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

["Dongseok Yi" <dseok.yi@samsung.com>]

On Wed, Apr 21, 2021 at 05:42:12PM +0800, Yunsheng Lin wrote:
> On 2021/4/19 8:35, Dongseok Yi wrote:
> > On Sat, Apr 17, 2021 at 11:44:35AM +0800, Yunsheng Lin wrote:
> >>
> >> 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@samsung.com> wrote:
> >>>>>
> >>>>> On 2021-01-05 06:03, Willem de Bruijn wrote:
> >>>>>>
> >>>>>> On Mon, Jan 4, 2021 at 4:00 AM Dongseok Yi <dseok.yi@samsung.com> 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@samsung.com>
> >>>>>>> ---
> >>>>>>>  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.
> >
> > A skb which detects use-after-free was not a part of frag_list. Please
> > check the commit msg once again.
> 
> I checked the commit msg again, but still have not figured it out yet:)
> 
> So I tried to see if I understand the skb'reference counting correctly:
> 
> skb->user is used to reference counting the "struct sk_buff", and
> skb_shinfo(skb)->dataref is used to reference counting head data.
> 
> skb_clone(): allocate a sperate "struct sk_buff" but share the head data
>              with the original skb, so skb_shinfo()->dataref need
>              incrmenting.
> 
> pskb_expand_head(): allocate a sperate head data(which includes the space
>                     for skb_shinfo(skb)), since the original head data
> 		    and the new head data' skb_shinfo()->frag_list both
>                     point to the same fraglist skb, so each fraglist_skb's
> 		    skb->users need incrmenting, and original head data's
> 		    skb_shinfo() need decrmenting.
> 
> 
> So after pf_packet called skb_clone() and pskb_expand_head(), we have:
> 
>     old skb              new skb
>       |                     |
>       |                     |
> old head data         new head data
>         \                   /
>           \                /
>            \              /
>              \           /
>               \         /
>              fraglist_skb1 -> fraglist_skb2 -> fraglist_skb3 .....
> 
> So both old and new skb' skb->user is one, both old and new head data's
> skb_shinfo()->dataref is one, and both old and new head data'
> skb_shinfo()->frag_list points to fraglist_skb1, and each fraglist_skb's
> skb->user is two.
> 
> Each fraglist_skb points to a head data, and its skb_shinfo()->dataref
> is one too.
> 
> Suppose old skb is called with skb_segment_list(), without this patch,
> we have:
> 
>                          new skb
>                             |
>                             |
>                      new head data
>                             /
>                            /
>                           /
>                          /
>                         /
>        old skb -> fraglist_skb1 -> fraglist_skb2 -> fraglist_skb3 .....
>           |
>           |
>     old head data
> 
> And old skb and each fraglist_skb become a regular packet, so freeing
> the old skb, new skb and each fraglist_skb here do not seems to have
> any reference counting problem, because each fraglist_skb's skb->user
> is two, right?
> 
> >
> > Both sk_receive_queue of PF_PACKET and PF_INET (or PF_INET6) can have
> > a link for the same frag_skbs chain.
> 
> Does "frag_skbs chain" means fraglist_skb1? It seems only new head data's
> skb_shinfo()->frag_list points to fraglist_skb1

Yes, right.

> 
> 
> If a new skb (*not frags*) is
> > queued to one of the sk_receive_queue, multiple ptypes can see and
> > release this. It causes use-after-free.
> 
> Does "a new skb" mean each fraglist_skb after skb_segment_list()? Or other
> new incoming skb?

I mean a new incoming skb.

> 
> I am not so familiar with the PF_PACKET and PF_INET, so still have hard
> time figuring how the reference counting goes wrong here:)

Let's assume a new incoming skb that is added to the next of the last
fraglist_skb. The new incoming skb->user is *one*.

                         new skb
                            |
                            |
                     new head data
                            /
                           /
                          /
                         /
                        /
       old skb -> fraglist_skb1 -> fraglist_skb2 -> ... -> new incoming skb
          |
          |
    old head data

Let's skb_queue_purge from old skb. kfree_skb from old skb will free
2 skbs (marked as xxx1 and xxx2). What happened if kfree_skb(new skb)?

                         new skb
                            |
                            |
                     new head data
                            /
                           /
                          /
                         /
                        /
          xxx1 -> fraglist_skb1 -> fraglist_skb2 -> ... -> xxx2

It will try to free xxx2.

> 
> >
> >>
> >>>>
> >>>> 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.
> >>>>
> >
> >
> >
> > .
> >