linux-kernel - Re: [PATCH v8 3/8] Use atomic_t for ucounts reference counting

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Message-ID: <202103161146.E118DE5@keescook>
Date:   Tue, 16 Mar 2021 11:49:20 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Alexey Gladkov <gladkov.alexey@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        io-uring <io-uring@...r.kernel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Linux-MM <linux-mm@...ck.org>,
        Alexey Gladkov <legion@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Jann Horn <jannh@...gle.com>, Jens Axboe <axboe@...nel.dk>,
        Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH v8 3/8] Use atomic_t for ucounts reference counting

On Mon, Mar 15, 2021 at 03:19:17PM -0700, Linus Torvalds wrote:
> It just saturates, and doesn't have the "don't do this" case, which
> the ucounts case *DOES* have.

Right -- I saw that when digging through the thread. I'm honestly
curious, though, why did the 0-day bot find a boot crash? (I can't
imagine ucounts wrapped in 0.4 seconds.) So it looked like an
increment-from-zero case, which seems like it would be a bug?

> I know you are attached to refcounts, but really: they are not only
> more expensive, THEY LITERALLY DO THE WRONG THING.

Heh, right -- I'm not arguing that refcount_t MUST be used, I just didn't
see the code path that made them unsuitable: hitting INT_MAX - 128 seems
very hard to do. Anyway, I'll go study it more to try to understand what
I'm missing.

-- 
Kees Cook