Merge branch 'fix-acyclic-ownership-checks'

Kumar Kartikeya Dwivedi says:

====================
Fix acyclic ownership checks

Bound and ensure acyclic ownership graphs for native data structures to
fix a bug reported by Nicholas. See commit logs and tests for details.

The existing list/rbtree rule already rejects graph-only cycles and bounds
those chains conservatively. It misses ownership through local referenced
kptrs, which can produce unbounded synchronous field destruction. Validate
all local ownership edges together, with an explicit depth bound, and allow
longer acyclic graph-only layouts within that bound.

Changelog:
----------
v1 -> v2
v1: https://lore.kernel.org/bpf/20260905090750.4064411-1-memxor@gmail.com/

 * Fold the graph-walk and local-kptr changes into one complete fix. (Alexei)
 * Explain why the original rule catches graph-only cycles, its three-type
   chain bound, and the missing local-kptr ownership edges. (Alexei)
 * Distinguish synchronous recursive field destruction from the deferred
   RCU freeing of object storage.
 * Add depth-boundary tests with child-first BTF ordering and a shared
   suffix reached with different remaining budgets.
 * Cover list and rbtree chains at the original three-type bound and at the
   new eight-type bound, including rejected over-limit cases.
====================

Link: https://patch.msgid.link/20260914132444.2564218-1-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

3 files changed, 384 insertions, 50 deletions

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index d67a169ba2bb..d870bc5e50bc 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -4268,13 +4268,10 @@ int btf_check_and_fixup_fields(const struct btf *btf, struct btf_record *rec)
 {
 	int i;
 
-	/* There are three types that signify ownership of some other type:
-	 *  kptr_ref, bpf_list_head, bpf_rb_root.
-	 * kptr_ref only supports storing kernel types, which can't store
-	 * references to program allocated local types.
-	 *
-	 * Hence we only need to ensure that bpf_{list_head,rb_root} ownership
-	 * does not form cycles.
+	/*
+	 * Check fields which require the complete BTF and initialize runtime
+	 * metadata. Ownership relationships are validated after every record has
+	 * been fixed up.
 	 */
 	if (IS_ERR_OR_NULL(rec) || !(rec->field_mask & (BPF_GRAPH_ROOT | BPF_UPTR)))
 		return 0;
@@ -4305,51 +4302,88 @@ int btf_check_and_fixup_fields(const struct btf *btf, struct btf_record *rec)
 		if (!meta)
 			return -EFAULT;
 		rec->fields[i].graph_root.value_rec = meta->record;
+	}
+	return 0;
+}
 
-		/* We need to set value_rec for all root types, but no need
-		 * to check ownership cycle for a type unless it's also a
-		 * node type.
-		 */
-		if (!(rec->field_mask & BPF_GRAPH_NODE))
+static int btf_owned_type_idx(const struct btf *btf, struct btf_struct_metas *tab,
+			      const struct btf_field *field)
+{
+	struct btf_struct_meta *meta;
+	u32 btf_id;
+
+	if (field->type & BPF_GRAPH_ROOT) {
+		btf_id = field->graph_root.value_btf_id;
+	} else if (field->type == BPF_KPTR_REF || field->type == BPF_KPTR_PERCPU) {
+		if (btf_is_kernel(field->kptr.btf))
+			return -ENOENT;
+		btf_id = field->kptr.btf_id;
+	} else {
+		return -ENOENT;
+	}
+
+	meta = btf_find_struct_meta(btf, btf_id);
+	if (!meta)
+		return field->type & BPF_GRAPH_ROOT ? -EFAULT : -ENOENT;
+	return meta - tab->types;
+}
+
+/*
+ * Each ownership edge adds kernel frames through bpf_obj_free_fields() and
+ * __bpf_obj_drop_impl(). Keep the bound deliberately small because object
+ * destruction can itself run below a BPF call chain. A final pointee without
+ * special fields is not present in the struct metadata table and adds only a
+ * non-recursing drop.
+ */
+#define BTF_MAX_OWNERSHIP_DEPTH 8
+
+static int btf_ownership_depth(const struct btf *btf,
+			       struct btf_struct_metas *tab, u8 *depth,
+			       int idx, int depth_left)
+{
+	const struct btf_record *rec = tab->types[idx].record;
+	int i, ret, max_depth = 0;
+
+	if (!depth_left)
+		return -ELOOP;
+	if (depth[idx])
+		goto done;
+
+	for (i = 0; i < rec->cnt; i++) {
+		ret = btf_owned_type_idx(btf, tab, &rec->fields[i]);
+		if (ret == -ENOENT)
 			continue;
+		if (ret < 0)
+			return ret;
+		ret = btf_ownership_depth(btf, tab, depth, ret, depth_left - 1);
+		if (ret < 0)
+			return ret;
+		max_depth = max(max_depth, ret);
+	}
+	depth[idx] = max_depth + 1;
+done:
+	return depth[idx] > depth_left ? -ELOOP : depth[idx];
+}
 
