Date: Tue, 12 Dec 2017 19:09:18 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Andy Lutomirski <luto@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Dave Hansen <dave.hansen@intel.com>, Borislav Petkov <bpetkov@suse.de>,
        Greg KH <gregkh@linuxfoundation.org>, Kees Cook <keescook@google.com>,
        Hugh Dickins <hughd@google.com>, Brian Gerst <brgerst@gmail.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Denys Vlasenko <dvlasenk@redhat.com>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Juergen Gross <jgross@suse.com>,
        David Laight <David.Laight@aculab.com>,
        Eduardo Valentin <eduval@amazon.com>, aliguori@amazon.com,
        Will Deacon <will.deacon@arm.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>
Subject: Re: [patch 11/16] x86/ldt: Force access bit for CS/SS
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On Tue, Dec 12, 2017 at 10:03:02AM -0800, Andy Lutomirski wrote:
> On Tue, Dec 12, 2017 at 9:32 AM, Thomas Gleixner <tglx@linutronix.de> wrote:

> > @@ -171,6 +172,9 @@ static void exit_to_usermode_loop(struct
> >                 /* Disable IRQs and retry */
> >                 local_irq_disable();
> >
> > +               if (cached_flags & _TIF_LDT)
> > +                       ldt_exit_user(regs);
> 
> Nope.  To the extent that this code actually does anything (which it
> shouldn't since you already forced the access bit),

Without this; even with the access bit set; IRET will go wobbly and
we'll #GP on the user-space side. Try it ;-)

> it's racy against
> flush_ldt() from another thread, and that race will be exploitable for
> privilege escalation.  It needs to be outside the loopy part.

The flush_ldt (__ldt_install after these patches) would re-set the TIF
flag. But sure, we can move this outside the loop I suppose.