Re: [PATCH v3 4/5] mm: Add a NO_INHERIT flag to the PR_SET_MDWE prctl

[Catalin Marinas <catalin.marinas@arm.com>]

On Fri, Aug 25, 2023 at 03:38:36PM -0700, Kees Cook wrote:
> On Tue, Jul 04, 2023 at 05:36:28PM +0200, Florent Revest wrote:
> >  static inline int prctl_set_mdwe(unsigned long bits, unsigned long arg3,
> >  				 unsigned long arg4, unsigned long arg5)
> >  {
> > +	unsigned long current_bits;
> > +
> >  	if (arg3 || arg4 || arg5)
> >  		return -EINVAL;
> >  
> > -	if (bits & ~(PR_MDWE_REFUSE_EXEC_GAIN))
> > +	if (bits & ~(PR_MDWE_REFUSE_EXEC_GAIN | PR_MDWE_NO_INHERIT))
> > +		return -EINVAL;
> > +
> > +	/* NO_INHERIT only makes sense with REFUSE_EXEC_GAIN */
> > +	if (bits & PR_MDWE_NO_INHERIT && !(bits & PR_MDWE_REFUSE_EXEC_GAIN))
> >  		return -EINVAL;
> >  
> > +	current_bits = get_current_mdwe();
> > +	if (current_bits && current_bits != bits)
> > +		return -EPERM; /* Cannot unset the flags */
> 
> I was pondering why PR_MDWE_NO_INHERIT can't be unset, but I guess it
> doesn't matter. Anything forked with have it off, and any process
> wanting to launch stuff before locking down can just skip running the
> prctl() until later.

I had a similar doubt initially but then realised that the no-inherit
mode won't be inherited and concluded it's ok.

-- 
Catalin