linux-kernel - Re: [PATCH v2 0/2] x86, refcount: Implement fast refcount overflow

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Message-ID: <CAGXu5j+N9x+jXB4ozDPnTSXT4EHTjuQ8mOuCmAtBEpwe8bLusQ@mail.gmail.com>
Date:   Tue, 25 Apr 2017 20:59:10 -0700
From:   Kees Cook <keescook@...omium.org>
To:     PaX Team <pageexec@...email.hu>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Jann Horn <jannh@...gle.com>,
        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 0/2] x86, refcount: Implement fast refcount overflow

On Tue, Apr 25, 2017 at 7:01 PM, PaX Team <pageexec@...email.hu> wrote:
> On 25 Apr 2017 at 15:56, Kees Cook 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.
>
> 'signed value' sounds somewhat ambiguous given that in C a signed type (such
> as the one beneath refcount_t) can have both negative and positive values yet
> you didn't mean the latter here i guess.

Yeah, the language for the CPU "sign flag" confuses this. I will
attempt to clarify for future versions.

>> Various differences from PaX:
>> - uses "js" instead of "jo" to trap all signed results instead of just
>>   under/overflow transitions
>
> there're differences in my 4.11 port but this isn't one of them.

Any changes you'd suggest for upstreaming?

-Kees

-- 
Kees Cook
Pixel Security