Date:   Thu, 7 Jan 2021 13:49:26 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Dongseok Yi <dseok.yi@...sung.com>,
        "'David S. Miller'" <davem@...emloft.net>,
        'Willem de Bruijn' <willemb@...gle.com>
Cc:     'Jakub Kicinski' <kuba@...nel.org>,
        'Miaohe Lin' <linmiaohe@...wei.com>,
        'Paolo Abeni' <pabeni@...hat.com>,
        'Florian Westphal' <fw@...len.de>,
        'Al Viro' <viro@...iv.linux.org.uk>,
        'Guillaume Nault' <gnault@...hat.com>,
        'Yunsheng Lin' <linyunsheng@...wei.com>,
        'Steffen Klassert' <steffen.klassert@...unet.com>,
        'Yadu Kishore' <kyk.segfault@...il.com>,
        'Marco Elver' <elver@...gle.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, namkyu78.kim@...sung.com
Subject: Re: [PATCH net v2] net: fix use-after-free when UDP GRO with shared
 fraglist

On 1/7/21 12:40 PM, Dongseok Yi wrote:
> On 2021-01-07 20:05, Daniel Borkmann wrote:
>> On 1/7/21 1:39 AM, Dongseok Yi wrote:
>>> skbs in fraglist could be shared by a BPF filter loaded at TC. It
>>> triggers skb_ensure_writable -> pskb_expand_head ->
>>> skb_clone_fraglist -> skb_get on each skb in the fraglist.
>>>
>>> While tcpdump, sk_receive_queue of PF_PACKET has the original fraglist.
>>> But the same fraglist is queued to PF_INET (or PF_INET6) as the fraglist
>>> chain made by skb_segment_list.
>>>
>>> If the new skb (not fraglist) is queued to one of the sk_receive_queue,
>>> multiple ptypes can see this. The skb could be released by ptypes and
>>> it causes use-after-free.
>>>
>>> [ 4443.426215] ------------[ cut here ]------------
>>> [ 4443.426222] refcount_t: underflow; use-after-free.
>>> [ 4443.426291] WARNING: CPU: 7 PID: 28161 at lib/refcount.c:190
>>> refcount_dec_and_test_checked+0xa4/0xc8
>>> [ 4443.426726] pstate: 60400005 (nZCv daif +PAN -UAO)
>>> [ 4443.426732] pc : refcount_dec_and_test_checked+0xa4/0xc8
>>> [ 4443.426737] lr : refcount_dec_and_test_checked+0xa0/0xc8
>>> [ 4443.426808] Call trace:
>>> [ 4443.426813]  refcount_dec_and_test_checked+0xa4/0xc8
>>> [ 4443.426823]  skb_release_data+0x144/0x264
>>> [ 4443.426828]  kfree_skb+0x58/0xc4
>>> [ 4443.426832]  skb_queue_purge+0x64/0x9c
>>> [ 4443.426844]  packet_set_ring+0x5f0/0x820
>>> [ 4443.426849]  packet_setsockopt+0x5a4/0xcd0
>>> [ 4443.426853]  __sys_setsockopt+0x188/0x278
>>> [ 4443.426858]  __arm64_sys_setsockopt+0x28/0x38
>>> [ 4443.426869]  el0_svc_common+0xf0/0x1d0
>>> [ 4443.426873]  el0_svc_handler+0x74/0x98
>>> [ 4443.426880]  el0_svc+0x8/0xc
>>>
>>> Fixes: 3a1296a38d0c (net: Support GRO/GSO fraglist chaining.)
>>> Signed-off-by: Dongseok Yi <dseok.yi@...sung.com>
>>> Acked-by: Willem de Bruijn <willemb@...gle.com>
>>> ---
>>>    net/core/skbuff.c | 20 +++++++++++++++++++-
>>>    1 file changed, 19 insertions(+), 1 deletion(-)
>>>
>>> v2: Expand the commit message to clarify a BPF filter loaded
>>>
>>> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
>>> index f62cae3..1dcbda8 100644
>>> --- a/net/core/skbuff.c
>>> +++ b/net/core/skbuff.c
>>> @@ -3655,7 +3655,8 @@ struct sk_buff *skb_segment_list(struct sk_buff *skb,
>>>    	unsigned int delta_truesize = 0;
>>>    	unsigned int delta_len = 0;
>>>    	struct sk_buff *tail = NULL;
>>> -	struct sk_buff *nskb;
>>> +	struct sk_buff *nskb, *tmp;
>>> +	int err;
>>>
>>>    	skb_push(skb, -skb_network_offset(skb) + offset);
>>>
>>> @@ -3665,11 +3666,28 @@ struct sk_buff *skb_segment_list(struct sk_buff *skb,
>>>    		nskb = list_skb;
>>>    		list_skb = list_skb->next;
>>>
>>> +		err = 0;
>>> +		if (skb_shared(nskb)) {
>>> +			tmp = skb_clone(nskb, GFP_ATOMIC);
>>> +			if (tmp) {
>>> +				kfree_skb(nskb);
>>
>> Should use consume_skb() to not trigger skb:kfree_skb tracepoint when looking
>> for drops in the stack.
> 
> I will use to consume_skb() on the next version.
> 
>>> +				nskb = tmp;
>>> +				err = skb_unclone(nskb, GFP_ATOMIC);
>>
>> Could you elaborate why you also need to unclone? This looks odd here. tc layer
>> (independent of BPF) from ingress & egress side generally assumes unshared skb,
>> so above clone + dropping ref of nskb looks okay to make the main skb struct private
>> for mangling attributes (e.g. mark) & should suffice. What is the exact purpose of
>> the additional skb_unclone() in this context?
> 
> Willem de Bruijn said:
> udp_rcv_segment later converts the udp-gro-list skb to a list of
> regular packets to pass these one-by-one to udp_queue_rcv_one_skb.
> Now all the frags are fully fledged packets, with headers pushed
> before the payload.

Yes.

> PF_PACKET handles untouched fraglist. To modify the payload only
> for udp_rcv_segment, skb_unclone is necessary.

I don't parse this last sentence here, please elaborate in more detail on why
it is necessary.

For example, if tc layer would modify mark on the skb, then __copy_skb_header()
in skb_segment_list() will propagate it. If tc layer would modify payload, the
skb_ensure_writable() will take care of that internally and if needed pull in
parts from fraglist into linear section to make it private. The purpose of the
skb_clone() above iff shared is to make the struct itself private (to safely
modify its struct members). What am I missing?

>>> +			} else {
>>> +				err = -ENOMEM;
>>> +			}
>>> +		}
>>> +
>>>    		if (!tail)
>>>    			skb->next = nskb;
>>>    		else
>>>    			tail->next = nskb;
>>>
>>> +		if (unlikely(err)) {
>>> +			nskb->next = list_skb;
>>> +			goto err_linearize;
>>> +		}
>>> +
>>>    		tail = nskb;
>>>
>>>    		delta_len += nskb->len;
>>>
> 
> 

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