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From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date: Wed, 16 Nov 2016 12:08:52 -0800
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Subject: Re: [RFC][PATCH 2/7] kref: Add kref_read()
To: Kees Cook <keescook@chromium.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
        Greg KH <gregkh@linuxfoundation.org>,
        Will Deacon <will.deacon@arm.com>,
        "Reshetova, Elena" <elena.reshetova@intel.com>,
        Arnd Bergmann <arnd@arndb.de>, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
        David Windsor <dave@progbits.org>, LKML <linux-kernel@vger.kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>
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On Wed, Nov 16, 2016 at 10:58 AM, Kees Cook <keescook@chromium.org> wrote:
> On Wed, Nov 16, 2016 at 2:09 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>> On Tue, Nov 15, 2016 at 12:53:35PM -0800, Kees Cook wrote:
>>>
>>> What should we do about things like this (bpf_prog_put() and callbacks
>>> from kernel/bpf/syscall.c):
>>>
>>>
>>> static void bpf_prog_uncharge_memlock(struct bpf_prog *prog)
>>> {
>>>         struct user_struct *user = prog->aux->user;
>>>
>>>         atomic_long_sub(prog->pages, &user->locked_vm);
>>>         free_uid(user);
>>> }
>>>
>>> static void __bpf_prog_put_rcu(struct rcu_head *rcu)
>>> {
>>>         struct bpf_prog_aux *aux = container_of(rcu, struct bpf_prog_aux, rcu);
>>>
>>>         free_used_maps(aux);
>>>         bpf_prog_uncharge_memlock(aux->prog);
>>>         bpf_prog_free(aux->prog);
>>> }
>>>
>>> void bpf_prog_put(struct bpf_prog *prog)
>>> {
>>>         if (atomic_dec_and_test(&prog->aux->refcnt))
>>>                 call_rcu(&prog->aux->rcu, __bpf_prog_put_rcu);
>>> }
>>>
>>>
>>> Not only do we want to protect prog->aux->refcnt, but I think we want
>>> to protect user->locked_vm too ... I don't think it's sane for
>>> user->locked_vm to be a stats_t ?
>>
>> Why would you want to mess with locked_vm? You seem of the opinion that
>> everything atomic_t is broken, this isn't the case.
>
> What I mean to say is that while the refcnt here should clearly be
> converted to kref or refcount_t, it looks like locked_vm should become
> a new stats_t. However, it seems weird for locked_vm to ever wrap
> either...

I prefer to avoid 'fixing' things that are not broken.
Note, prog->aux->refcnt already has explicit checks for overflow.
locked_vm is used for resource accounting and not refcnt,
so I don't see issues there either.