[PATCH ipsec] xfrm: esp: reset outer TCP GSO metadata on espintcp input

[Guoshuai Yang <yangguoshuai@gmail.com>]

When ESP packets are received over TCP (RFC 8229 espintcp), strparser
slices individual length-prefixed ESP records from the outer TCP stream by
cloning the incoming TCP skb(s) in __strp_recv().

If the outer TCP skb was GRO-coalesced before reaching strparser, its
skb_shinfo carries the outer TCP stream's GSO metadata (gso_size, gso_segs,
and gso_type). When an ESP record spans two outer TCP segments, strparser
chains the spillover clone onto skb_shinfo(head)->frag_list.

Subsequently in esp_input() and esp6_input(), skb_cow_data() unshares the
head skb via pskb_expand_head() (preserving the outer TCP gso_size and
gso_type in the new private skb_shinfo) and calls skb_copy() on the cloned
frag_list member, producing a linear frag_list child skb allocated from
slab with head_frag == 0.

After ESP decryption and header removal, the resulting single inner packet
still has skb_is_gso(skb) == true with the stale outer TCP gso_size, while
skb_headlen(head_skb) is now smaller than that gso_size due to stripping
the outer framing and ESP header. When this packet passes through
gro_cells_receive() (which flushes frag_list packets via GRO_NORMAL
without resetting gso_size) and is forwarded, validate_xmit_skb() invokes
tcp_gso_segment() -> skb_segment(). Because head_skb is shorter than the
stale gso_size and SKB_GSO_DODGY is not set, skb_segment() attempts to map
the slab-backed frag_list member as a scatter-gather page fragment and hits
BUG_ON(!list_skb->head_frag):

  Internal error: Oops - undefined instruction: 0 [#1] SMP ARM
  PC is at skb_segment+0x574/0xd20
  LR is at skb_segment+0x884/0xd20
  Call trace:
   skb_segment from tcp_gso_segment
   tcp_gso_segment from inet_gso_segment
   inet_gso_segment from skb_mac_gso_segment
   skb_mac_gso_segment from __skb_gso_segment
   __skb_gso_segment from validate_xmit_skb
   validate_xmit_skb from __dev_queue_xmit
   __dev_queue_xmit from ip_finish_output2
   ip_finish_output2 from ip_output
   ip_output from ip_sublist_rcv_finish
   ip_sublist_rcv_finish from ip_sublist_rcv
   ip_sublist_rcv from ip_list_rcv
   ip_list_rcv from __netif_receive_skb_list_core
   __netif_receive_skb_list_core from __netif_receive_skb_list
   __netif_receive_skb_list from gro_normal_list
   gro_normal_list from gro_cell_poll

Even when an ESP record does not have a frag_list, leaving the stale outer
TCP gso_size on the decrypted inner skb prevents dev_gro_receive() from
initializing gso_size to the inner packet's skb_gro_len().

We cannot reset GSO state earlier in espintcp_rcv() / handle_esp() because
the skb is still a clone sharing skb_shinfo with the outer TCP skb held by
strparser. Instead, call skb_gso_reset(skb) at skip_cow in esp_input() and
esp6_input(), where the skb is guaranteed to be uncloned and its
skb_shinfo is private.

Fixes: e27cca96cd68 ("xfrm: add espintcp (RFC 8229)")
Cc: stable@vger.kernel.org
Signed-off-by: Guoshuai Yang <yangguoshuai@gmail.com>
---
 net/ipv4/esp4.c | 3 +++
 net/ipv6/esp6.c | 3 +++
 2 files changed, 6 insertions(+)

diff --git a/net/ipv4/esp4.c b/net/ipv4/esp4.c
index e76db58..416638c 100644
--- a/net/ipv4/esp4.c
+++ b/net/ipv4/esp4.c
@@ -900,6 +900,9 @@ static int esp_input(struct xfrm_state *x, struct sk_buff *skb)
 	nfrags = err;
 
 skip_cow:
+	if (x->encap && x->encap->encap_type == TCP_ENCAP_ESPINTCP)
+		skb_gso_reset(skb);
+
 	err = -ENOMEM;
 	tmp = esp_alloc_tmp(aead, nfrags, seqhilen);
 	if (!tmp)
diff --git a/net/ipv6/esp6.c b/net/ipv6/esp6.c
index b1c9b36..02cfc85 100644
--- a/net/ipv6/esp6.c
+++ b/net/ipv6/esp6.c
@@ -943,6 +943,9 @@ static int esp6_input(struct xfrm_state *x, struct sk_buff *skb)
 	}
 
 skip_cow:
+	if (x->encap && x->encap->encap_type == TCP_ENCAP_ESPINTCP)
+		skb_gso_reset(skb);
+
 	ret = -ENOMEM;
 	tmp = esp_alloc_tmp(aead, nfrags, seqhilen);
 	if (!tmp)
-- 
2.54.0 (Apple Git-157)