linux-kernel - Re: [RFC PATCH v1 0/5] Add support for O

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Message-ID: <20181213121645.GN6830@bombadil.infradead.org>
Date:   Thu, 13 Dec 2018 04:16:46 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Mimi Zohar <zohar@...ux.ibm.com>
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>,
        Mickaël Salaün <mickael.salaun@....gouv.fr>,
        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 06:04:20AM -0500, Mimi Zohar wrote:
> > 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.
> 
> The indication needs to be set during file open, before the open
> returns to the caller.  This is the point where ima_file_check()
> verifies the file's signature.  On failure, access to the file is
> denied.

I understand that's what happens today, but do we need to do it that way?
There's no harm in the interpreter having an fd to a file if it knows
not to execute it.  This is different from a program opening a file and
having the LSM deny access to it because it violates the security model.