linux-kernel - Re: [RFC][PATCH 2/7] kref: Add kref

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Date:   Wed, 16 Nov 2016 10:58:38 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Greg KH <gregkh@...uxfoundation.org>,
        Will Deacon <will.deacon@....com>,
        "Reshetova, Elena" <elena.reshetova@...el.com>,
        Arnd Bergmann <arnd@...db.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        David Windsor <dave@...gbits.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [RFC][PATCH 2/7] kref: Add kref_read()

On Wed, Nov 16, 2016 at 2:09 AM, Peter Zijlstra <peterz@...radead.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...

-Kees

-- 
Kees Cook
Nexus Security