bpf: Reject struct_ops registration that uses module ptr and the module btf_id is missing

There is a UAF report in the bpf_struct_ops when CONFIG_MODULES=n.
In particular, the report is on tcp_congestion_ops that has
a "struct module *owner" member.

For struct_ops that has a "struct module *owner" member,
it can be extended either by the regular kernel module or
by the bpf_struct_ops. bpf_try_module_get() will be used
to do the refcounting and different refcount is done
based on the owner pointer. When CONFIG_MODULES=n,
the btf_id of the "struct module" is missing:

WARN: resolve_btfids: unresolved symbol module

Thus, the bpf_try_module_get() cannot do the correct refcounting.

Not all subsystem's struct_ops requires the "struct module *owner" member.
e.g. the recent sched_ext_ops.

This patch is to disable bpf_struct_ops registration if
the struct_ops has the "struct module *" member and the
"struct module" btf_id is missing. The btf_type_is_fwd() helper
is moved to the btf.h header file for this test.

This has happened since the beginning of bpf_struct_ops which has gone
through many changes. The Fixes tag is set to a recent commit that this
patch can apply cleanly. Considering CONFIG_MODULES=n is not
common and the age of the issue, targeting for bpf-next also.

Fixes: 1611603537a4 ("bpf: Create argument information for nullable arguments.")
Reported-by: Robert Morris <rtm@csail.mit.edu>
Closes: https://lore.kernel.org/bpf/74665.1733669976@localhost/
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Tested-by: Eduard Zingerman <eddyz87@gmail.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20241220201818.127152-1-martin.lau@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/linux/btf.h

@@ -353,6 +353,11 @@ static inline bool btf_type_is_scalar(const struct btf_type *t)
 	return btf_type_is_int(t) || btf_type_is_enum(t);
 }
 
+static inline bool btf_type_is_fwd(const struct btf_type *t)
+{
+	return BTF_INFO_KIND(t->info) == BTF_KIND_FWD;
+}
+
 static inline bool btf_type_is_typedef(const struct btf_type *t)
 {
 	return BTF_INFO_KIND(t->info) == BTF_KIND_TYPEDEF;

kernel/bpf/bpf_struct_ops.c

@@ -310,6 +310,20 @@ void bpf_struct_ops_desc_release(struct bpf_struct_ops_desc *st_ops_desc)
 	kfree(arg_info);
 }
 
+static bool is_module_member(const struct btf *btf, u32 id)
+{
+	const struct btf_type *t;
+
+	t = btf_type_resolve_ptr(btf, id, NULL);
+	if (!t)
+		return false;
+
+	if (!__btf_type_is_struct(t) && !btf_type_is_fwd(t))
+		return false;
+
+	return !strcmp(btf_name_by_offset(btf, t->name_off), "module");
+}
+
 int bpf_struct_ops_desc_init(struct bpf_struct_ops_desc *st_ops_desc,
 			     struct btf *btf,
 			     struct bpf_verifier_log *log)
@@ -389,6 +403,13 @@ int bpf_struct_ops_desc_init(struct bpf_struct_ops_desc *st_ops_desc,
 			goto errout;
 		}
 
+		if (!st_ops_ids[IDX_MODULE_ID] && is_module_member(btf, member->type)) {
+			pr_warn("'struct module' btf id not found. Is CONFIG_MODULES enabled? bpf_struct_ops '%s' needs module support.\n",
+				st_ops->name);
+			err = -EOPNOTSUPP;
+			goto errout;
+		}
+
 		func_proto = btf_type_resolve_func_ptr(btf,
 						       member->type,
 						       NULL);

kernel/bpf/btf.c

@@ -498,11 +498,6 @@ bool btf_type_is_void(const struct btf_type *t)
 	return t == &btf_void;
 }
 
-static bool btf_type_is_fwd(const struct btf_type *t)
-{
-	return BTF_INFO_KIND(t->info) == BTF_KIND_FWD;
-}
-
 static bool btf_type_is_datasec(const struct btf_type *t)
 {
 	return BTF_INFO_KIND(t->info) == BTF_KIND_DATASEC;