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From: Kees Cook <keescook@chromium.org>
To: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Will Deacon <will.deacon@arm.com>,
	Jayachandran Chandrasekharan Nair <jnair@marvell.com>,
	"catalin.marinas@arm.com" <catalin.marinas@arm.com>,
	Jan Glauber <jglauber@marvell.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>
Subject: Re: [RFC] Disable lockref on arm64
Date: Sun, 16 Jun 2019 14:31:24 -0700	[thread overview]
Message-ID: <201906161429.BCE1083@keescook> (raw)
In-Reply-To: <CAKv+Gu9-rZ16Nb9t3=knzW0BHu0eNxQoPwWS4c8UMMm=2iqiuw@mail.gmail.com>

On Sat, Jun 15, 2019 at 04:18:21PM +0200, Ard Biesheuvel wrote:
> Yes, I am using the same saturation point as x86. In this example, I
> am not entirely sure I understand why it matters, though: the atomics
> guarantee that the write by CPU2 fails if CPU1 changed the value in
> the mean time, regardless of which value it wrote.
> 
> I think the concern is more related to the likelihood of another CPU
> doing something nasty between the moment that the refcount overflows
> and the moment that the handler pins it at INT_MIN/2, e.g.,
> 
> > CPU 1                   CPU 2
> > inc()
> >   load INT_MAX
> >   about to overflow?
> >   yes
> >
> >   set to 0
> >                          <insert exploit here>
> >   set to INT_MIN/2

Ah, gotcha, but the "set to 0" is really "set to INT_MAX+1" (not zero)
if you're using the same saturation.

-- 
Kees Cook

  reply	other threads:[~2019-06-16 21:31 UTC|newest]

Thread overview: 37+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-04-29 14:52 [RFC] Disable lockref on arm64 Jan Glauber
2019-05-01 16:01 ` Will Deacon
2019-05-02  8:38   ` Jan Glauber
2019-05-01 16:41 ` Linus Torvalds
2019-05-02  8:27   ` Jan Glauber
2019-05-02 16:12     ` Linus Torvalds
2019-05-02 23:19       ` Jayachandran Chandrasekharan Nair
2019-05-03 19:40         ` Linus Torvalds
2019-05-06  6:13           ` [EXT] " Jayachandran Chandrasekharan Nair
2019-05-06 17:13             ` Linus Torvalds
2019-05-06 18:10             ` Will Deacon
2019-05-18  4:24               ` Jayachandran Chandrasekharan Nair
2019-05-18 10:00                 ` Ard Biesheuvel
2019-05-22 16:04                   ` Will Deacon
2019-06-12  4:10                     ` Jayachandran Chandrasekharan Nair
2019-06-12  9:31                       ` Will Deacon
2019-06-14  7:09                         ` Jayachandran Chandrasekharan Nair
2019-06-14  9:58                           ` Will Deacon
2019-06-14 10:24                             ` Ard Biesheuvel
2019-06-14 10:38                               ` Will Deacon
2019-06-15  4:21                                 ` Kees Cook
2019-06-15  8:47                                   ` Ard Biesheuvel
2019-06-15 13:59                                     ` Kees Cook
2019-06-15 14:18                                       ` Ard Biesheuvel
2019-06-16 21:31                                         ` Kees Cook [this message]
2019-06-17 11:33                                           ` Ard Biesheuvel
2019-06-17 17:26                                             ` Will Deacon
2019-06-17 20:07                                               ` Jayachandran Chandrasekharan Nair
2019-06-18  5:41                                               ` Kees Cook
2019-06-13  9:53                       ` Hanjun Guo
2019-06-05 13:48   ` [PATCH] lockref: Limit number of cmpxchg loop retries Jan Glauber
2019-06-05 20:16     ` Linus Torvalds
2019-06-06  8:03       ` Jan Glauber
2019-06-06  9:41         ` Will Deacon
2019-06-06 10:28           ` Jan Glauber
2019-06-07  7:27             ` Jan Glauber
2019-06-07 20:14               ` Linus Torvalds

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