Date: Fri, 6 Sep 2019 12:46:20 -0600 From: Tycho Andersen <tycho@...ho.ws> To: Florian Weimer <fweimer@...hat.com> Cc: Christian Brauner <christian.brauner@...ntu.com>, Aleksa Sarai <cyphar@...har.com>, Mickaël Salaün <mickael.salaun@....gouv.fr>, Mickaël Salaün <mic@...ikod.net>, linux-kernel@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>, Al Viro <viro@...iv.linux.org.uk>, Andy Lutomirski <luto@...nel.org>, Christian Heimes <christian@...hon.org>, Daniel Borkmann <daniel@...earbox.net>, Eric Chiang <ericchiang@...gle.com>, James Morris <jmorris@...ei.org>, Jan Kara <jack@...e.cz>, Jann Horn <jannh@...gle.com>, Jonathan Corbet <corbet@....net>, Kees Cook <keescook@...omium.org>, Matthew Garrett <mjg59@...gle.com>, Matthew Wilcox <willy@...radead.org>, Michael Kerrisk <mtk.manpages@...il.com>, Mimi Zohar <zohar@...ux.ibm.com>, Philippe Trébuchet <philippe.trebuchet@....gouv.fr>, Scott Shell <scottsh@...rosoft.com>, Sean Christopherson <sean.j.christopherson@...el.com>, Shuah Khan <shuah@...nel.org>, Song Liu <songliubraving@...com>, Steve Dower <steve.dower@...hon.org>, Steve Grubb <sgrubb@...hat.com>, Thibaut Sautereau <thibaut.sautereau@....gouv.fr>, Vincent Strubel <vincent.strubel@....gouv.fr>, Yves-Alexis Perez <yves-alexis.perez@....gouv.fr>, kernel-hardening@...ts.openwall.com, linux-api@...r.kernel.org, linux-security-module@...r.kernel.org, linux-fsdevel@...r.kernel.org Subject: Re: [PATCH v2 1/5] fs: Add support for an O_MAYEXEC flag on sys_open() 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@....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
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