[PATCH v5 3/6] fs: Enable to enforce noexec mounts or file exec through O_MAYEXEC

[Kees Cook]

On Thu, May 14, 2020 at 08:22:01AM -0400, Stephen Smalley wrote:
> On Wed, May 13, 2020 at 11:05 PM Kees Cook <keescook at chromium.org> wrote:
> >
> > On Wed, May 13, 2020 at 04:27:39PM -0700, Kees Cook wrote:
> > > Like, couldn't just the entire thing just be:
> > >
> > > diff --git a/fs/namei.c b/fs/namei.c
> > > index a320371899cf..0ab18e19f5da 100644
> > > --- a/fs/namei.c
> > > +++ b/fs/namei.c
> > > @@ -2849,6 +2849,13 @@ static int may_open(const struct path *path, int acc_mode, int flag)
> > >               break;
> > >       }
> > >
> > > +     if (unlikely(mask & MAY_OPENEXEC)) {
> > > +             if (sysctl_omayexec_enforce & OMAYEXEC_ENFORCE_MOUNT &&
> > > +                 path_noexec(path))
> > > +                     return -EACCES;
> > > +             if (sysctl_omayexec_enforce & OMAYEXEC_ENFORCE_FILE)
> > > +                     acc_mode |= MAY_EXEC;
> > > +     }
> > >       error = inode_permission(inode, MAY_OPEN | acc_mode);
> > >       if (error)
> > >               return error;
> > >
> >
> > FYI, I've confirmed this now. Effectively with patch 2 dropped, patch 3
> > reduced to this plus the Kconfig and sysctl changes, the self tests
> > pass.
> >
> > I think this makes things much cleaner and correct.
> 
> I think that covers inode-based security modules but not path-based
> ones (they don't implement the inode_permission hook).  For those, I
> would tentatively guess that we need to make sure FMODE_EXEC is set on
> the open file and then they need to check for that in their file_open
> hooks.

Does there need to be an FMODE_OPENEXEC, or is the presence of
FMODE_OPEN with FMODE_EXEC sufficient?

-- 
Kees Cook