linux-kernel - Re: [PATCH v2 2/2] x86, refcount: Implement fast refcount overflow protection

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Date:   Tue, 25 Apr 2017 20:52:26 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Jann Horn <jannh@...gle.com>
Cc:     LKML <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 Tue, Apr 25, 2017 at 5:25 PM, Jann Horn <jannh@...gle.com> wrote:
> 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?

Yeah, I think that's true, but as Peter has mentioned: it's better
than not having it. The inc path can be deterministic, and the dec
path can be lucky? :)

-Kees

-- 
Kees Cook
Pixel Security