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Date:   Thu, 13 Dec 2018 09:13:10 -0800
From:   Matthew Wilcox <willy@infradead.org>
To:     =?iso-8859-1?Q?Micka=EBl_Sala=FCn?= <mickael.salaun@ssi.gouv.fr>
Cc:     =?iso-8859-1?Q?Micka=EBl_Sala=FCn?= <mic@digikod.net>,
        linux-kernel@vger.kernel.org, Al Viro <viro@zeniv.linux.org.uk>,
        James Morris <jmorris@namei.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Kees Cook <keescook@chromium.org>,
        Matthew Garrett <mjg59@google.com>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Mimi Zohar <zohar@linux.ibm.com>,
        Philippe =?iso-8859-1?Q?Tr=E9buchet?= 
        <philippe.trebuchet@ssi.gouv.fr>, Shuah Khan <shuah@kernel.org>,
        Thibaut Sautereau <thibaut.sautereau@ssi.gouv.fr>,
        Vincent Strubel <vincent.strubel@ssi.gouv.fr>,
        Yves-Alexis Perez <yves-alexis.perez@ssi.gouv.fr>,
        kernel-hardening@lists.openwall.com, linux-api@vger.kernel.org,
        linux-security-module@vger.kernel.org,
        linux-fsdevel@vger.kernel.org
Subject: Re: [RFC PATCH v1 0/5] Add support for O_MAYEXEC
Message-ID: <20181213171310.GR6830@bombadil.infradead.org>
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On Thu, Dec 13, 2018 at 04:17:29PM +0100, Micka?l Sala?n wrote:
> On 13/12/2018 04:02, Matthew Wilcox wrote:
> > On Wed, Dec 12, 2018 at 09:17:07AM +0100, Micka?l Sala?n wrote:
> >> The goal of this patch series is to control script interpretation.  A
> >> new O_MAYEXEC flag used by sys_open() is added to enable userland script
> >> interpreter to delegate to the kernel (and thus the system security
> >> policy) the permission to interpret scripts or other files containing
> >> what can be seen as commands.
> > 
> > I don't have a problem with the concept, but we're running low on O_ bits.
> > Does this have to be done before the process gets a file descriptor,
> > or could we have a new syscall?  Since we're going to be changing the
> > interpreters anyway, it doesn't seem like too much of an imposition to
> > ask them to use:
> > 
> > 	int verify_for_exec(int fd)
> > 
> > instead of adding an O_MAYEXEC.
> 
> Adding a new syscall for this simple use case seems excessive. I think

We have somewhat less than 400 syscalls today.  We have 20 O_ bits defined.
Obviously there's a lower practical limit on syscalls, but in principle
we could have up to 2^32 syscalls, and there are only 12 O_ bits remaining.

> that the open/openat syscall familly are the right place to do an atomic
> open and permission check, the same way the kernel does for other file
> access. Moreover, it will be easier to patch upstream interpreters
> without the burden of handling a (new) syscall that may not exist on the
> running system, whereas unknown open flags are ignored.

Ah, but that's the problem.  The interpreter can see an -ENOSYS response
and handle it appropriately.  If the flag is silently ignored, the
interpreter has no idea whether it can do a racy check or whether to
skip even trying to do the check.