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Date: Tue, 12 Dec 2017 19:09:18 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Andy Lutomirski <luto@...nel.org>
Cc: Thomas Gleixner <tglx@...utronix.de>,
LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Dave Hansen <dave.hansen@...el.com>,
Borislav Petkov <bpetkov@...e.de>,
Greg KH <gregkh@...uxfoundation.org>,
Kees Cook <keescook@...gle.com>,
Hugh Dickins <hughd@...gle.com>,
Brian Gerst <brgerst@...il.com>,
Josh Poimboeuf <jpoimboe@...hat.com>,
Denys Vlasenko <dvlasenk@...hat.com>,
Boris Ostrovsky <boris.ostrovsky@...cle.com>,
Juergen Gross <jgross@...e.com>,
David Laight <David.Laight@...lab.com>,
Eduardo Valentin <eduval@...zon.com>, aliguori@...zon.com,
Will Deacon <will.deacon@....com>,
"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [patch 11/16] x86/ldt: Force access bit for CS/SS
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@...utronix.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.
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