[PATCH bpf v2 2/2] bpf: revalidate assigned sockets after protocol change

[Shihuang Liu <shlomojune6@gmail.com>]

An IPv4 packet can be assigned to an AF_INET socket and then translated
to IPv6 by bpf_skb_change_proto(). Since the translation preserves the
socket assignment, the IPv6 packet can still be delivered to the IPv4
socket, bypassing the family check in bpf_sk_assign().

After a successful protocol change, recheck any prefetched socket
against the new protocol and call skb_orphan() if its address family
is incompatible. This releases the assignment through the existing
skb destructor, allowing normal socket lookup or a new assignment
by the BPF program.

Fixes: cf7fbe660f2d ("bpf: Add socket assign support")
Assisted-by: LLM
Signed-off-by: Shihuang Liu <shlomojune6@gmail.com>
---
Changes since v1:
- Revalidate prefetched sockets after bpf_skb_change_proto() changes the
  packet protocol, closing a bypass of assignment-time validation.
- Preserve compatible dual-stack assignments and release incompatible
  assignments through their existing skb destructor.
- Split the fix into two patches and target the BPF fixes tree.

v1:
https://lore.kernel.org/netdev/20260823101809.26802-1-shlomojune6@gmail.com/

 include/uapi/linux/bpf.h       | 4 ++++
 net/core/filter.c              | 5 +++++
 tools/include/uapi/linux/bpf.h | 4 ++++
 3 files changed, 13 insertions(+)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 5d8f5e2c8db38..0de7967077a2e 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -2659,6 +2659,10 @@ union bpf_attr {
  * 		checked and segments are recalculated by the GSO/GRO engine.
  * 		The size for GSO target is adapted as well.
  *
+ *		On success, an assigned socket is released if its address
+ *		family is incompatible with the new protocol. Assign a
+ *		compatible socket after translation if required.
+ *
  * 		All values for *flags* are reserved for future usage, and must
  * 		be left at zero.
  *
diff --git a/net/core/filter.c b/net/core/filter.c
index e9cc76b775c0c..79a0e484d9dd4 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -3547,6 +3547,11 @@ BPF_CALL_3(bpf_skb_change_proto, struct sk_buff *, skb, __be16, proto,
 	if (ret)
 		return ret;
 
+	/* Protocol translation can invalidate an earlier socket assignment. */
+	if (skb_sk_is_prefetched(skb) &&
+	    !bpf_sk_assign_family_ok(skb, skb->sk))
+		skb_orphan(skb);
+
 	if (skb_valid_dst(skb))
 		skb_dst_drop(skb);
 
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index 5d8f5e2c8db38..0de7967077a2e 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -2659,6 +2659,10 @@ union bpf_attr {
  * 		checked and segments are recalculated by the GSO/GRO engine.
  * 		The size for GSO target is adapted as well.
  *
+ *		On success, an assigned socket is released if its address
+ *		family is incompatible with the new protocol. Assign a
+ *		compatible socket after translation if required.
+ *
  * 		All values for *flags* are reserved for future usage, and must
  * 		be left at zero.
  *
-- 
2.43.0