Re: [RFC PATCH v8 1/3] fs: Introduce AT_INTERPRETED flag for faccessat2(2)

From: Stephen Smalley <stephen.smalley.work@gmail.com>
To: "Mickaël Salaün" <mic@digikod.net>
Cc: "Mimi Zohar" <zohar@linux.ibm.com>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	"Aleksa Sarai" <cyphar@cyphar.com>,
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	kernel-hardening@lists.openwall.com, linux-api@vger.kernel.org,
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	"LSM List" <linux-security-module@vger.kernel.org>,
	"Linux FS Devel" <linux-fsdevel@vger.kernel.org>,
	"Thibaut Sautereau" <thibaut.sautereau@ssi.gouv.fr>,
	"Mickaël Salaün" <mic@linux.microsoft.com>,
	"John Johansen" <john.johansen@canonical.com>
Subject: Re: [RFC PATCH v8 1/3] fs: Introduce AT_INTERPRETED flag for faccessat2(2)
Date: Tue, 8 Sep 2020 08:50:14 -0400	[thread overview]
Message-ID: <CAEjxPJ49_BgGX50ZAhHh79Qy3OMN6sssnUHT_2yXqdmgyt==9w@mail.gmail.com> (raw)
In-Reply-To: <75451684-58f3-b946-dca4-4760fa0d7440@digikod.net>

On Tue, Sep 8, 2020 at 8:43 AM Mickaël Salaün <mic@digikod.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.