netdev - Re: [PATCH bpf] bpf: fix null pointer deref in bpf_prog_test_run

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Date:   Wed, 31 Jan 2018 11:42:16 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     alexei.starovoitov@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH bpf] bpf: fix null pointer deref in bpf_prog_test_run_xdp

On 01/31/2018 08:24 AM, Jesper Dangaard Brouer wrote:
> On Wed, 31 Jan 2018 01:31:11 +0100
> Daniel Borkmann <daniel@...earbox.net> wrote:
> 
>> syzkaller was able to generate the following XDP program ...
>>
>>   (18) r0 = 0x0
>>   (61) r5 = *(u32 *)(r1 +12)
>>   (04) (u32) r0 += (u32) 0
>>   (95) exit
>>
>> ... and trigger a NULL pointer dereference in ___bpf_prog_run()
>> via bpf_prog_test_run_xdp() where this was attempted to run.
>>
>> Reason is that recent xdp_rxq_info addition to XDP programs
>> updated all drivers, but not bpf_prog_test_run_xdp(), where
>> xdp_buff is set up. Thus when context rewriter does the deref
>> on the netdev it's NULL at runtime. Fix it by using xdp_rxq
>> from loopback dev. netif_get_rx_queue() helper can also be
>> reused in various other locations later on.
>>
>> Fixes: 02dd3291b2f0 ("bpf: finally expose xdp_rxq_info to XDP bpf-programs")
>> Reported-by: syzbot+1eb094057b338eb1fc00@...kaller.appspotmail.com
>> Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
>> Cc: Jesper Dangaard Brouer <brouer@...hat.com>
>> ---
>>  [ Note: Needs to wait till Linus pulled Dave's net-next so
>>    the affected commit lands in bpf tree. ]
>>
>>  include/linux/netdevice.h                   |  6 ++++++
>>  net/bpf/test_run.c                          |  4 ++++
>>  tools/testing/selftests/bpf/test_verifier.c | 14 ++++++++++++++
>>  3 files changed, 24 insertions(+)
>>
>> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
>> index cd46d3d..9630f4e 100644
>> --- a/include/linux/netdevice.h
>> +++ b/include/linux/netdevice.h
>> @@ -3228,6 +3228,12 @@ static inline int netif_set_real_num_rx_queues(struct net_device *dev,
>>  }
>>  #endif
>>  
>> +static inline struct netdev_rx_queue *
>> +netif_get_rx_queue(struct net_device *dev, unsigned int rxq)
> 
> This function name is very close to (net/core/dev.c):
> 
>  static struct netdev_rx_queue *
>  netif_get_rxqueue(struct sk_buff *skb)
> 
> 
>> +{
>> +	return dev->_rx + rxq;
>> +}
> 
> I know above is correct, is just annoys my eyes as I find it more
> natural to write as dev->_rx[rxq].  I'm not saying you should change
> it, because it does preserve the access style elsewhere (and I also
> kept this style in some of my changes).
> 
> You mention netif_get_rx_queue() helper can also be reused in various
> other locations later on, but there is not boundary checks (on this

>From a quick grep, few more could use this (didn't add this into
the fix here, though, but can be later as cleanup):

$ git grep -n "dev->_rx +"
net/core/dev.c:3644:            rxqueue = dev->_rx + rxq_index;
net/core/dev.c:3784:    struct netdev_rx_queue *rxqueue = dev->_rx + rxq_index;
net/core/net-sysfs.c:940:       struct netdev_rx_queue *queue = dev->_rx + index;

> dynamically allocated array of size dev->num_rx_queues).  We could add
> a comment saying caller is responsible for boundary checks, or prefix
> the function name with "__" to indicate this indirectly.

Okay, I don't mind changing the name, would __netif_get_rx_queue()
be a better one?

Thanks,
Daniel