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Message-ID: <CAGXu5j+QU7OXWA_oMaLUsDnS4yyfNx8-1f_ywHX=o88kqE7=JQ@mail.gmail.com> Date: Fri, 21 Jul 2017 20:33:17 -0700 From: Kees Cook <keescook@...omium.org> To: Andrew Morton <akpm@...ux-foundation.org> Cc: Ingo Molnar <mingo@...nel.org>, Peter Zijlstra <peterz@...radead.org>, Josh Poimboeuf <jpoimboe@...hat.com>, Christoph Hellwig <hch@...radead.org>, "Eric W. Biederman" <ebiederm@...ssion.com>, 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>, LKML <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 On Fri, Jul 21, 2017 at 2:22 PM, Andrew Morton <akpm@...ux-foundation.org> wrote: > On Thu, 20 Jul 2017 11:11:06 +0200 Ingo Molnar <mingo@...nel.org> wrote: > >> >> * 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! >> > > Yes, this is a relief. > > Do we have a feeling for how feasible/difficult it will be for other > architectures to implement such a thing? The PaX atomic_t overflow protection this is heavily based on was ported to a number of architectures (arm, powerpc, mips, sparc), so I suspect it shouldn't be too hard to adapt those for the more narrow refcount_t protection: https://forums.grsecurity.net/viewtopic.php?f=7&t=4173 And an arm64 port of the fast refcount_t protection is already happening too. -Kees -- Kees Cook Pixel Security
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