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

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Date:   Mon, 21 Nov 2016 07:47:09 -0500
From:   David Windsor <dwindsor@...il.com>
To:     "Reshetova, Elena" <elena.reshetova@...el.com>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Kees Cook <keescook@...omium.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Will Deacon <will.deacon@....com>,
        Arnd Bergmann <arnd@...db.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [RFC][PATCH 2/7] kref: Add kref_read()

On Fri, Nov 18, 2016 at 12:33 PM, Reshetova, Elena
<elena.reshetova@...el.com> wrote:
> On Thu, Nov 17, 2016 at 09:53:42AM +0100, Peter Zijlstra wrote:
>> On Wed, Nov 16, 2016 at 12:08:52PM -0800, Alexei Starovoitov wrote:
>>
>> > 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.
>>
>> The idea is to use something along the lines of:
>>
>>
>> http://lkml.kernel.org/r/20161115104608.GH3142@twins.programming.kicks
>> -ass.net
>>
>> for all refcounts in the kernel.
>
>>I understand the idea. I'm advocating to fix refcnts explicitly the way we did in bpf land instead of leaking memory, making processes unkillable and so on.
>>If refcnt can be bounds checked, it should be done that way, since it's a clean error path without odd side effects.
>>Therefore I'm against unconditionally applying refcount to all atomics.
>
>> Also note that your:
>>
>> struct bpf_prog *bpf_prog_add(struct bpf_prog *prog, int i) {
>>         if (atomic_add_return(i, &prog->aux->refcnt) > BPF_MAX_REFCNT) {
>>                 atomic_sub(i, &prog->aux->refcnt);
>>                 return ERR_PTR(-EBUSY);
>>         }
>>         return prog;
>> }
>>
>> is actually broken in the face of an actual overflow. Suppose @i is
>> big enough to wrap refcnt into negative space.
>
>>'i' is not controlled by user. It's a number of nic hw queues and BPF_MAX_REFCNT is 32k, so above is always safe.
>
> If I understand your code right, you export the bpf_prog_add() and anyone is free to use it
> (some crazy buggy driver for example).
> Currently only drivers/net/ethernet/mellanox/mlx4/en_netdev.c uses it, but you should
> consider any externally exposed interface as an attack vector from security point of view.
> So, I would not claim that above construction is always safe since there is a way using API to
> supply "i" that would overflow.
>
> Next question is how to convert the above code sanely to refcount_t interface... Loop of inc(s)? Iikk...
>

By the way, there are several sites where the use of
atomic_t/atomic_wrap_t as a counter ventures beyond the standard (inc,
dec, add, sub, read, set) operations we're planning on implementing
for both refcount_t and stats_t.  While performing the conversion to
stats_t, I've found usage of atomic_xchg(), for instance.  From
kernel/trace/trace_mmiotrace.c:123:

unsigned long cnt = atomic_xchg(&dropped_count, 0);

stats_xchg() isn't anticipated to go into the stats_t API, and
dropped_count clearly appears to be a statistical counter, so we will
have to further audit this site to determine whether the atomicity of
the atomic_xchg() operation is truly  necessary here.  If it is, we
can either decide to implement stats_xchg(), or we could use a
combination of locking, stats_read() and stats_set() to accomplish the
same thing as stats_xchg().

>
>