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

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

[Cc'ing Casey]

On Tue, 2020-09-08 at 16:14 +0200, Mickaël Salaün wrote:
> On 08/09/2020 15:42, Stephen Smalley wrote:
> > 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.
> > 
> 
> We could create a new hook path_permission(struct path *path, int mask)
> as a superset of inode_permission(). To be more convenient, his new hook
> could then just call inode_permission() for every LSMs not implementing
> path_permission().

The LSM maintainers need to chime in here on this suggestion.  In terms
of the name, except for one hook, all the security_path_XXXX() hooks
are dependent on CONFIG_SECURITY_PATH being configured.

Mimi