Date:   Thu, 7 Jan 2021 14:33:34 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Dongseok Yi <dseok.yi@...sung.com>,
        "David S. Miller" <davem@...emloft.net>,
        Willem de Bruijn <willemb@...gle.com>,
        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>,
        Network Development <netdev@...r.kernel.org>,
        LKML <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 2:05 PM, Willem de Bruijn wrote:
> On Thu, Jan 7, 2021 at 7:52 AM Daniel Borkmann <daniel@...earbox.net> wrote:
>> 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?
> 
> If tc writes, it will call skb_make_writable and thus pskb_expand_head
> to create a private linear section for the head_skb.
> 
> skb_segment_list overwrites part of the skb linear section of each
> fragment itself. Even after skb_clone, the frag_skbs share their
> linear section with their clone in pf_packet, so we need a
> pskb_expand_head here, too.

Ok, got it, thanks for the explanation. Would be great to have it in the v3 commit
log as well as that was more clear than the above. Too bad in that case (otoh
the pf_packet situation could be considered corner case ...); ether way, fix makes
sense then.

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