netdev - Re: [PATCH 3/5] net/packet: fix overflow in check for tp_frame

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Date:   Wed, 29 Mar 2017 09:43:49 -0400
From:   Craig Gallek <kraigatgoog@...il.com>
To:     Andrey Konovalov <andreyknvl@...gle.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>,
        netdev <netdev@...r.kernel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Kostya Serebryany <kcc@...gle.com>
Subject: Re: [PATCH 3/5] net/packet: fix overflow in check for tp_frame_nr

On Tue, Mar 28, 2017 at 1:19 PM, Andrey Konovalov <andreyknvl@...gle.com> wrote:
> On Tue, Mar 28, 2017 at 5:54 PM, Craig Gallek <kraigatgoog@...il.com> wrote:
>> On Tue, Mar 28, 2017 at 10:00 AM, Andrey Konovalov
>> <andreyknvl@...gle.com> wrote:
>>> When calculating rb->frames_per_block * req->tp_block_nr the result
>>> can overflow.
>>>
>>> Add a check that tp_block_size * tp_block_nr <= UINT_MAX.
>>>
>>> Since frames_per_block <= tp_block_size, the expression would
>>> never overflow.
>>>
>>> Signed-off-by: Andrey Konovalov <andreyknvl@...gle.com>
>>> ---
>>>  net/packet/af_packet.c | 3 +++
>>>  1 file changed, 3 insertions(+)
>>>
>>> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
>>> index 506348abdf2f..c5c43fff8c01 100644
>>> --- a/net/packet/af_packet.c
>>> +++ b/net/packet/af_packet.c
>>> @@ -4197,6 +4197,9 @@ static int packet_set_ring(struct sock *sk, union tpacket_req_u *req_u,
>>>                         goto out;
>>>                 if (unlikely(req->tp_frame_size == 0))
>>>                         goto out;
>>> +               if (unlikely((u64)req->tp_block_size * req->tp_block_nr >
>>> +                                       UINT_MAX))
>>> +                       goto out;
>> So this may be pedantic, but really the only guarantee that you have
>> for the 'unsigned int' type of these fields is that they are _at
>> least_ 16 bits.  There is no guarantee on the upper bound size, so
>> casting to a u64 will be problematic on a compiler that happens to use
>> 64 bits for 'unsigned int'.  I'm not aware of any that use greater
>> than 32 bits right now and using one that does may very well break
>> other things in the kernel, but here we are...  Perhaps a alternative
>> fix would be to do the multiplication into an 'unsigned int' type and
>> ensure that the result is larger than each of the original two values?
>
> I don't mind changing the check, but I've never encountered such compilers.
>
> Would this alternative work? It doesn't seem obvious.
>
> Other alternatives that I see for this check are:
>
> 1. req->tp_block_size > UINT_MAX / req->tp_block_nr
>
> 2. (req->tp_block_size * req->tp_block_nr) / req->tp_block_nr !=
> req->tp_block_size
>
> I'm not sure which one is better.
I'm by no means the style expert here, but I would go with whichever
makes the intention of the check (preventing overflow) most obvious.
Maybe #1 in your example?  I'm also not sure what the acceptable
assumptions about the size of 'int' are in the kernel code.  I'm sure
there's a thread out there with Linus expressing a strong feeling one
way or another, but I haven't found it yet ;)

>
>>
>> The real issue is that explicit size types should have been used in
>> this userspace structure.