[PATCH bpf-next v1 1/2] bpf: Restrict usage scope of bpf_get_cgroup_classid

[Jiayuan Chen <jiayuan.chen@linux.dev>]

A previous commit expanded the usage scope of bpf_get_cgroup_classid() to
all contexts (see Fixes tag), but this was inappropriate.

First, syzkaller reported a bug [1].
Second, it uses skb as an argument, but its implementation varies across
different bpf prog types. For example, in sock_filter and sock_addr, it
retrieves the classid from the current context
(bpf_get_cgroup_classid_curr_proto) instead of from skb. In tc egress and
lwt, it fetches the classid from skb->sk, but in tc ingress, it returns 0.

In summary, the definition of bpf_get_cgroup_classid() is ambiguous and
its usage scenarios are limited. It should not be treated as a
general-purpose helper. This patch reverts part of the previous commit.

[1] https://syzkaller.appspot.com/bug?extid=9767c7ed68b95cfa69e6

Fixes: ee971630f20f ("bpf: Allow some trace helpers for all prog types")
Reported-by: syzbot+9767c7ed68b95cfa69e6@syzkaller.appspotmail.com
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
 include/linux/bpf-cgroup.h |  8 ++++++++
 kernel/bpf/cgroup.c        | 25 +++++++++++++++++++++++++
 kernel/bpf/helpers.c       |  4 ----
 3 files changed, 33 insertions(+), 4 deletions(-)

diff --git a/include/linux/bpf-cgroup.h b/include/linux/bpf-cgroup.h
index 4847dcade917..9de7adb68294 100644
--- a/include/linux/bpf-cgroup.h
+++ b/include/linux/bpf-cgroup.h
@@ -427,6 +427,8 @@ int cgroup_bpf_prog_query(const union bpf_attr *attr,
 
 const struct bpf_func_proto *
 cgroup_common_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog);
+const struct bpf_func_proto *
+cgroup_current_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog);
 #else
 
 static inline int cgroup_bpf_inherit(struct cgroup *cgrp) { return 0; }
@@ -463,6 +465,12 @@ cgroup_common_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 	return NULL;
 }
 
+static inline const struct bpf_func_proto *
+cgroup_current_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
+{
+	return NULL;
+}
+
 static inline int bpf_cgroup_storage_assign(struct bpf_prog_aux *aux,
 					    struct bpf_map *map) { return 0; }
 static inline struct bpf_cgroup_storage *bpf_cgroup_storage_alloc(
diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
index 62a1d8deb3dc..a99b72e6f1c9 100644
--- a/kernel/bpf/cgroup.c
+++ b/kernel/bpf/cgroup.c
@@ -1653,6 +1653,10 @@ cgroup_dev_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 	if (func_proto)
 		return func_proto;
 
+	func_proto = cgroup_current_func_proto(func_id, prog);
+	if (func_proto)
+		return func_proto;
+
 	switch (func_id) {
 	case BPF_FUNC_perf_event_output:
 		return &bpf_event_output_data_proto;
@@ -2200,6 +2204,10 @@ sysctl_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 	if (func_proto)
 		return func_proto;
 
+	func_proto = cgroup_current_func_proto(func_id, prog);
+	if (func_proto)
+		return func_proto;
+
 	switch (func_id) {
 	case BPF_FUNC_sysctl_get_name:
 		return &bpf_sysctl_get_name_proto;
@@ -2343,6 +2351,10 @@ cg_sockopt_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 	if (func_proto)
 		return func_proto;
 
+	func_proto = cgroup_current_func_proto(func_id, prog);
+	if (func_proto)
+		return func_proto;
+
 	switch (func_id) {
 #ifdef CONFIG_NET
 	case BPF_FUNC_get_netns_cookie:
@@ -2589,3 +2601,16 @@ cgroup_common_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 		return NULL;
 	}
 }
+
+const struct bpf_func_proto *
+cgroup_current_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
+{
+	switch (func_id) {
+#ifdef CONFIG_CGROUP_NET_CLASSID
+	case BPF_FUNC_get_cgroup_classid:
+		return &bpf_get_cgroup_classid_curr_proto;
+#endif
+	default:
+		return NULL;
+	}
+}
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index b71e428ad936..9d0d54f4f0de 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -2024,10 +2024,6 @@ bpf_base_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 		return &bpf_get_current_ancestor_cgroup_id_proto;
 	case BPF_FUNC_current_task_under_cgroup:
 		return &bpf_current_task_under_cgroup_proto;
-#endif
-#ifdef CONFIG_CGROUP_NET_CLASSID
-	case BPF_FUNC_get_cgroup_classid:
-		return &bpf_get_cgroup_classid_curr_proto;
 #endif
 	case BPF_FUNC_task_storage_get:
 		if (bpf_prog_check_recur(prog))
-- 
2.47.1