netdev - Re: [PATCH 4/5] net/packet: fix overflow in check for tp

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Message-ID: <CAAeHK+yX=3TTX+fYZCi5vPqa9=Fi2hyfybcbJo_iEFM4CLvWrw@mail.gmail.com>
Date:   Tue, 28 Mar 2017 17:11:54 +0200
From:   Andrey Konovalov <andreyknvl@...gle.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Craig Gallek <cgallek@...gle.com>,
        Network Development <netdev@...r.kernel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Kostya Serebryany <kcc@...gle.com>
Subject: Re: [PATCH 4/5] net/packet: fix overflow in check for tp_reserve

On Tue, Mar 28, 2017 at 5:00 PM, Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
> On Tue, Mar 28, 2017 at 10:00 AM, Andrey Konovalov
> <andreyknvl@...gle.com> wrote:
>> When calculating po->tp_hdrlen + po->tp_reserve the result can overflow.
>>
>> Fix by checking that tp_reserve <= INT_MAX on assign.
>>
>> This also takes cared of an overflow when calculating
>>  macoff = TPACKET_ALIGN(po->tp_hdrlen) + 16 + po->tp_reserve
>>  snaplen = skb->len
>>  macoff + snaplen
>> since macoff ~ INT_MAX and snaplen < SKB_MAX_ALLOC.
>
> This refers to the overflow of macoff + snaplen?
>
> Note that macoff is unsigned short, so will truncate any overflow from
> tp_reserve.

Yes, you're right.
Should I make macoff unsigned int to fix this?

>
>> Signed-off-by: Andrey Konovalov <andreyknvl@...gle.com>
>> ---
>>  net/packet/af_packet.c | 7 ++++---
>>  1 file changed, 4 insertions(+), 3 deletions(-)
>>
>> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
>> index c5c43fff8c01..28b49749d1af 100644
>> --- a/net/packet/af_packet.c
>> +++ b/net/packet/af_packet.c
>> @@ -3665,6 +3665,8 @@ packet_setsockopt(struct socket *sock, int level, int optname, char __user *optv
>>                         return -EBUSY;
>>                 if (copy_from_user(&val, optval, sizeof(val)))
>>                         return -EFAULT;
>> +               if (val > INT_MAX)
>> +                       return -EINVAL;
>
> This change on its own is sufficient to avoid the overflow. For net
> and backports to stable, this minimal patch is preferable.

I will put it into a separate patch then.

>
>>                 po->tp_reserve = val;
>>                 return 0;
>>         }
>> @@ -4200,6 +4202,8 @@ static int packet_set_ring(struct sock *sk, union tpacket_req_u *req_u,
>>                 if (unlikely((u64)req->tp_block_size * req->tp_block_nr >
>>                                         UINT_MAX))
>>                         goto out;
>> +               if (unlikely(po->tp_reserve >= req->tp_frame_size))
>> +                       goto out;
>>
>>                 if (unlikely(!PAGE_ALIGNED(req->tp_block_size)))
>>                         goto out;
>> @@ -4207,9 +4211,6 @@ static int packet_set_ring(struct sock *sk, union tpacket_req_u *req_u,
>>                     req->tp_block_size <=
>>                           BLK_PLUS_PRIV((u64)req_u->req3.tp_sizeof_priv))
>>                         goto out;
>> -               if (unlikely(req->tp_frame_size < po->tp_hdrlen +
>> -                                       po->tp_reserve))
>> -                       goto out;
>
> Is there a reason that the test is moved up? It is probably not
> correct to remove tp_hdrlen from the test.

Just to group together all checks of tp_frame_size and tp_block_size.

I'm not sure there's any difference between checking against
po->tp_hdrlen + po->tp_reserve and just po->tp_reserve.
I guess the correct check should be against
TPACKET_ALIGN(po->tp_hdrlen) + 16 + po->tp_reserve.

Should I use this value?

>
>>                 if (unlikely(req->tp_frame_size & (TPACKET_ALIGNMENT - 1)))
>>                         goto out;
>>
>> --
>> 2.12.2.564.g063fe858b8-goog
>>