-		/* We need to ensure ownership acyclicity among all types. The
-		 * proper way to do it would be to topologically sort all BTF
-		 * IDs based on the ownership edges, since there can be multiple
-		 * bpf_{list_head,rb_node} in a type. Instead, we use the
-		 * following resaoning:
-		 *
-		 * - A type can only be owned by another type in user BTF if it
-		 *   has a bpf_{list,rb}_node. Let's call these node types.
-		 * - A type can only _own_ another type in user BTF if it has a
-		 *   bpf_{list_head,rb_root}. Let's call these root types.
-		 *
-		 * We ensure that if a type is both a root and node, its
-		 * element types cannot be root types.
-		 *
-		 * To ensure acyclicity:
-		 *
-		 * When A is an root type but not a node, its ownership
-		 * chain can be:
-		 *	A -> B -> C
-		 * Where:
-		 * - A is an root, e.g. has bpf_rb_root.
-		 * - B is both a root and node, e.g. has bpf_rb_node and
-		 *   bpf_list_head.
-		 * - C is only an root, e.g. has bpf_list_node
-		 *
-		 * When A is both a root and node, some other type already
-		 * owns it in the BTF domain, hence it can not own
-		 * another root type through any of the ownership edges.
-		 *	A -> B
-		 * Where:
-		 * - A is both an root and node.
-		 * - B is only an node.
-		 */
-		if (meta->record->field_mask & BPF_GRAPH_ROOT)
-			return -ELOOP;
+static int btf_check_ownership_depth(const struct btf *btf,
+				     struct btf_struct_metas *tab)
+{
+	u8 *depth;
+	int i, ret = 0;
+
+	depth = kvcalloc(tab->cnt, sizeof(*depth), GFP_KERNEL | __GFP_NOWARN);
+	if (!depth)
+		return -ENOMEM;
+
+	for (i = 0; i < tab->cnt; i++) {
+		ret = btf_ownership_depth(btf, tab, depth, i,
+					  BTF_MAX_OWNERSHIP_DEPTH);
+		if (ret < 0)
+			break;
+		ret = 0;
 	}
-	return 0;
+	kvfree(depth);
+	return ret;
 }
 
 static void __btf_struct_show(const struct btf *btf, const struct btf_type *t,
@@ -6044,6 +6078,10 @@ static struct btf *btf_parse(const union bpf_attr *attr, bpfptr_t uattr,
 			if (err < 0)
 				goto errout_meta;
 		}
+
+		err = btf_check_ownership_depth(btf, struct_meta_tab);
+		if (err < 0)
+			goto errout_meta;
 	}
 
 	err = bpf_log_attr_finalize(attr_log, &env->log);
diff --git a/tools/testing/selftests/bpf/prog_tests/linked_list.c b/tools/testing/selftests/bpf/prog_tests/linked_list.c
index c3d133c6a00d..52fabbee3dd5 100644
--- a/tools/testing/selftests/bpf/prog_tests/linked_list.c
+++ b/tools/testing/selftests/bpf/prog_tests/linked_list.c
@@ -714,7 +714,7 @@ static void test_btf(void)
 			break;
 
 		err = btf__load_into_kernel(btf);
-		ASSERT_EQ(err, -ELOOP, "check btf");
+		ASSERT_EQ(err, 0, "check btf");
 		btf__free(btf);
 		break;
 	}
@@ -773,7 +773,7 @@ static void test_btf(void)
 			break;
 
