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Date: Tue, 9 May 2017 14:33:02 -0500
From: Josh Poimboeuf <>
To: Kees Cook <>
Cc:, Peter Zijlstra <>,
	PaX Team <>, Jann Horn <>,
	Eric Biggers <>,
	Christoph Hellwig <>,
	"" <>,
	James Bottomley <>,
	Elena Reshetova <>,
	Hans Liljestrand <>,
	David Windsor <>,
	"" <>, Ingo Molnar <>,
	Arnd Bergmann <>,
	Greg Kroah-Hartman <>,
	"David S. Miller" <>,
	Rik van Riel <>,
	linux-arch <>,
Subject: Re: [PATCH v4 2/2] x86/refcount: Implement fast refcount overflow

On Tue, May 09, 2017 at 12:01:23PM -0700, Kees Cook wrote:
> This protection is a modified version of the x86 PAX_REFCOUNT defense
> from PaX/grsecurity. This speeds up the refcount_t API by duplicating
> the existing atomic_t implementation with a single instruction added to
> detect if the refcount has wrapped past INT_MAX (or below 0) resulting
> in a negative value, where the handler then restores the refcount_t to
> INT_MAX. With this overflow protection, the use-after-free following a
> refcount_t wrap is blocked from happening, avoiding the vulnerability
> entirely.
> While this defense only perfectly protects the overflow case, as that
> can be detected and stopped before the reference is freed and left to be
> abused by an attacker, it also notices some of the "inc from 0" and "below
> 0" cases. However, these only indicate that a use-after-free has 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 overflow detection (actually "negative value" detection), the refcount
> value is reset to INT_MAX, the offending process is killed, and a report
> and stack trace are generated. This allows the system to attempt to
> keep operating. Another option, though not done in this patch, would be
> to reset the counter to (INT_MIN / 2) to trap all future refcount inc
> or dec actions, but this would result in even legitimate uses getting
> blocked. Yet another option would be to choose (INT_MAX - N) with some
> small N to provide some headroom for legitimate users of the reference
> counter.
> On the matter of races, since the entire range beyond INT_MAX but before 0
> is negative, every inc 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 regular atomic_t operations.
> 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 no measurable change in performance over standard
> atomic_t operations. The error path, located in .text.unlikely, uses
> UD0 to fire a refcount exception handler, which reports 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.
> Assembly comparison:
> atomic_inc
> .text:
> ffffffff81546149:       f0 ff 45 f4             lock incl -0xc(%rbp)
> refcount_inc
> .text:
> ffffffff81546149:       f0 ff 45 f4             lock incl -0xc(%rbp)
> ffffffff8154614d:       0f 88 80 d5 17 00       js     ffffffff816c36d3
> ...
> .text.unlikely:
> ffffffff816c36d3:       c7 45 f4 ff ff ff 7f    movl   $0x7fffffff,-0xc(%rbp)
> ffffffff816c36da:       0f ff                   (bad)
> Various differences from PaX:
> - uses earlier value reset implementation in assembly
> - uses UD0 and refcount exception handler instead of new int vector
> - uses .text.unlikely instead of custom named text sections
> - applied only to refcount_t, not atomic_t (single size, only overflow)
> - reorganized refcount error handler
> - uses "js" instead of "jo" to trap all negative results instead of
>   just under/overflow transitions
> Signed-off-by: Kees Cook <>

Reviewed-by: Josh Poimboeuf <>


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