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From: Jann Horn <jannh@google.com>
Date: Wed, 26 Apr 2017 02:25:23 +0200
Message-ID: <CAG48ez0udSMcD2GhgVj4amsqwVJ8qEw31A8SDnDjzgjMjAsHbg@mail.gmail.com>
Subject: Re: [PATCH v2 2/2] x86, refcount: Implement fast refcount overflow protection
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>,
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        Rik van Riel <riel@redhat.com>,
        linux-arch <linux-arch@vger.kernel.org>,
        "kernel-hardening@lists.openwall.com" 
        <kernel-hardening@lists.openwall.com>
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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?