 		err = btf__load_into_kernel(btf);
-		ASSERT_EQ(err, -ELOOP, "check btf");
+		ASSERT_EQ(err, 0, "check btf");
 		btf__free(btf);
 		break;
 	}
diff --git a/tools/testing/selftests/bpf/prog_tests/local_kptr_ownership.c b/tools/testing/selftests/bpf/prog_tests/local_kptr_ownership.c
new file mode 100644
index 000000000000..a487aa68f2ee
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/local_kptr_ownership.c
@@ -0,0 +1,296 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2026 Meta Platforms, Inc. and affiliates. */
+
+#include <bpf/btf.h>
+#include <linux/btf.h>
+#include <test_progs.h>
+
+#define SPIN_LOCK 2
+#define LIST_HEAD 3
+#define LIST_NODE 4
+/* Keep in sync with BTF_MAX_OWNERSHIP_DEPTH. */
+#define MAX_OWNERSHIP_DEPTH 8
+
+static struct btf *init_btf(void)
+{
+	struct btf *btf;
+	int id;
+
+	btf = btf__new_empty();
+	if (!ASSERT_OK_PTR(btf, "btf__new_empty"))
+		return NULL;
+	id = btf__add_int(btf, "int", 4, BTF_INT_SIGNED);
+	if (!ASSERT_EQ(id, 1, "btf__add_int"))
+		goto err_out;
+	id = btf__add_struct(btf, "bpf_spin_lock", 4);
+	if (!ASSERT_EQ(id, SPIN_LOCK, "btf__add_struct bpf_spin_lock"))
+		goto err_out;
+	id = btf__add_struct(btf, "bpf_list_head", 16);
+	if (!ASSERT_EQ(id, LIST_HEAD, "btf__add_struct bpf_list_head"))
+		goto err_out;
+	id = btf__add_struct(btf, "bpf_list_node", 24);
+	if (!ASSERT_EQ(id, LIST_NODE, "btf__add_struct bpf_list_node"))
+		goto err_out;
+	return btf;
+
+err_out:
+	btf__free(btf);
+	return NULL;
+}
+
+static int add_local_kptr(struct btf *btf, int pointee_id, const char *tag)
+{
+	int id;
+
+	id = btf__add_type_tag(btf, tag, pointee_id);
+	if (!ASSERT_GT(id, 0, "btf__add_type_tag"))
+		return id;
+	id = btf__add_ptr(btf, id);
+	ASSERT_GT(id, 0, "btf__add_ptr");
+	return id;
+}
+
+static void test_self_cycle(const char *tag, int expected_err)
+{
+	struct btf *btf;
+	int id, err;
+
+	btf = init_btf();
+	if (!ASSERT_OK_PTR(btf, "init_btf"))
+		return;
+	id = add_local_kptr(btf, 7, tag);
+	if (id <= 0)
+		goto out;
+	id = btf__add_struct(btf, "self_cycle", 8);
+	if (!ASSERT_EQ(id, 7, "btf__add_struct self_cycle"))
+		goto out;
+	err = btf__add_field(btf, "next", 6, 0, 0);
+	if (!ASSERT_OK(err, "btf__add_field self_cycle::next"))
+		goto out;
+
+	err = btf__load_into_kernel(btf);
+	ASSERT_EQ(err, expected_err, "check btf");
+out:
+	btf__free(btf);
+}
+
+static void test_aba_cycle(void)
+{
+	struct btf *btf;
+	int id, err;
+
+	btf = init_btf();
+	if (!ASSERT_OK_PTR(btf, "init_btf"))
+		return;
+	id = add_local_kptr(btf, 10, "kptr");
+	if (id <= 0)
+		goto out;
+	id = add_local_kptr(btf, 9, "kptr");
+	if (id <= 0)
+		goto out;
+	id = btf__add_struct(btf, "cycle_a", 8);
+	if (!ASSERT_EQ(id, 9, "btf__add_struct cycle_a"))
+		goto out;
+	err = btf__add_field(btf, "b", 6, 0, 0);
+	if (!ASSERT_OK(err, "btf__add_field cycle_a::b"))
+		goto out;
+	id = btf__add_struct(btf, "cycle_b", 8);
+	if (!ASSERT_EQ(id, 10, "btf__add_struct cycle_b"))
+		goto out;
+	err = btf__add_field(btf, "a", 8, 0, 0);
+	if (!ASSERT_OK(err, "btf__add_field cycle_b::a"))
+		goto out;
+
+	err = btf__load_into_kernel(btf);
+	ASSERT_EQ(err, -ELOOP, "check btf");
+out:
+	btf__free(btf);
+}
+
+static void test_mixed_cycle(void)
