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From: Jann Horn <jannh@google.com>
To: Kees Cook <keescook@chromium.org>
Cc: linux-kernel@vger.kernel.org,
	Peter Zijlstra <peterz@infradead.org>,
	PaX Team <pageexec@freemail.hu>,
	Eric Biggers <ebiggers3@gmail.com>,
	Christoph Hellwig <hch@infradead.org>,
	"axboe@kernel.dk" <axboe@kernel.dk>,
	James Bottomley <James.Bottomley@hansenpartnership.com>,
	Elena Reshetova <elena.reshetova@intel.com>,
	Hans Liljestrand <ishkamiel@gmail.com>,
	David Windsor <dwindsor@gmail.com>,
	"x86@kernel.org" <x86@kernel.org>, Ingo Molnar <mingo@kernel.org>,
	Arnd Bergmann <arnd@arndb.de>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	"David S. Miller" <davem@davemloft.net>,
	Rik van Riel <riel@redhat.com>,
	linux-arch <linux-arch@vger.kernel.org>,
	"kernel-hardening@lists.openwall.com" 
	<kernel-hardening@lists.openwall.com>
Subject: Re: [PATCH v2 2/2] x86, refcount: Implement fast refcount overflow protection
Date: Wed, 26 Apr 2017 02:25:23 +0200	[thread overview]
Message-ID: <CAG48ez0udSMcD2GhgVj4amsqwVJ8qEw31A8SDnDjzgjMjAsHbg@mail.gmail.com> (raw)
In-Reply-To: <1493160997-126108-3-git-send-email-keescook@chromium.org>

On Wed, Apr 26, 2017 at 12:56 AM, Kees Cook <keescook@chromium.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?

  reply	other threads:[~2017-04-26  0:25 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-04-25 22:56 [PATCH v2 0/2] x86, refcount: Implement fast refcount overflow Kees Cook
2017-04-25 22:56 ` [PATCH v2 1/2] x86, asm: Add suffix macro for GEN_*_RMWcc() Kees Cook
2017-04-25 22:56 ` [PATCH v2 2/2] x86, refcount: Implement fast refcount overflow protection Kees Cook
2017-04-26  0:25   ` Jann Horn [this message]
2017-04-26  3:52     ` Kees Cook
2017-04-27  1:31   ` kbuild test robot
2017-04-27 20:22     ` Kees Cook
2017-05-01 15:54       ` Josh Poimboeuf
2017-05-01 17:28         ` Kees Cook
2017-05-01 22:33           ` Josh Poimboeuf
2017-05-01 16:30   ` Josh Poimboeuf
2017-05-01 17:36     ` Kees Cook
2017-05-01 22:45       ` Josh Poimboeuf
2017-04-26  2:01 ` [PATCH v2 0/2] x86, refcount: Implement fast refcount overflow PaX Team
2017-04-26  3:59   ` Kees Cook

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