[PATCH bpf-next v1 3/3] bpf, arena: check range_tree_set return in arena_free_pages and arena_free_worker

[chenyuan_fl]

From: Yuan Chen <chenyuan@xxxxxxxxxx>

arena_free_pages() and arena_free_worker() ignore range_tree_set()
errors and proceed to clear PTEs and free pages, even though the range
tree still considers the range allocated.  The range then becomes
permanently unavailable.

Check the return value: in arena_free_pages() abort the free on error;
in arena_free_worker() move range_tree_set() before PTE clearing and
skip the span on error.

Fixes: b795379757eb ("bpf: Introduce range_tree data structure and use it in bpf arena")
Signed-off-by: Yuan Chen <chenyuan@xxxxxxxxxx>
---
 kernel/bpf/arena.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/kernel/bpf/arena.c b/kernel/bpf/arena.c
index 555ee2531ef9..30745da9277b 100644
--- a/kernel/bpf/arena.c
+++ b/kernel/bpf/arena.c
@@ -881,7 +881,12 @@ static void arena_free_pages(struct bpf_arena *arena, long uaddr, long page_cnt,
 	if (ret)
 		goto defer;
 
-	range_tree_set(&arena->rt, pgoff, page_cnt);
+	ret = range_tree_set(&arena->rt, pgoff, page_cnt);
+	if (ret) {
+		raw_res_spin_unlock_irqrestore(&arena->spinlock, flags);
+		bpf_map_memcg_exit(old_memcg, new_memcg);
+		return;
+	}
 
 	init_llist_head(&free_pages);
 	cdata.arena = arena;
@@ -1004,11 +1009,17 @@ static void arena_free_worker(struct work_struct *work)
 		kaddr = arena_vm_start + s->uaddr;
 		pgoff = compute_pgoff(arena, s->uaddr);
 
+		/* Mark the range as free before clearing PTEs so that
+		 * if the range tree update fails we leave the PTEs
+		 * intact, avoiding an unrecoverable state where pages
+		 * are freed but the arena free tree does not track them.
+		 */
+		if (range_tree_set(&arena->rt, pgoff, page_cnt))
+			continue;
+
 		/* clear ptes and collect pages in free_pages llist */
 		apply_to_existing_page_range(&init_mm, kaddr, page_cnt << PAGE_SHIFT,
 					     apply_range_clear_cb, &cdata);
-
-		range_tree_set(&arena->rt, pgoff, page_cnt);
 	}
 	raw_res_spin_unlock_irqrestore(&arena->spinlock, flags);
 
-- 
2.54.0