+{
+	struct btf *btf;
+	int id, err;
+
+	btf = init_btf();
+	if (!ASSERT_OK_PTR(btf, "init_btf"))
+		return;
+	id = add_local_kptr(btf, 7, "kptr");
+	if (id <= 0)
+		goto out;
+	id = btf__add_struct(btf, "mixed_owner", 20);
+	if (!ASSERT_EQ(id, 7, "btf__add_struct mixed_owner"))
+		goto out;
+	err = btf__add_field(btf, "root", LIST_HEAD, 0, 0);
+	if (!ASSERT_OK(err, "btf__add_field mixed_owner::root"))
+		goto out;
+	err = btf__add_field(btf, "lock", SPIN_LOCK, 128, 0);
+	if (!ASSERT_OK(err, "btf__add_field mixed_owner::lock"))
+		goto out;
+	id = btf__add_decl_tag(btf, "contains:mixed_node:node", 7, 0);
+	if (!ASSERT_EQ(id, 8, "btf__add_decl_tag mixed_owner"))
+		goto out;
+	id = btf__add_struct(btf, "mixed_node", 32);
+	if (!ASSERT_EQ(id, 9, "btf__add_struct mixed_node"))
+		goto out;
+	err = btf__add_field(btf, "node", LIST_NODE, 0, 0);
+	if (!ASSERT_OK(err, "btf__add_field mixed_node::node"))
+		goto out;
+	err = btf__add_field(btf, "owner", 6, 192, 0);
+	if (!ASSERT_OK(err, "btf__add_field mixed_node::owner"))
+		goto out;
+
+	err = btf__load_into_kernel(btf);
+	ASSERT_EQ(err, -ELOOP, "check btf");
+out:
+	btf__free(btf);
+}
+
+static void test_acyclic_depth(int depth, bool child_first, bool shared_suffix, int expected_err)
+{
+	int ptr_id[MAX_OWNERSHIP_DEPTH + 1];
+	int first_struct_id;
+	struct btf *btf;
+	int id, err, i, n, pointee_id;
+
+	btf = init_btf();
+	if (!ASSERT_OK_PTR(btf, "init_btf"))
+		return;
+	first_struct_id = 5 + 2 * depth;
+	for (i = 0; i < depth; i++) {
+		if (i == depth - 1)
+			pointee_id = first_struct_id + depth;
+		else
+			pointee_id = first_struct_id + (child_first ? depth - 2 - i : i + 1);
+		ptr_id[i] = add_local_kptr(btf, pointee_id, "kptr");
+		if (ptr_id[i] <= 0)
+			goto out;
+	}
+	for (n = 0; n < depth; n++) {
+		char name[32];
+		int offset = 0;
+
+		i = child_first ? depth - 1 - n : n;
+		snprintf(name, sizeof(name), "owner_%d", i);
+		id = btf__add_struct(btf, name, shared_suffix && !i ? 16 : 8);
+		if (!ASSERT_EQ(id, first_struct_id + n, "btf__add_struct owner"))
+			goto out;
+		if (shared_suffix && !i) {
+			/*
+			 * Visit the shared suffix through the shorter path before
+			 * reaching it again with less remaining depth.
+			 */
+			err = btf__add_field(btf, "suffix", ptr_id[1], 0, 0);
+			if (!ASSERT_OK(err, "btf__add_field owner::suffix"))
+				goto out;
+			offset = 64;
+		}
+		err = btf__add_field(btf, "next", ptr_id[i], offset, 0);
+		if (!ASSERT_OK(err, "btf__add_field owner::next"))
+			goto out;
+	}
+	id = btf__add_struct(btf, "plain_leaf", 4);
+	if (!ASSERT_EQ(id, first_struct_id + depth, "btf__add_struct plain_leaf"))
+		goto out;
+
+	err = btf__load_into_kernel(btf);
+	ASSERT_EQ(err, expected_err, "check btf");
+out:
+	btf__free(btf);
+}
+
+static void test_graph_depth(bool rbtree, int depth, int expected_err)
+{
+	int root_type = LIST_HEAD, node_type = LIST_NODE, node_size = 24;
+	int id, err, i, lock_off, root_off, size;
+	struct btf *btf;
+
+	btf = init_btf();
+	if (!ASSERT_OK_PTR(btf, "init_btf"))
+		return;
+	if (rbtree) {
+		root_type = btf__add_struct(btf, "bpf_rb_root", 16);
+		if (!ASSERT_GT(root_type, 0, "btf__add_struct bpf_rb_root"))
