[PATCH bpf-next v2] net: Fix slab-out-of-bounds in inet[6]_steal_sock

[Lorenz Bauer]

Kumar reported a KASAN splat in tcp_v6_rcv:

  bash-5.2# ./test_progs -t btf_skc_cls_ingress
  ...
  [   51.810085] BUG: KASAN: slab-out-of-bounds in tcp_v6_rcv+0x2d7d/0x3440
  [   51.810458] Read of size 2 at addr ffff8881053f038c by task test_progs/226

The problem is that inet[6]_steal_sock accesses sk->sk_protocol without
accounting for request or timewait sockets. To fix this we can't just
check sock_common->skc_reuseport since that flag is present on timewait
sockets.

Instead, add a fullsock check to avoid the out of bands access of sk_protocol.

Fixes: 9c02bec95954 ("bpf, net: Support SO_REUSEPORT sockets with bpf_sk_assign")
Reported-by: Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx>
Signed-off-by: Lorenz Bauer <lmb@xxxxxxxxxxxxx>
---
Changes in v2:
- Do a sk_fullsock check instead (Martin Lau)
- Link to v1: https://lore.kernel.org/r/20230809-bpf-next-v1-1-c1b80712e83b@xxxxxxxxxxxxx
---
 include/net/inet6_hashtables.h | 2 +-
 include/net/inet_hashtables.h  | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/net/inet6_hashtables.h b/include/net/inet6_hashtables.h
index 284b5ce7205d..533a7337865a 100644
--- a/include/net/inet6_hashtables.h
+++ b/include/net/inet6_hashtables.h
@@ -116,7 +116,7 @@ struct sock *inet6_steal_sock(struct net *net, struct sk_buff *skb, int doff,
 	if (!sk)
 		return NULL;
 
-	if (!prefetched)
+	if (!prefetched || !sk_fullsock(sk))
 		return sk;
 
 	if (sk->sk_protocol == IPPROTO_TCP) {
diff --git a/include/net/inet_hashtables.h b/include/net/inet_hashtables.h
index 1177effabed3..b277f7ef423a 100644
--- a/include/net/inet_hashtables.h
+++ b/include/net/inet_hashtables.h
@@ -462,7 +462,7 @@ struct sock *inet_steal_sock(struct net *net, struct sk_buff *skb, int doff,
 	if (!sk)
 		return NULL;
 
-	if (!prefetched)
+	if (!prefetched || !sk_fullsock(sk))
 		return sk;
 
 	if (sk->sk_protocol == IPPROTO_TCP) {

---
base-commit: eb62e6aef940fcb1879100130068369d4638088f
change-id: 20230808-bpf-next-a442a095562b

Best regards,
-- 
Lorenz Bauer <lmb@xxxxxxxxxxxxx>