[PATCH bpf-next 1/2] bpf: Look at frame 0 when telling async callback entries apart

[Puranjay Mohan <puranjay@kernel.org>]

is_state_visited() skips the infinite loop check when two states differ
in async_entry_cnt, because seeing the same state on a second entry into
an async callback is not a loop. It reads that from the innermost frame,
but push_async_cb() sets in_async_callback_fn and async_entry_cnt on the
callback's own frame, and setup_func_entry() copies neither, so a subprog
called by the callback carries neither.

A callback which re-arms itself and calls a subprog therefore compares
the two entries at a loop inside that subprog, where the innermost frame
is the subprog's, and the state is rejected:

  infinite loop detected at insn 60

Read the flag and the count from frame 0, which push_async_cb() makes the
callback's frame. A loop within a single entry still has a matching count
and is still caught, and frame 0 of a non-async state does not have the
flag set, so nothing else changes.

Fixes: bfc6bb74e4f1 ("bpf: Implement verifier support for validation of async callbacks.")
Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
---
 kernel/bpf/states.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/states.c b/kernel/bpf/states.c
index 66fb11b6c6a76..32e141aa6a117 100644
--- a/kernel/bpf/states.c
+++ b/kernel/bpf/states.c
@@ -1273,10 +1273,10 @@ int bpf_is_state_visited(struct bpf_verifier_env *env, int insn_idx)
 			continue;
 
 		if (sl->state.branches) {
-			struct bpf_func_state *frame = sl->state.frame[sl->state.curframe];
+			struct bpf_func_state *frame = sl->state.frame[0];
 
 			if (frame->in_async_callback_fn &&
-			    frame->async_entry_cnt != cur->frame[cur->curframe]->async_entry_cnt) {
+			    frame->async_entry_cnt != cur->frame[0]->async_entry_cnt) {
 				/* Different async_entry_cnt means that the verifier is
 				 * processing another entry into async callback.
 				 * Seeing the same state is not an indication of infinite
-- 
2.53.0-Meta