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

From: Dongseok Yi
Date: Wed Apr 21 2021 - 07:04:20 EST

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In reply to: Yunsheng Lin: "Re: [PATCH net] net: fix use-after-free when UDP GRO with shared fraglist"
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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@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.
> >
> > 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.
> >>>>
> >
> >
> >
> > .
> >

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In reply to: Yunsheng Lin: "Re: [PATCH net] net: fix use-after-free when UDP GRO with shared fraglist"
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