+			goto out;
+		node_type = btf__add_struct(btf, "bpf_rb_node", 32);
+		if (!ASSERT_GT(node_type, 0, "btf__add_struct bpf_rb_node"))
+			goto out;
+		node_size = 32;
+	}
+
+	for (i = 0; i < depth; i++) {
+		char name[32], tag[64];
+
+		lock_off = i ? node_size : 0;
+		root_off = lock_off + 8;
+		size = i == depth - 1 ? node_size : root_off + 16;
+		snprintf(name, sizeof(name), "graph_owner_%d", i);
+		id = btf__add_struct(btf, name, size);
+		if (!ASSERT_GT(id, 0, "btf__add_struct graph_owner"))
+			goto out;
+		if (i) {
+			err = btf__add_field(btf, "node", node_type, 0, 0);
+			if (!ASSERT_OK(err, "btf__add_field graph_owner::node"))
+				goto out;
+		}
+		if (i == depth - 1)
+			continue;
+		err = btf__add_field(btf, "lock", SPIN_LOCK, lock_off * 8, 0);
+		if (!ASSERT_OK(err, "btf__add_field graph_owner::lock"))
+			goto out;
+		err = btf__add_field(btf, "root", root_type, root_off * 8, 0);
+		if (!ASSERT_OK(err, "btf__add_field graph_owner::root"))
+			goto out;
+		snprintf(tag, sizeof(tag), "contains:graph_owner_%d:node", i + 1);
+		err = btf__add_decl_tag(btf, tag, id, i ? 2 : 1);
+		if (!ASSERT_GT(err, 0, "btf__add_decl_tag graph_owner"))
+			goto out;
+	}
+
+	err = btf__load_into_kernel(btf);
+	ASSERT_EQ(err, expected_err, "check btf");
+out:
+	btf__free(btf);
+}
+
+void test_local_kptr_ownership(void)
+{
+	if (test__start_subtest("self_cycle"))
+		test_self_cycle("kptr", -ELOOP);
+	if (test__start_subtest("untrusted_self_cycle"))
+		test_self_cycle("kptr_untrusted", 0);
+	if (test__start_subtest("percpu_self_cycle"))
+		test_self_cycle("percpu_kptr", -ELOOP);
+	if (test__start_subtest("ABA_cycle"))
+		test_aba_cycle();
+	if (test__start_subtest("mixed_graph_root_cycle"))
+		test_mixed_cycle();
+	if (test__start_subtest("max_acyclic"))
+		test_acyclic_depth(MAX_OWNERSHIP_DEPTH, false, false, 0);
+	if (test__start_subtest("too_deep_acyclic"))
+		test_acyclic_depth(MAX_OWNERSHIP_DEPTH + 1, false, false, -ELOOP);
+	if (test__start_subtest("max_acyclic_child_first"))
+		test_acyclic_depth(MAX_OWNERSHIP_DEPTH, true, false, 0);
+	if (test__start_subtest("too_deep_acyclic_child_first"))
+		test_acyclic_depth(MAX_OWNERSHIP_DEPTH + 1, true, false, -ELOOP);
+	if (test__start_subtest("max_acyclic_shared_suffix"))
+		test_acyclic_depth(MAX_OWNERSHIP_DEPTH, false, true, 0);
+	if (test__start_subtest("too_deep_acyclic_shared_suffix"))
+		test_acyclic_depth(MAX_OWNERSHIP_DEPTH + 1, false, true, -ELOOP);
+	if (test__start_subtest("list_three_types"))
+		test_graph_depth(false, 3, 0);
+	if (test__start_subtest("list_four_types"))
+		test_graph_depth(false, 4, 0);
+	if (test__start_subtest("list_max_depth"))
+		test_graph_depth(false, MAX_OWNERSHIP_DEPTH, 0);
+	if (test__start_subtest("list_too_deep"))
+		test_graph_depth(false, MAX_OWNERSHIP_DEPTH + 1, -ELOOP);
+	if (test__start_subtest("rbtree_three_types"))
+		test_graph_depth(true, 3, 0);
+	if (test__start_subtest("rbtree_four_types"))
+		test_graph_depth(true, 4, 0);
+	if (test__start_subtest("rbtree_max_depth"))
+		test_graph_depth(true, MAX_OWNERSHIP_DEPTH, 0);
+	if (test__start_subtest("rbtree_too_deep"))
+		test_graph_depth(true, MAX_OWNERSHIP_DEPTH + 1, -ELOOP);
+}