From: Ard Biesheuvel <ard.biesheuvel@linaro.org>
To: Kees Cook <keescook@chromium.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: Mon, 17 Jun 2019 13:33:19 +0200 [thread overview]
Message-ID: <CAKv+Gu_8ibO4D01DZv6KjL2GnvKuVBVnt=doxkN0w=4utJ7NvQ@mail.gmail.com> (raw)
In-Reply-To: <201906161429.BCE1083@keescook>
On Sun, 16 Jun 2019 at 23:31, Kees Cook <keescook@chromium.org> wrote:
>
> 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.
>
Of course. So there is no issue here: whatever manipulations are
racing with the overflow handler can never result in the counter to
unsaturate.
And actually, moving the checks before the stores is not as trivial as
I thought, E.g., for the LSE refcount_add case, we have
" ldadd %w[i], w30, %[cval]\n" \
" adds %w[i], %w[i], w30\n" \
REFCOUNT_PRE_CHECK_ ## pre (w30)) \
REFCOUNT_POST_CHECK_ ## post \
and changing this into load/test/store defeats the purpose of using
the LSE atomics in the first place.
On my single core TX2, the comparative performance is as follows
Baseline: REFCOUNT_TIMING test using REFCOUNT_FULL (LSE cmpxchg)
191057942484 cycles # 2.207 GHz
148447589402 instructions # 0.78 insn per
cycle
86.568269904 seconds time elapsed
Upper bound: ATOMIC_TIMING
116252672661 cycles # 2.207 GHz
28089216452 instructions # 0.24 insn per
cycle
52.689793525 seconds time elapsed
REFCOUNT_TIMING test using LSE atomics
127060259162 cycles # 2.207 GHz
0 instructions # 0.00 insn per
cycle
57.243690077 seconds time elapsed
next prev parent reply other threads:[~2019-06-17 11:33 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
2019-06-17 11:33 ` Ard Biesheuvel [this message]
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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