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Message-ID: <CAEjxPJ6ZTKeunzJvWf_kS3QYjca6v1yJq=ad-jCCuDSgG6n60g@mail.gmail.com>
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>, 
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	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.

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