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.
The patch has been tested in the following setup:
ipv6_host <-> nat_server <-> ipv4_host
where nat_server has a bpf program doing ipv4<->ipv6
translation and forwarding through clsact hook
bpf_skb_change_proto.
Changelog:
v3 -> v4:
. Remove dynamic memory allocation and use rewinding
for both index and frag to remove one branch in fast path,
from Alexander.
. Fix a bunch of issues in test_bpf skb_segment() test,
including proper way to allocate skb, proper function
argument for skb_add_rx_frag and not freeint skb, etc.,
from Eric.
v2 -> v3:
. Use starting frag index -1 (instead of 0) to
special process head_frag before other frags in the skb,
from Alexander Duyck.
v1 -> v2:
. Removed never-hit BUG_ON, spotted by Linyu Yuan.
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 | 91 +++++++++++++++++++++++++++++++++++++++++++++++++++++--
net/core/skbuff.c | 36 +++++++++++++++-------
2 files changed, 114 insertions(+), 13 deletions(-)
--
2.9.5
