skb_mpls_push() records the pre-encapsulation network header once, gated
on !skb->inner_protocol. skb_mpls_pop() never clears that record, so it
outlives the encapsulation it describes.
Open vSwitch can then re-push MPLS onto a packet whose
inner_network_header still points at the older, deeper offset: push a
label, pop every label, recirculate (ovs_flow_key_update() re-derives
key->eth.type and resets network_header, but leaves inner_*), then push
again. ovs_fragment() trusts the record:
skb->network_header = skb->inner_network_header;
so skb_network_offset() goes negative. The bound check is signed:
if (skb_network_offset(skb) > MAX_L2_LEN)
a negative offset passes it, and prepare_frag() widens the value:
unsigned int hlen = skb_network_offset(skb);
memcpy(&data->l2_data, skb->data, hlen);
which is a ~4GiB memcpy out of a 30-byte per-CPU buffer.
Reproduced on v7.3-rc1. RDX is the truncated length, (unsigned int)(-8):
BUG: unable to handle page fault for address: ffffe8ffffc16000
#PF: supervisor write access in kernel mode
Oops: 0002 [#1] SMP KASAN NOPTI
RIP: 0010:memcpy+0x8/0x20
RDX: 00000000fffffff8 RSI: ffff888105d732db RDI: ffffe8ffffc16000
prepare_frag+0x3df/0x4e0
ovs_fragment+0x589/0x7e0
do_output+0x4ce/0x5e0
do_execute_actions+0x55d2/0x7b30
ovs_execute_actions+0xea/0x450
Same root-cause shape as commit 975b5b067f52 ("ipv6: sr: restore network
header before routing and forwarding"): a stale network header offset
reaching a consumer that widens it. Here it originates in the MPLS
push/pop path.
Clear inner_protocol once the packet is no longer MPLS, so a later push
re-records the current header. net/sched/act_mpls.c is the only other
skb_mpls_pop() caller and gets the same fix; sch_frag.c saves and
restores inner_protocol around fragmentation in the same way OVS does.
Fixes: c66549ffd666 ("openvswitch: correctly fragment packet with mpls headers")
Cc: [email protected]
Assisted-by: tencentos-corvus-ai:hy4-preview
Signed-off-by: Fourie Zhang <[email protected]>
---
A KASAN reproducer for this issue is available if requested.
net/core/skbuff.c | 7 +++++++
1 file changed, 7 insertions(+)
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 966af3beed94..cc3b4b70288b 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -6690,6 +6690,13 @@ int skb_mpls_pop(struct sk_buff *skb, __be16 next_proto,
int mac_len,
}
skb->protocol = next_proto;
+ /* The last label is gone, so the inner header recorded by
+ * skb_mpls_push() no longer describes this packet. Drop it, or a
+ * later push keeps the stale offset.
+ */
+ if (!eth_p_mpls(next_proto))
+ skb->inner_protocol = 0;
+
return 0;
}
EXPORT_SYMBOL_GPL(skb_mpls_pop);
--
2.43.7
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