Message-ID: <20200811140203.GQ17456@casper.infradead.org>
Date:   Tue, 11 Aug 2020 15:02:03 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Mimi Zohar <zohar@...ux.ibm.com>
Cc:     Mickaël Salaün <mic@...ikod.net>,
        Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Deven Bowers <deven.desai@...ux.microsoft.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Aleksa Sarai <cyphar@...har.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Christian Heimes <christian@...hon.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Eric Biggers <ebiggers@...nel.org>,
        Eric Chiang <ericchiang@...gle.com>,
        Florian Weimer <fweimer@...hat.com>,
        James Morris <jmorris@...ei.org>, Jan Kara <jack@...e.cz>,
        Jonathan Corbet <corbet@....net>,
        Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>,
        Matthew Garrett <mjg59@...gle.com>,
        Michael Kerrisk <mtk.manpages@...il.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>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Thibaut Sautereau <thibaut.sautereau@...p-os.org>,
        Vincent Strubel <vincent.strubel@....gouv.fr>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Linux API <linux-api@...r.kernel.org>,
        linux-integrity@...r.kernel.org,
        linux-security-module <linux-security-module@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH v7 0/7] Add support for O_MAYEXEC

On Tue, Aug 11, 2020 at 09:56:50AM -0400, Mimi Zohar wrote:
> On Tue, 2020-08-11 at 10:48 +0200, Mickaël Salaün wrote:
> > On 11/08/2020 01:03, Jann Horn wrote:
> > > On Tue, Aug 11, 2020 at 12:43 AM Mickaël Salaün <mic@...ikod.net> wrote:
> > > > On 10/08/2020 22:21, Al Viro wrote:
> > > > > On Mon, Aug 10, 2020 at 10:11:53PM +0200, Mickaël Salaün wrote:
> > > > > > It seems that there is no more complains nor questions. Do you want me
> > > > > > to send another series to fix the order of the S-o-b in patch 7?
> > > > > 
> > > > > There is a major question regarding the API design and the choice of
> > > > > hooking that stuff on open().  And I have not heard anything resembling
> > > > > a coherent answer.
> > > > 
> > > > Hooking on open is a simple design that enables processes to check files
> > > > they intend to open, before they open them. From an API point of view,
> > > > this series extends openat2(2) with one simple flag: O_MAYEXEC. The
> > > > enforcement is then subject to the system policy (e.g. mount points,
> > > > file access rights, IMA, etc.).
> > > > 
> > > > Checking on open enables to not open a file if it does not meet some
> > > > requirements, the same way as if the path doesn't exist or (for whatever
> > > > reasons, including execution permission) if access is denied.
> > > 
> > > You can do exactly the same thing if you do the check in a separate
> > > syscall though.
> > > 
> > > And it provides a greater degree of flexibility; for example, you can
> > > use it in combination with fopen() without having to modify the
> > > internals of fopen() or having to use fdopen().
> > > 
> > > > It is a
> > > > good practice to check as soon as possible such properties, and it may
> > > > enables to avoid (user space) time-of-check to time-of-use (TOCTOU)
> > > > attacks (i.e. misuse of already open resources).
> > > 
> > > The assumption that security checks should happen as early as possible
> > > can actually cause security problems. For example, because seccomp was
> > > designed to do its checks as early as possible, including before
> > > ptrace, we had an issue for a long time where the ptrace API could be
> > > abused to bypass seccomp filters.
> > > 
> > > Please don't decide that a check must be ordered first _just_ because
> > > it is a security check. While that can be good for limiting attack
> > > surface, it can also create issues when the idea is applied too
> > > broadly.
> > 
> > I'd be interested with such security issue examples.
> > 
> > I hope that delaying checks will not be an issue for mechanisms such as
> > IMA or IPE:
> > https://lore.kernel.org/lkml/1544699060.6703.11.camel@linux.ibm.com/
> > 
> > Any though Mimi, Deven, Chrome OS folks?
> 
> One of the major gaps, defining a system wide policy requiring all code
> being executed to be signed, is interpreters.  The kernel has no
> context for the interpreter's opening the file.  From an IMA
> perspective, this information needs to be conveyed to the kernel prior
> to ima_file_check(), which would allow IMA policy rules to be defined
> in terms of O_MAYEXEC.

This is kind of evading the point -- Mickaël is proposing a new flag
to open() to tell IMA to measure the file being opened before the fd
is returned to userspace, and Al is suggesting a new syscall to allow
a previously-obtained fd to be measured.

I think what you're saying is that you don't see any reason to prefer
one over the other.

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