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

[Stephen Smalley <stephen.smalley.work@gmail.com>]

On Tue, Sep 8, 2020 at 9:29 AM Mimi Zohar <zohar@linux.ibm.com> wrote:
>
> On Tue, 2020-09-08 at 08:52 -0400, Stephen Smalley wrote:
> > On Tue, Sep 8, 2020 at 8:50 AM Stephen Smalley
> > <stephen.smalley.work@gmail.com> wrote:
> > >
> > > 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.
>
> I wasn't suggesting replacing the existing security_inode_permission
> hook later, but adding a new security_file_permission hook here.
>
> > >
> > > 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.
>
> My question is whether adding a new security_file_permission call here
> would break either SELinux or Apparmor?

selinux_inode_permission() has special handling for MAY_ACCESS so we'd
need to duplicate that into selinux_file_permission() ->
selinux_revalidate_file_permission().  Also likely need to adjust
selinux_file_permission() to explicitly check whether the mask
includes any permissions not checked at open time.  So some changes
would be needed here.  By default, it would be a no-op unless there
was a policy reload or the file was relabeled between the open(2) and
the faccessat(2) call.