linux-kernel - Re: [PATCH v3 0/5] Add support for RESOLVE

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Message-ID: <CAG48ez2TphTj-VdDaSjvnr0Q8BhNmT3n86xYz4bF3wRJmAMsMw@mail.gmail.com>
Date:   Wed, 29 Apr 2020 00:01:38 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Florian Weimer <fw@...eb.enyo.de>
Cc:     Mickaël Salaün <mic@...ikod.net>,
        kernel list <linux-kernel@...r.kernel.org>,
        Aleksa Sarai <cyphar@...har.com>,
        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>,
        Deven Bowers <deven.desai@...ux.microsoft.com>,
        Eric Chiang <ericchiang@...gle.com>,
        James Morris <jmorris@...ei.org>, Jan Kara <jack@...e.cz>,
        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>,
        Mickaël Salaün <mickael.salaun@....gouv.fr>,
        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>,
        Steve Dower <steve.dower@...hon.org>,
        Steve Grubb <sgrubb@...hat.com>,
        Thibaut Sautereau <thibaut.sautereau@....gouv.fr>,
        Vincent Strubel <vincent.strubel@....gouv.fr>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Linux API <linux-api@...r.kernel.org>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH v3 0/5] Add support for RESOLVE_MAYEXEC

On Tue, Apr 28, 2020 at 11:21 PM Florian Weimer <fw@...eb.enyo.de> wrote:
> * Jann Horn:
>
> > Just as a comment: You'd probably also have to use RESOLVE_MAYEXEC in
> > the dynamic linker.
>
> Absolutely.  In typical configurations, the kernel does not enforce
> that executable mappings must be backed by files which are executable.
> It's most obvious with using an explicit loader invocation to run
> executables on noexec mounts.  RESOLVE_MAYEXEC is much more useful
> than trying to reimplement the kernel permission checks (or what some
> believe they should be) in userspace.

Oh, good point.

That actually seems like something Mickaël could add to his series? If
someone turns on that knob for "When an interpreter wants to execute
something, enforce that we have execute access to it", they probably
also don't want it to be possible to just map files as executable? So
perhaps when that flag is on, the kernel should either refuse to map
anything as executable if it wasn't opened with RESOLVE_MAYEXEC or
(less strict) if RESOLVE_MAYEXEC wasn't used, print a warning, then
check whether the file is executable and bail out if not?

A configuration where interpreters verify that scripts are executable,
but other things can just mmap executable pages, seems kinda
inconsistent...