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

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Date:   Tue, 12 Dec 2017 18:00:48 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...uxfoundation.org>, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        David Laight <David.Laight@...lab.com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [patch V2 2/2] x86/ldt: Prevent ldt inheritance on exec

On Mon, 11 Dec 2017, Peter Zijlstra wrote:
> 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.

Yeah, found that and fixed that already. Next version will be perfect :)