[PATCH bpf-next 0/2] bpf: Compare iterator types during state pruning

[PATCH bpf-next 0/2] bpf: Compare iterator types during state pruning

[Ning Ding]

Iterator stack slots can be marked MEM_RCU or PTR_UNTRUSTED. The
STACK_ITER check in stacksafe() does not compare this type, so state
pruning can treat these states as equal and prune an unsafe path.

Compare the type and add a test where RCU protection has a gap.

Ning Ding (2):
  bpf: Compare iterator types during state pruning
  selftests/bpf: Test RCU iterator state pruning

 kernel/bpf/states.c                           |  3 ++-
 .../selftests/bpf/progs/iters_task_failure.c  | 27 +++++++++++++++++++
 2 files changed, 29 insertions(+), 1 deletion(-)

-- 
2.43.0

[PATCH bpf-next 1/2] bpf: Compare iterator types during state pruning

[Ning Ding]

An iterator stack slot can be MEM_RCU or PTR_UNTRUSTED. These states
must not be equal, or the verifier can prune an unsafe path.

Compare the pointer type for STACK_ITER slots.

Fixes: dfab99df147b ("bpf: teach the verifier to enforce css_iter and task_iter in RCU CS")
Signed-off-by: Ning Ding <dingning04@gmail.com>
---
 kernel/bpf/states.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/states.c b/kernel/bpf/states.c
index ea2153cf28d0a..4e6aafad33bd2 100644
--- a/kernel/bpf/states.c
+++ b/kernel/bpf/states.c
@@ -812,7 +812,8 @@ static bool stacksafe(struct bpf_verifier_env *env, struct bpf_func_state *old,
 			 * infinite loop check triggering, see
 			 * iter_active_depths_differ()
 			 */
-			if (old_reg->iter.btf != cur_reg->iter.btf ||
+			if (old_reg->type != cur_reg->type ||
+			    old_reg->iter.btf != cur_reg->iter.btf ||
 			    old_reg->iter.btf_id != cur_reg->iter.btf_id ||
 			    old_reg->iter.state != cur_reg->iter.state ||
 			    /* ignore {old_reg,cur_reg}->iter.depth, see above */
-- 
2.43.0

[PATCH bpf-next 2/2] selftests/bpf: Test RCU iterator state pruning

[Ning Ding]

Add a path where RCU protection reaches zero and then starts again.
The iterator is untrusted after this gap and must be rejected.

Signed-off-by: Ning Ding <dingning04@gmail.com>
---
 .../selftests/bpf/progs/iters_task_failure.c  | 27 +++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/tools/testing/selftests/bpf/progs/iters_task_failure.c b/tools/testing/selftests/bpf/progs/iters_task_failure.c
index fe3663dedbe14..bac394ae77f44 100644
--- a/tools/testing/selftests/bpf/progs/iters_task_failure.c
+++ b/tools/testing/selftests/bpf/progs/iters_task_failure.c
@@ -61,6 +61,33 @@ int BPF_PROG(iter_tasks_lock_and_unlock)
 	return 0;
 }
 
+SEC("?fentry.s/" SYS_PREFIX "sys_getpgid")
+__failure __msg("expected an RCU CS when using bpf_iter_task_next")
+__flag(BPF_F_TEST_STATE_FREQ)
+int BPF_PROG(iter_tasks_rcu_state_pruning)
+{
+	struct bpf_iter_task it;
+
+	bpf_rcu_read_lock();
+	bpf_iter_task_new(&it, NULL, BPF_TASK_ITER_ALL_PROCS);
+
+	if (likely(bpf_get_prandom_u32())) {
+		/* Keep the outer RCU lock active. */
+		bpf_rcu_read_lock();
+		bpf_rcu_read_unlock();
+	} else {
+		/* Create an unprotected gap. */
+		bpf_rcu_read_unlock();
+		bpf_rcu_read_lock();
+	}
+
+	bpf_iter_task_next(&it);
+	bpf_iter_task_destroy(&it);
+	bpf_rcu_read_unlock();
+
+	return 0;
+}
+
 SEC("?fentry.s/" SYS_PREFIX "sys_getpgid")
 __failure __msg("expected an RCU CS when using bpf_iter_css_next")
 int BPF_PROG(iter_css_lock_and_unlock)
-- 
2.43.0