From: Will Deacon <will@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org, Kees Cook <keescook@chromium.org>,
Ingo Molnar <mingo@kernel.org>,
Elena Reshetova <elena.reshetova@intel.com>,
Ard Biesheuvel <ard.biesheuvel@linaro.org>,
Hanjun Guo <guohanjun@huawei.com>,
Jan Glauber <jglauber@marvell.com>
Subject: Re: [PATCH 0/6] Rework REFCOUNT_FULL using atomic_fetch_* operations
Date: Fri, 9 Aug 2019 16:34:28 +0100 [thread overview]
Message-ID: <20190809153427.7dsds3u5j2gegp7z@willie-the-truck> (raw)
In-Reply-To: <20190802184947.GC2349@hirez.programming.kicks-ass.net>
On Fri, Aug 02, 2019 at 08:49:47PM +0200, Peter Zijlstra wrote:
> On Fri, Aug 02, 2019 at 11:09:54AM +0100, Will Deacon wrote:
>
> > Although the revised implementation passes all of the lkdtm REFCOUNT
> > tests, there is a race condition introduced by the deferred saturation
> > whereby if INT_MIN + 2 tasks take a reference on a refcount at
> > REFCOUNT_MAX and are each preempted between detecting overflow and
> > writing the saturated value without being rescheduled, then another task
> > may end up erroneously freeing the object when it drops the refcount and
> > sees zero. It doesn't feel like a particularly realistic case to me, but
> > I thought I should mention it in case somebody else knows better.
>
> So my OCD has always found that hole objectionable. Also I suppose the
> cmpxchg ones are simpler to understand.
>
> Maybe make this fancy stuff depend on !FULL ?
Hmm.
Right now, arm64 selects REFCOUNT_FULL, since I think it's important for
us to have this hardening enabled given the sorts of places we find
ourselves deployed. If the race above is a viable attack vector, then I'd
stick with the status quo, however Kees previously wrote this off as
"unrealistic":
https://lkml.kernel.org/r/CAGXu5jLXFA4=X5mC9ph9dZ0ZJaVkGXd2p1Vh8jH_EE15kVL6Hw@mail.gmail.com
and I'm inclined to agree with him given the conditions involved.
My understanding is that the current !FULL implementation (x86 only)
simply doesn't detect certain cases such as increment-from-zero, which
I think is different from being exposed to a theoretical race condition
involving billions of tasks each preempting each other one-by-one within
a handful of instructions. Even then, we'll still WARN!
Will
prev parent reply other threads:[~2019-08-09 15:34 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-02 10:09 [PATCH 0/6] Rework REFCOUNT_FULL using atomic_fetch_* operations Will Deacon
2019-08-02 10:09 ` [PATCH 1/6] lib/refcount: Define constants for saturation and max refcount values Will Deacon
2019-08-02 10:09 ` [PATCH 2/6] lib/refcount: Ensure integer operands are treated as signed Will Deacon
2019-08-02 10:09 ` [PATCH 3/6] lib/refcount: Remove unused refcount_*_checked() variants Will Deacon
2019-08-02 10:09 ` [PATCH 4/6] lib/refcount: Move bulk of REFCOUNT_FULL implementation into header Will Deacon
2019-08-02 18:52 ` Peter Zijlstra
2019-08-03 2:23 ` Kees Cook
2019-08-09 16:04 ` Will Deacon
2019-08-12 18:08 ` Kees Cook
2019-08-02 10:09 ` [PATCH 5/6] lib/refcount: Improve performance of generic REFCOUNT_FULL code Will Deacon
2019-08-02 18:55 ` Peter Zijlstra
2019-08-09 15:38 ` Will Deacon
2019-08-02 10:10 ` [PATCH 6/6] lib/refcount: Consolidate REFCOUNT_{MAX,SATURATED} definitions Will Deacon
2019-08-02 18:49 ` [PATCH 0/6] Rework REFCOUNT_FULL using atomic_fetch_* operations Peter Zijlstra
2019-08-09 15:34 ` Will Deacon [this message]
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