[PATCH bpf v2 6/8] selftests/bpf: Reject graph kptr use after RCU unlock

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

Add a sleepable verifier test that loads a graph-node local kptr in an
explicit RCU read-side critical section, then passes its node to
bpf_rbtree_remove() after the section ends.

Before the verifier fix, the stale NON_OWN_REF flag makes the node look like
a live borrowed reference and the program is accepted. After the fix, the
pointer is demoted without NON_OWN_REF and the graph kfunc argument is
rejected.

Also exercise a graph kptr loaded while a spin lock provides implicit RCU
protection. The pointer must be invalidated when the lock is released, which
guards the required ordering between non-owning-reference invalidation and
RCU demotion.

Update the existing fault-protected load test state description. The
post-unlock pointer no longer carries NON_OWN_REF, but remains readable
because the load is rewritten to use BPF_PROBE_MEM.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 .../selftests/bpf/progs/rcu_read_lock.c       |  6 +-
 .../bpf/progs/refcounted_kptr_fail.c          | 75 +++++++++++++++++++
 2 files changed, 78 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/bpf/progs/rcu_read_lock.c b/tools/testing/selftests/bpf/progs/rcu_read_lock.c
index 31d4081c3a9f..cdb255addbc3 100644
--- a/tools/testing/selftests/bpf/progs/rcu_read_lock.c
+++ b/tools/testing/selftests/bpf/progs/rcu_read_lock.c
@@ -592,9 +592,9 @@ int non_own_ref_untrusted_ld(void *ctx)
 	}
 	bpf_rcu_read_unlock();
 	/*
-	 * The unlock leaves node as PTR_TO_BTF_ID | MEM_ALLOC | PTR_UNTRUSTED
-	 * | NON_OWN_REF, and the load below has to get the BPF_PROBE_MEM
-	 * rewrite for it, otherwise a bad address panics the kernel.
+	 * The unlock leaves node as PTR_TO_BTF_ID | MEM_ALLOC | PTR_UNTRUSTED,
+	 * and the load below has to get the BPF_PROBE_MEM rewrite for it,
+	 * otherwise a bad address panics the kernel.
 	 */
 	non_own_ref_key = node->key;
 	return 0;
diff --git a/tools/testing/selftests/bpf/progs/refcounted_kptr_fail.c b/tools/testing/selftests/bpf/progs/refcounted_kptr_fail.c
index 7d2f8897e5ad..f787ecf189d8 100644
--- a/tools/testing/selftests/bpf/progs/refcounted_kptr_fail.c
+++ b/tools/testing/selftests/bpf/progs/refcounted_kptr_fail.c
@@ -28,6 +28,17 @@ struct map_value_refcount_only {
 	struct node_refcount_only __kptr *node;
 };
 
+struct rcu_graph_node {
+	struct bpf_rb_node node;
+	long data;
+};
+
+struct rcu_graph_node *just_here_because_btf_bug;
+
+struct map_value_rcu_graph {
+	struct rcu_graph_node __kptr *node;
+};
+
 extern void bpf_rcu_read_lock(void) __ksym;
 extern void bpf_rcu_read_unlock(void) __ksym;
 
@@ -36,6 +47,8 @@ private(A) struct bpf_spin_lock glock;
 private(A) struct bpf_rb_root groot __contains(node_acquire, node);
 private(B) struct bpf_spin_lock lock;
 private(B) struct bpf_list_head head __contains(node_refcounted, list);
+private(C) struct bpf_spin_lock graph_lock;
+private(C) struct bpf_rb_root graph_root __contains(rcu_graph_node, node);
 
 struct {
 	__uint(type, BPF_MAP_TYPE_ARRAY);
@@ -44,6 +57,13 @@ struct {
 	__uint(max_entries, 1);
 } stashed_refcount_only SEC(".maps");
 
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__type(key, int);
+	__type(value, struct map_value_rcu_graph);
+	__uint(max_entries, 1);
+} stashed_rcu_graph SEC(".maps");
+
 static bool less(struct bpf_rb_node *a, const struct bpf_rb_node *b)
 {
 	struct node_acquire *node_a;
@@ -137,6 +157,61 @@ long refcount_acquire_rcu_map_kptr_unchecked_drop(void *ctx)
 	return 0;
 }
 
+SEC("?syscall")
+__failure
+__msg("bpf_rbtree_remove can only take non-owning or refcounted "
+      "bpf_rb_node pointer")
+long rbtree_remove_after_rcu_unlock(void *ctx)
+{
+	struct map_value_rcu_graph *mapval;
+	struct bpf_rb_node *rb_node;
+	struct rcu_graph_node *node;
+	int idx = 0;
+
+	mapval = bpf_map_lookup_elem(&stashed_rcu_graph, &idx);
+	if (!mapval)
+		return 0;
+
+	bpf_rcu_read_lock();
+	node = mapval->node;
+	if (!node) {
+		bpf_rcu_read_unlock();
+		return 0;
+	}
+	bpf_rcu_read_unlock();
+
+	bpf_spin_lock(&graph_lock);
+	rb_node = bpf_rbtree_remove(&graph_root, &node->node);
+	bpf_spin_unlock(&graph_lock);
+	if (rb_node)
+		bpf_obj_drop(container_of(rb_node, struct rcu_graph_node, node));
+
+	return 0;
+}
+
+SEC("?syscall")
+__failure __msg("invalid mem access 'scalar'")
+long graph_kptr_after_spin_unlock(void *ctx)
+{
+	struct map_value_rcu_graph *mapval;
+	struct rcu_graph_node *node;
+	int idx = 0;
+
+	mapval = bpf_map_lookup_elem(&stashed_rcu_graph, &idx);
+	if (!mapval)
+		return 0;
+
+	bpf_spin_lock(&graph_lock);
+	node = mapval->node;
+	if (!node) {
+		bpf_spin_unlock(&graph_lock);
+		return 0;
+	}
+	bpf_spin_unlock(&graph_lock);
+
+	return node->data;
+}
+
 SEC("?tc")
 __failure __msg("Unreleased reference id=3 alloc_insn={{[0-9]+}}")
 long rbtree_refcounted_node_ref_escapes_owning_input(void *ctx)
-- 
2.53.0