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

[Stephen Smalley]

On Tue, Sep 8, 2020 at 8:50 AM Stephen Smalley
<stephen.smalley.work@xxxxxxxxx> wrote:
>
> On Tue, Sep 8, 2020 at 8:43 AM Mickaël Salaün <mic@xxxxxxxxxxx> 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.