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Date: Mon, 11 Dec 2017 13:13:02 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linuxfoundation.org>, x86@kernel.org,
        Andy Lutomirski <luto@kernel.org>, Borislav Petkov <bp@alien8.de>,
        Brian Gerst <brgerst@gmail.com>,
        David Laight <David.Laight@aculab.com>,
        Kees Cook <keescook@chromium.org>
Subject: Re: [patch V2 2/2] x86/ldt: Prevent ldt inheritance on exec
Message-ID: <20171211121302.bdjkhnfv2ncclrht@hirez.programming.kicks-ass.net>
References: <20171208223241.367344665@linutronix.de>
 <20171208223421.711848352@linutronix.de>
 <alpine.DEB.2.20.1712091922340.2301@nanos>
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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.