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Date: Thu, 13 Dec 2018 09:44:08 -0800
From: Matthew Wilcox <willy@...radead.org>
To: Mickaël Salaün <mickael.salaun@....gouv.fr>
Cc: 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
Subject: Re: [RFC PATCH v1 0/5] Add support for O_MAYEXEC

On Thu, Dec 13, 2018 at 06:36:15PM +0100, Mickaël Salaün wrote:
> On 13/12/2018 18:13, Matthew Wilcox wrote:
> > On Thu, Dec 13, 2018 at 04:17:29PM +0100, Mickaël Salaün wrote:
> >> 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.
> 
> Right, but the interpreter should interpret the script if the open with
> O_MAYEXEC succeed (but not otherwise): it may be because the flag is
> known by the kernel and the system policy allow this call, or because
> the (old) kernel doesn't known about this flag (which is fine and needed
> for backward compatibility). The script interpretation must not failed
> if the kernel doesn't support O_MAYEXEC, it is then useless for the
> interpreter to do any additional check.

If that's the way interpreters want to work, then that's fine.  They
can just call the verify() syscall and ignore the -ENOSYS.  Done.

Or somebody who cares very, very deeply can change the interpreter to
decline to run any scripts if the kernel returns -ENOSYS.

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