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From: Andy Lutomirski <luto@kernel.org>
Date: Tue, 12 Dec 2017 10:10:40 -0800
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Subject: Re: [patch 11/16] x86/ldt: Force access bit for CS/SS
To: Peter Zijlstra <peterz@infradead.org>
Cc: Andy Lutomirski <luto@kernel.org>,
        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>,
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        Will Deacon <will.deacon@arm.com>,
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On Tue, Dec 12, 2017 at 10:09 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> 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 ;-)

Maybe later.

But that means that we need Intel and AMD to confirm WTF is going on
before this blows up even with LAR on some other CPU.

>
>> 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.