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

[Mimi Zohar <zohar@linux.ibm.com>]

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?

Mimi