Date: Wed, 26 Apr 2017 04:14:06 +0200 From: "PaX Team" <pageexec@...email.hu> To: Kees Cook <keescook@...omium.org> CC: Peter Zijlstra <peterz@...radead.org>, LKML <linux-kernel@...r.kernel.org>, Eric Biggers <ebiggers3@...il.com>, Christoph Hellwig <hch@...radead.org>, "axboe@...nel.dk" <axboe@...nel.dk>, James Bottomley <James.Bottomley@...senpartnership.com>, Elena Reshetova <elena.reshetova@...el.com>, Hans Liljestrand <ishkamiel@...il.com>, David Windsor <dwindsor@...il.com>, "x86@...nel.org" <x86@...nel.org>, Ingo Molnar <mingo@...nel.org>, Arnd Bergmann <arnd@...db.de>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Jann Horn <jann@...jh.net>, "David S. Miller" <davem@...emloft.net>, linux-arch <linux-arch@...r.kernel.org>, "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com> Subject: Re: [PATCH] x86/refcount: Implement fast refcount_t handling On 25 Apr 2017 at 9:39, Kees Cook wrote: > On Tue, Apr 25, 2017 at 4:26 AM, PaX Team <pageexec@...email.hu> wrote: > > INT_MAX threads would be needed when the leaking path is locked so > > that it can only be exercised once and you'll need to get normal > > (balanced) paths preempted just after the increment. if the leaking > > path is lockless (can be exercised in parallel without bounds) then > > 2 threads are enough where the one triggering the signed overflow > > would have to be preempted while the other one does INT_MAX increments > > and trigger the UAF. this is where the other mechanisms i talked about > > in the past become relevant: preemption or interrupts can be disabled > > or negative refcount values can be detected and acted upon (your blind > > copy-pasting effort passed upon this latter opportunity by not > > specializing the 'jo' into 'js' for the refcount case). > > Well, it's not "blind" -- I'm trying to bring the code as-is to > upstream for discussion/examination with as little functional > differences as possible so it's easier to compare apples to apples. you copied code from a version which is at least 2 major kernel revisions behind (so much for those apples), you chose the one version which had a bug that you didn't spot nor fix properly, you didn't realize the opportunity that a special refcount type represents, you claimed refcount underflows aren't exploitable but copied code that would detect signed underflow, you didn't understand the limits and edge cases i explained above... need i go on? doesn't leave one with great confidence in your ability to understand and maintain this code...
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