Re: [patch V2 2/2] x86/ldt: Prevent ldt inheritance on exec

[Peter Zijlstra]

On Sat, Dec 09, 2017 at 07:24:35PM +0100, Thomas Gleixner wrote:
> On Fri, 8 Dec 2017, Thomas Gleixner wrote:
> > +int ldt_dup_context(struct mm_struct *old_mm, struct mm_struct *mm)
> >  {
> >  	struct ldt_struct *new_ldt;
> > -	struct mm_struct *old_mm;
> >  	int retval = 0;
> >  
> > -	mutex_init(&mm->context.lock);
> > -	old_mm = current->mm;
> > -	if (!old_mm) {
> > -		mm->context.ldt = NULL;
> > +	if (!old_mm)
> >  		return 0;
> > -	}
> >  
> >  	mutex_lock(&old_mm->context.lock);
> 
> Bah. That's broken. It now nests into old_mm->mmap_sem which is the reverse
> lock order than in ldt_write. Will fix.

But read_ldt() will still nest mmap_sem inside context.lock, no? Lockdep
doesn't care about old_mm vs new_mm.