@yavin.dot.cyphar.com> <20190906172050.v44f43psd6qc6awi@wittgenstein> <20190906174041.GH7627@cisco> <87v9u5cmb0.fsf@oldenburg2.str.redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: 8bit Return-path: Content-Disposition: inline In-Reply-To: <87v9u5cmb0.fsf@oldenburg2.str.redhat.com> Sender: linux-kernel-owner@vger.kernel.org To: Florian Weimer Cc: Christian Brauner , Aleksa Sarai , =?iso-8859-1?Q?Micka=EBl_Sala=FCn?= , =?iso-8859-1?Q?Micka=EBl_Sala=FCn?= , linux-kernel@vger.kernel.org, Alexei Starovoitov , Al Viro , Andy Lutomirski , Christian Heimes , Daniel Borkmann , Eric Chiang , James Morris , Jan Kara , Jann Horn , Jonathan Corbet , Kees Cook , Matthew Garrett , Matthew Wilcox , Michael Kerrisk , Mimi Zohar List-Id: linux-api@vger.kernel.org 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 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