linux-kernel - Re: [PATCH v2 0/5] Add support for O

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Date:   Fri, 6 Sep 2019 12:26:51 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Steve Grubb <sgrubb@...hat.com>
Cc:     Florian Weimer <fweimer@...hat.com>,
        Mickaël Salaün <mic@...ikod.net>,
        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>,
        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>,
        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>,
        Song Liu <songliubraving@...com>,
        Steve Dower <steve.dower@...hon.org>,
        Thibaut S autereau <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 0/5] Add support for O_MAYEXEC



> On Sep 6, 2019, at 12:07 PM, Steve Grubb <sgrubb@...hat.com> wrote:
> 
>> On Friday, September 6, 2019 2:57:00 PM EDT Florian Weimer wrote:
>> * Steve Grubb:
>>> Now with LD_AUDIT
>>> $ LD_AUDIT=/home/sgrubb/test/openflags/strip-flags.so.0 strace ./test
>>> 2>&1 | grep passwd openat(3, "passwd", O_RDONLY)           = 4
>>> 
>>> No O_CLOEXEC flag.
>> 
>> I think you need to explain in detail why you consider this a problem.
> 
> Because you can strip the O_MAYEXEC flag from being passed into the kernel. 
> Once you do that, you defeat the security mechanism because it never gets 
> invoked. The issue is that the only thing that knows _why_ something is being 
> opened is user space. With this mechanism, you can attempt to pass this 
> reason to the kernel so that it may see if policy permits this. But you can 
> just remove the flag.

I’m with Florian here. Once you are executing code in a process, you could just emulate some other unapproved code. This series is not intended to provide the kind of absolute protection you’re imagining.

What the kernel *could* do is prevent mmapping a non-FMODE_EXEC file with PROT_EXEC, which would indeed have a real effect (in an iOS-like world, for example) but would break many, many things.