[PATCH net-next 0/2] net: permit skb_segment on head_frag frag_list skb

[Yonghong Song <yhs@fb.com>]

One of our in-house projects, bpf-based NAT, hits a kernel BUG_ON at
function skb_segment(), line 3667. The bpf program attaches to
clsact ingress, calls bpf_skb_change_proto to change protocol
from ipv4 to ipv6 or from ipv6 to ipv4, and then calls bpf_redirect
to send the changed packet out.
 ...
    3665                 while (pos < offset + len) {
    3666                         if (i >= nfrags) {
    3667                                 BUG_ON(skb_headlen(list_skb));
 ...

The triggering input skb has the following properties:
    list_skb = skb->frag_list;
    skb->nfrags != NULL && skb_headlen(list_skb) != 0
and skb_segment() is not able to handle a frag_list skb
if its headlen (list_skb->len - list_skb->data_len) is not 0.

Patch #1 provides a simple solution to avoid BUG_ON. If
list_skb->head_frag is true, its page-backed frag will
be processed before the list_skb->frags.
Patch #2 provides a test case in test_bpf module which
constructs a skb and calls skb_segment() directly. The test
case is able to trigger the BUG_ON without Patch #1.

Yonghong Song (2):
  net: permit skb_segment on head_frag frag_list skb
  net: bpf: add a test for skb_segment in test_bpf module

 lib/test_bpf.c    | 71 ++++++++++++++++++++++++++++++++++++++++++++++++++++++-
 net/core/skbuff.c | 42 ++++++++++++++++++++++----------
 2 files changed, 100 insertions(+), 13 deletions(-)

-- 
2.9.5