linux-kernel - Re: [RFC PATCH v8 1/3] fs: Introduce AT

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Date:   Tue, 8 Sep 2020 08:52:01 -0400
From:   Stephen Smalley <stephen.smalley.work@...il.com>
To:     Mickaël Salaün <mic@...ikod.net>
Cc:     Mimi Zohar <zohar@...ux.ibm.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Aleksa Sarai <cyphar@...har.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        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>,
        Matthew Wilcox <willy@...radead.org>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Miklos Szeredi <mszeredi@...hat.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@...ts.openwall.com, linux-api@...r.kernel.org,
        linux-integrity@...r.kernel.org,
        LSM List <linux-security-module@...r.kernel.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        Thibaut Sautereau <thibaut.sautereau@....gouv.fr>,
        Mickaël Salaün <mic@...ux.microsoft.com>,
        John Johansen <john.johansen@...onical.com>
Subject: Re: [RFC PATCH v8 1/3] fs: Introduce AT_INTERPRETED flag for faccessat2(2)

On Tue, Sep 8, 2020 at 8:50 AM Stephen Smalley
<stephen.smalley.work@...il.com> wrote:
>
> On Tue, Sep 8, 2020 at 8:43 AM Mickaël Salaün <mic@...ikod.net> wrote:
> >
> >
> > On 08/09/2020 14:28, Mimi Zohar wrote:
> > > Hi Mickael,
> > >
> > > On Tue, 2020-09-08 at 09:59 +0200, Mickaël Salaün wrote:
> > >> +                    mode |= MAY_INTERPRETED_EXEC;
> > >> +                    /*
> > >> +                     * For compatibility reasons, if the system-wide policy
> > >> +                     * doesn't enforce file permission checks, then
> > >> +                     * replaces the execute permission request with a read
> > >> +                     * permission request.
> > >> +                     */
> > >> +                    mode &= ~MAY_EXEC;
> > >> +                    /* To be executed *by* user space, files must be readable. */
> > >> +                    mode |= MAY_READ;
> > >
> > > After this change, I'm wondering if it makes sense to add a call to
> > > security_file_permission().  IMA doesn't currently define it, but
> > > could.
> >
> > Yes, that's the idea. We could replace the following inode_permission()
> > with file_permission(). I'm not sure how this will impact other LSMs though.
>
> They are not equivalent at least as far as SELinux is concerned.
> security_file_permission() was only to be used to revalidate
> read/write permissions previously checked at file open to support
> policy changes and file or process label changes.  We'd have to modify
> the SELinux hook if we wanted to have it check execute access even if
> nothing has changed since open time.

Also Smack doesn't appear to implement file_permission at all, so it
would skip Smack checking.