Re: [PATCH net] net: fix use-after-free when UDP GRO with shared fraglist

[Yunsheng Lin]

On 2021/1/6 11:32, Dongseok Yi wrote:
> On 2021-01-06 12:07, Willem de Bruijn wrote:
>>
>> On Tue, Jan 5, 2021 at 8:29 PM Dongseok Yi <dseok.yi@xxxxxxxxxxx> wrote:
>>>
>>> On 2021-01-05 06:03, Willem de Bruijn wrote:
>>>>
>>>> On Mon, Jan 4, 2021 at 4:00 AM Dongseok Yi <dseok.yi@xxxxxxxxxxx> wrote:
>>>>>
>>>>> skbs in frag_list could be shared by pskb_expand_head() from BPF.
>>>>
>>>> Can you elaborate on the BPF connection?
>>>
>>> With the following registered ptypes,
>>>
>>> /proc/net # cat ptype
>>> Type Device      Function
>>> ALL           tpacket_rcv
>>> 0800          ip_rcv.cfi_jt
>>> 0011          llc_rcv.cfi_jt
>>> 0004          llc_rcv.cfi_jt
>>> 0806          arp_rcv
>>> 86dd          ipv6_rcv.cfi_jt
>>>
>>> BPF checks skb_ensure_writable between tpacket_rcv and ip_rcv
>>> (or ipv6_rcv). And it calls pskb_expand_head.
>>>
>>> [  132.051228] pskb_expand_head+0x360/0x378
>>> [  132.051237] skb_ensure_writable+0xa0/0xc4
>>> [  132.051249] bpf_skb_pull_data+0x28/0x60
>>> [  132.051262] bpf_prog_331d69c77ea5e964_schedcls_ingres+0x5f4/0x1000
>>> [  132.051273] cls_bpf_classify+0x254/0x348
>>> [  132.051284] tcf_classify+0xa4/0x180
>>
>> Ah, you have a BPF program loaded at TC. That was not entirely obvious.
>>
>> This program gets called after packet sockets with ptype_all, before
>> those with a specific protocol.
>>
>> Tcpdump will have inserted a program with ptype_all, which cloned the
>> skb. This triggers skb_ensure_writable -> pskb_expand_head ->
>> skb_clone_fraglist -> skb_get.
>>
>>> [  132.051294] __netif_receive_skb_core+0x590/0xd28
>>> [  132.051303] __netif_receive_skb+0x50/0x17c
>>> [  132.051312] process_backlog+0x15c/0x1b8
>>>
>>>>
>>>>> While tcpdump, sk_receive_queue of PF_PACKET has the original frag_list.
>>>>> But the same frag_list is queued to PF_INET (or PF_INET6) as the fraglist
>>>>> chain made by skb_segment_list().
>>>>>
>>>>> If the new skb (not frag_list) 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.
>>>>
>>>> If I understand correctly, a udp-gro-list skb makes it up the receive
>>>> path with one or more active packet sockets.
>>>>
>>>> The packet socket will call skb_clone after accepting the filter. This
>>>> replaces the head_skb, but shares the skb_shinfo and thus frag_list.
>>>>
>>>> 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. This does not change their refcount anymore than
>>>> the skb_clone in pf_packet did. This should be 1.
>>>>
>>>> Eventually udp_recvmsg will call skb_consume_udp on each packet.
>>>>
>>>> The packet socket eventually also frees its cloned head_skb, which triggers
>>>>
>>>>   kfree_skb_list(shinfo->frag_list)
>>>>     kfree_skb
>>>>       skb_unref
>>>>         refcount_dec_and_test(&skb->users)
>>>
>>> Every your understanding is right, but
>>>
>>>>
>>>>>
>>>>> [ 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@xxxxxxxxxxx>
>>>>> ---
>>>>>  net/core/skbuff.c | 20 +++++++++++++++++++-
>>>>>  1 file changed, 19 insertions(+), 1 deletion(-)
>>>>>
>>>>> 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)) {
>>>>
>>>> I must be missing something still. This does not square with my
>>>> understanding that the two sockets are operating on clones, with each
>>>> frag_list skb having skb->users == 1.
>>>>
>>>> Unless the packet socket patch previously also triggered an
>>>> skb_unclone/pskb_expand_head, as that call skb_clone_fraglist, which
>>>> calls skb_get on each frag_list skb.
>>>
>>> A cloned skb after tpacket_rcv cannot go through skb_ensure_writable
>>> with the original shinfo. pskb_expand_head reallocates the shinfo of
>>> the skb and call skb_clone_fraglist. skb_release_data in
>>> pskb_expand_head could not reduce skb->users of the each frag_list skb
>>> if skb_shinfo(skb)->dataref == 2.
>>>
>>> After the reallocation, skb_shinfo(skb)->dataref == 1 but each frag_list
>>> skb could have skb->users == 2.

Hi, Dongseok
   I understand there is liner head data shared between the frag_list skb in the
cloned skb(cloned by pf_packet?) and original skb, which should not be shared
when skb_segment_list() converts the frag_list skb into regular packet.

   But both skb->users of original and cloned skb is one(skb_shinfo(skb)->dataref
is one for both skb too), and skb->users of each fraglist skb is two because both
original and cloned skb is linking to the same fraglist pointer, and there is
"skb_shinfo(skb)->frag_list = NULL" for original skb in the begin of skb_segment_list(),
if kfree_skb() is called with original skb, the fraglist skb will not be freed.
If kfree_skb is called with original skb,cloned skb and each fraglist skb here, the
reference counter for three of them seem right here, so why is there a refcount_t
warning in the commit log? am I missing something obvious here?

Sorry for bringing up this thread again.

>>
>> Yes, that makes sense. skb_clone_fraglist just increments the
>> frag_list skb's refcounts.
>>
>> skb_segment_list must create an unshared struct sk_buff before it
>> changes skb data to insert the protocol headers.
>>