linux-kernel - Re: [PATCH v14 1/3] fs: Add trusted_for(2) syscall implementation and related sysctl

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Date:   Mon, 11 Oct 2021 10:26:58 +0200
From:   Mickaël Salaün <mic@...ikod.net>
To:     Florian Weimer <fw@...eb.enyo.de>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Aleksa Sarai <cyphar@...har.com>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Christian Heimes <christian@...hon.org>,
        Deven Bowers <deven.desai@...ux.microsoft.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Eric Biggers <ebiggers@...nel.org>,
        Eric Chiang <ericchiang@...gle.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        James Morris <jmorris@...ei.org>, Jan Kara <jack@...e.cz>,
        Jann Horn <jannh@...gle.com>, Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>,
        "Madhavan T . Venkataraman" <madvenka@...ux.microsoft.com>,
        Matthew Garrett <mjg59@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        Paul Moore <paul@...l-moore.com>,
        Philippe Trébuchet 
        <philippe.trebuchet@....gouv.fr>,
        Scott Shell <scottsh@...rosoft.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@...ts.openwall.com, linux-api@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-integrity@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        Mickaël Salaün <mic@...ux.microsoft.com>
Subject: Re: [PATCH v14 1/3] fs: Add trusted_for(2) syscall implementation and
 related sysctl


On 10/10/2021 16:10, Florian Weimer wrote:
> * Mickaël Salaün:
> 
>> Being able to restrict execution also enables to protect the kernel by
>> restricting arbitrary syscalls that an attacker could perform with a
>> crafted binary or certain script languages.  It also improves multilevel
>> isolation by reducing the ability of an attacker to use side channels
>> with specific code.  These restrictions can natively be enforced for ELF
>> binaries (with the noexec mount option) but require this kernel
>> extension to properly handle scripts (e.g. Python, Perl).  To get a
>> consistent execution policy, additional memory restrictions should also
>> be enforced (e.g. thanks to SELinux).
> 
> One example I have come across recently is that code which can be
> safely loaded as a Perl module is definitely not a no-op as a shell
> script: it downloads code and executes it, apparently over an
> untrusted network connection and without signature checking.
> 
> Maybe in the IMA world, the expectation is that such ambiguous code
> would not be signed in the first place, but general-purpose
> distributions are heading in a different direction with
> across-the-board signing:
> 
>   Signed RPM Contents
>   <https://fedoraproject.org/wiki/Changes/Signed_RPM_Contents>
> 
> So I wonder if we need additional context information for a potential
> LSM to identify the intended use case.
> 

This is an interesting use case. I think such policy enforcement could
be done either with an existing LSM (e.g. IMA) or a new one (e.g. IPE),
but it could also partially be enforced by the script interpreter. The
kernel should have enough context: interpreter process (which could be
dedicated to a specific usage) and the opened script file, or we could
add a new usage flag to the trusted_for syscall if that makes sense.
Either way, this doesn't seem to be an issue for the current patch series.