On 2021-01-05 06:03, Willem de Bruijn wrote:
>
> On Mon, Jan 4, 2021 at 4:00 AM Dongseok Yi <dseok...@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
[ 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...@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.
>
>
> > + tmp = skb_clone(nskb, GFP_ATOMIC);
> > + if (tmp) {
> > + kfree_skb(nskb);
> > + nskb = tmp;
> > + err = skb_unclone(nskb, GFP_ATOMIC);
> > + } else {
> > + err = -ENOMEM;
> > + }
> > + }
> > +
> > if (!tail)
> > skb->next = nskb;
> > else
> > tail->next = nskb;
> >
> > + if (unlikely(err)) {
> > + nskb->next = list_skb;
> > + goto err_linearize;
> > + }
> > +
> > tail = nskb;
> >
> > delta_len += nskb->len;
> > --
> > 2.7.4
> >
