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

[Willem de Bruijn]

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.

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.

> >
> >
> > > +                       tmp = skb_clone(nskb, GFP_ATOMIC);
> > > +                       if (tmp) {
> > > +                               kfree_skb(nskb);
> > > +                               nskb = tmp;
> > > +                               err = skb_unclone(nskb, GFP_ATOMIC);

Calling clone and unclone in quick succession looks odd.

But you need the first to create a private skb and to trigger the
second to create a private copy of the linear data (as well as frags,
if any, but these are not touched). So this looks okay.

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

To avoid leaking these skbs when calling kfree_skb_list(skb->next). Is
that concern new with this patch, or also needed for the existing
error case?

> > > +                       goto err_linearize;
> > > +               }
> > > +
> > >                 tail = nskb;
> > >
> > >                 delta_len += nskb->len;
> > > --
> > > 2.7.4
> > >
>