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Date: Thu, 20 Jul 2017 11:11:06 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Kees Cook <keescook@...omium.org>
Cc: Peter Zijlstra <peterz@...radead.org>,
Josh Poimboeuf <jpoimboe@...hat.com>,
Christoph Hellwig <hch@...radead.org>,
"Eric W. Biederman" <ebiederm@...ssion.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Jann Horn <jannh@...gle.com>, Eric Biggers <ebiggers3@...il.com>,
Elena Reshetova <elena.reshetova@...el.com>,
Hans Liljestrand <ishkamiel@...il.com>,
Greg KH <gregkh@...uxfoundation.org>,
Alexey Dobriyan <adobriyan@...il.com>,
"Serge E. Hallyn" <serge@...lyn.com>, arozansk@...hat.com,
Davidlohr Bueso <dave@...olabs.net>,
Manfred Spraul <manfred@...orfullife.com>,
"axboe@...nel.dk" <axboe@...nel.dk>,
James Bottomley <James.Bottomley@...senpartnership.com>,
"x86@...nel.org" <x86@...nel.org>, Arnd Bergmann <arnd@...db.de>,
"David S. Miller" <davem@...emloft.net>,
Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
linux-arch <linux-arch@...r.kernel.org>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v6 0/2] x86: Implement fast refcount overflow protection
* Kees Cook <keescook@...omium.org> wrote:
> This implements refcount_t overflow protection on x86 without a noticeable
> performance impact, though without the fuller checking of REFCOUNT_FULL.
> This is done by duplicating the existing atomic_t refcount implementation
> but with normally a single instruction added to detect if the refcount
> has gone negative (i.e. wrapped past INT_MAX or below zero). When
> detected, the handler saturates the refcount_t to INT_MIN / 2. With this
> overflow protection, the erroneous reference release that would follow
> a wrap back to zero is blocked from happening, avoiding the class of
> refcount-over-increment use-after-free vulnerabilities entirely.
>
> Only the overflow case of refcounting can be perfectly protected, since it
> can be detected and stopped before the reference is freed and left to be
> abused by an attacker. This implementation also notices some of the "dec
> to 0 without test", and "below 0" cases. However, these only indicate that
> a use-after-free may have already happened. Such notifications are likely
> avoidable by an attacker that has already exploited a use-after-free
> vulnerability, but it's better to have them than allow such conditions to
> remain universally silent.
>
> On first overflow detection, the refcount value is reset to INT_MIN / 2
> (which serves as a saturation value), the offending process is killed,
> and a report and stack trace are produced. When operations detect only
> negative value results (such as changing an already saturated value),
> saturation still happens but no notification is performed (since the
> value was already saturated).
>
> On the matter of races, since the entire range beyond INT_MAX but before
> 0 is negative, every operation at INT_MIN / 2 will trap, leaving no
> overflow-only race condition.
>
> As for performance, this implementation adds a single "js" instruction
> to the regular execution flow of a copy of the standard atomic_t refcount
> operations. (The non-"and_test" refcount_dec() function, which is uncommon
> in regular refcount design patterns, has an additional "jz" instruction
> to detect reaching exactly zero.) Since this is a forward jump, it is by
> default the non-predicted path, which will be reinforced by dynamic branch
> prediction. The result is this protection having virtually no measurable
> change in performance over standard atomic_t operations. The error path,
> located in .text.unlikely, saves the refcount location and then uses UD0
> to fire a refcount exception handler, which resets the refcount, handles
> reporting, and returns to regular execution. This keeps the changes to
> .text size minimal, avoiding return jumps and open-coded calls to the
> error reporting routine.
Pretty nice!
Could you please also create a tabulated quick-comparison of the three variants,
of all key properties, about behavior, feature and tradeoff differences?
Something like:
!ARCH_HAS_REFCOUNT ARCH_HAS_REFCOUNT=y REFCOUNT_FULL=y
avg fast path instructions: 5 3 10
behavior on overflow: unsafe, silent safe, verbose safe, verbose
behavior on underflow: unsafe, silent unsafe, verbose unsafe, verbose
...
etc. - note that this table is just a quick mockup with wild guesses. (Please add
more comparisons of other aspects as well.)
Such a comparison would make it easier for arch, subsystem and distribution
maintainers to decide on which variant to use/enable.
Thanks,
Ingo
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