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Date: Wed, 26 Apr 2017 02:25:23 +0200
From: Jann Horn <jannh@...gle.com>
To: Kees Cook <keescook@...omium.org>
Cc: linux-kernel@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
PaX Team <pageexec@...email.hu>, 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>,
"David S. Miller" <davem@...emloft.net>, Rik van Riel <riel@...hat.com>,
linux-arch <linux-arch@...r.kernel.org>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v2 2/2] x86, refcount: Implement fast refcount overflow protection
On Wed, Apr 26, 2017 at 12:56 AM, Kees Cook <keescook@...omium.org> wrote:
> This protection is a modified version of the x86 PAX_REFCOUNT
> implementation 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 signed value.
[...]
> +static __always_inline void refcount_dec(refcount_t *r)
> +{
> + asm volatile(LOCK_PREFIX "decl %0\n\t"
> + REFCOUNT_CHECK_UNDERFLOW(4)
> + : [counter] "+m" (r->refs.counter)
> + : : "cc", "cx");
> +}
What purpose do checks on decrement now have? The mitigation is only
intended to deal with (positive) overflows, right? AFAICS if you hit this code,
similar to the inc-from-0 case, you're already in a UAF situation?
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