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From:   Daniel Micay <danielmicay@gmail.com>
Date:   Thu, 8 Feb 2018 14:33:58 -0500
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Subject: Re: [RFC] Warn the user when they could overflow mapcount
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Jann Horn <jannh@google.com>, linux-mm@kvack.org,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        kernel list <linux-kernel@vger.kernel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
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I don't think the kernel can get away with the current approach.
Object sizes and counts on 64-bit should be 64-bit unless there's a
verifiable reason they can get away with 32-bit. Having it use leak
memory isn't okay, just much less bad than vulnerabilities exploitable
beyond just denial of service.

Every 32-bit reference count should probably have a short comment
explaining why it can't overflow on 64-bit... if that can't be written
or it's too complicated to demonstrate, it probably needs to be
64-bit. It's one of many pervasive forms of integer overflows in the
kernel... :(