linux-kernel - Re: [RFC PATCH v1 1/5] fs: Add support for an O_MAYEXEC flag on sys

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Message-ID: <7da3a491-232a-5a02-63a2-65aa9df413ea@ssi.gouv.fr>
Date:   Wed, 17 Apr 2019 17:04:41 +0200
From:   Mickaël Salaün <mickael.salaun@....gouv.fr>
To:     Florian Weimer <fweimer@...hat.com>,
        Steve Grubb <sgrubb@...hat.com>
CC:     Jan Kara <jack@...e.cz>,
        Mickaël Salaün <mic@...ikod.net>,
        <linux-kernel@...r.kernel.org>, Al Viro <viro@...iv.linux.org.uk>,
        James Morris <jmorris@...ei.org>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Matthew Garrett <mjg59@...gle.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        Philippe Trébuchet 
        <philippe.trebuchet@....gouv.fr>, Shuah Khan <shuah@...nel.org>,
        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>,
        Matthew Bobrowski <mbobrowski@...browski.org>
Subject: Re: [RFC PATCH v1 1/5] fs: Add support for an O_MAYEXEC flag on
 sys_open()

On 17/04/2019 12:01, Florian Weimer wrote:
> * Steve Grubb:
>
>> On Tuesday, April 16, 2019 7:49:39 AM EDT Florian Weimer wrote:
>>> * Steve Grubb:
>>>> This flag that is being proposed means that you would have to patch all
>>>> interpreters to use it. If you are sure that upstreams will accept that,
>>>> why not just change the policy to interpreters shouldn't execute
>>>> anything unless the execute bit is set? That is simpler and doesn't need
>>>> a kernel change. And setting the execute bit is an auditable event.
>>>
>>> I think we need something like O_MAYEXEC so that security policies can
>>> be enforced and noexec mounts can be detected.
>>
>> Application whitelisting can already today stop unknown software without
>> needing O_MAYEXEC.

Whitelisting may be a lot of thing (path/TPE, signed binaries…), but
being able to handle this with a global system configuration (instead of
app-specific hardcoded configuration) is a good idea. ;)

>
> I'm somewhat interested in using this to add a proper check for
> executability to explicit dynamic loader invocations.  In other words,
> this
>
>   /lib64/ld-linux-x86-64.so.2 /path/to/noexec/fs/program
>
> should refuse to run the program if the program is located on a file
> system mounted with the noexec attribute.

What if a sysadmin need to do this on an executable mount point? Being
able to enforce a security policy according to a configuration may fit
to much more use cases.

>
>> The problem is that passing O_MAYEXEC is opt-in. You can use ptrace/seccomp/
>> bpf/LD_PRELOAD/LD_AUDIT to remove that bit from an otherwise normal program.
>> This does not require privs to do so.
>
> That doesn't really help with the above.

Right, ptrace/LD_PRELOAD and so on must be addressed by something else
than only O_MAYEXEC.

>
>> But let's consider that this comes to pass and every interpreter is
>> updated and IMA can see the O_MAYEXEC flag. Attackers now simply pivot
>> to running programs via stdin. It never touches disk and therefore
>> nothing enforces security policy. This already is among the most
>> common ways that malware runs today to evade detection.

As my previous reply, use cases like stdin may be restricted as well.

>
> Are you referring to Windows malware using Powershell?
>
> I'm not sure this is applicable to Linux.  We do not have much
> behavioral monitoring anyway.
>
> Thanks,
> Florian
>

--
Mickaël Salaün
ANSSI/SDE/ST/LAM

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