linux-kernel - Re: [RFC PATCH v8 0/3] Add support for AT_INTERPRETED (was O

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Message-ID: <20200910092656.GA800@gandi.net>
Date:   Thu, 10 Sep 2020 11:26:56 +0200
From:   Thibaut Sautereau <thibaut.sautereau@...p-os.org>
To:     Matthew Wilcox <willy@...radead.org>,
        Al Viro <viro@...iv.linux.org.uk>
Cc:     Mickaël Salaün <mic@...ikod.net>,
        linux-kernel@...r.kernel.org, Aleksa Sarai <cyphar@...har.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Christian Heimes <christian@...hon.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Deven Bowers <deven.desai@...ux.microsoft.com>,
        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>,
        Jann Horn <jannh@...gle.com>, Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>,
        Matthew Garrett <mjg59@...gle.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Mimi Zohar <zohar@...ux.ibm.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>,
        Vincent Strubel <vincent.strubel@....gouv.fr>,
        kernel-hardening@...ts.openwall.com, linux-api@...r.kernel.org,
        linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>
Subject: Re: [RFC PATCH v8 0/3] Add support for AT_INTERPRETED (was O_MAYEXEC)

On Wed, Sep 09, 2020 at 06:08:51PM +0100, Matthew Wilcox wrote:
> On Wed, Sep 09, 2020 at 09:19:11AM +0200, Mickaël Salaün wrote:
> > 
> > On 08/09/2020 20:50, Al Viro wrote:
> > > On Tue, Sep 08, 2020 at 09:59:53AM +0200, Mickaël Salaün wrote:
> > >> Hi,
> > >>
> > >> This height patch series rework the previous O_MAYEXEC series by not
> > >> adding a new flag to openat2(2) but to faccessat2(2) instead.  As
> > >> suggested, this enables to perform the access check on a file descriptor
> > >> instead of on a file path (while opening it).  This may require two
> > >> checks (one on open and then with faccessat2) but it is a more generic
> > >> approach [8].
> > > 
> > > Again, why is that folded into lookup/open/whatnot, rather than being
> > > an operation applied to a file (e.g. O_PATH one)?
> > 
> > I don't understand your question. AT_INTERPRETED can and should be used
> > with AT_EMPTY_PATH. The two checks I wrote about was for IMA.
> 
> Al is saying you should add a new syscall, not try to fold it into
> some existing syscall.
> 
> I agree with him.  Add a new syscall, just like you were told to do it
> last time.

Sure, we'll do it. In the meantime, could we at least get an explanation
about why using faccessat2() instead of a new syscall is wrong? I could
see the reasons for separating the exec checks from the file opening,
but this time I don't understand. Is it because it brings too much
complexity to do_faccessat()?

-- 
Thibaut Sautereau
CLIP OS developer