[PATCH bpf] net/sched: cls_bpf: prevent unbounded recursion in offload rollback

[Jiayuan Chen <jiayuan.chen@linux.dev>]

Quan Sun reported [1] a stack overflow in cls_bpf_offload_cmd().

Reproducer on netdevsim: add a skip_sw cls_bpf filter, set the
bpf_tc_accept debugfs knob to 0, then `tc filter replace`. The replace
calls tc_setup_cb_replace() which fails. cls_bpf_offload_cmd() then
swaps prog/oldprog and recursively calls itself to roll back. But
bpf_tc_accept=0 makes the rollback fail too, which triggers yet another
rollback frame with the same arguments, and so on until the stack is
exhausted.

bpf_tc_accept is just a convenient knob for the reproducer. Any driver
whose tc_setup_cb_replace() fails twice in a row can hit the same loop,
so this is not a netdevsim-only issue.

Two ways to fix it:

  1) Have the rollback call tc_setup_cb_add() on oldprog instead of
     re-entering cls_bpf_offload_cmd().
  2) Mark the rollback frame with a flag and skip a second-level
     rollback from inside it.

Go with (2). It is the smaller change and keeps the original behaviour:
the rollback still goes through tc_setup_cb_replace(), so the driver
gets one real chance to restore its state. If that attempt also fails,
we just return the original error instead of recursing.

[1]: https://lore.kernel.org/bpf/ce5a6005-3c5e-4696-9e05-eba9461dc860@std.uestc.edu.cn/T/#u

Fixes: 102740bd9436 ("cls_bpf: fix offload assumptions after callback conversion")
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
 net/sched/cls_bpf.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/net/sched/cls_bpf.c b/net/sched/cls_bpf.c
index 9a346b6221b35..d71fbf7cb407e 100644
--- a/net/sched/cls_bpf.c
+++ b/net/sched/cls_bpf.c
@@ -142,7 +142,8 @@ static bool cls_bpf_is_ebpf(const struct cls_bpf_prog *prog)
 
 static int cls_bpf_offload_cmd(struct tcf_proto *tp, struct cls_bpf_prog *prog,
 			       struct cls_bpf_prog *oldprog,
-			       struct netlink_ext_ack *extack)
+			       struct netlink_ext_ack *extack,
+			       bool is_rollback)
 {
 	struct tcf_block *block = tp->chain->block;
 	struct tc_cls_bpf_offload cls_bpf = {};
@@ -176,8 +177,8 @@ static int cls_bpf_offload_cmd(struct tcf_proto *tp, struct cls_bpf_prog *prog,
 					  skip_sw, &oldprog->gen_flags,
 					  &oldprog->in_hw_count, true);
 
-	if (prog && err) {
-		cls_bpf_offload_cmd(tp, oldprog, prog, extack);
+	if (prog && err && !is_rollback) {
+		cls_bpf_offload_cmd(tp, oldprog, prog, extack, true);
 		return err;
 	}
 
@@ -208,7 +209,7 @@ static int cls_bpf_offload(struct tcf_proto *tp, struct cls_bpf_prog *prog,
 	if (!prog && !oldprog)
 		return 0;
 
-	return cls_bpf_offload_cmd(tp, prog, oldprog, extack);
+	return cls_bpf_offload_cmd(tp, prog, oldprog, extack, false);
 }
 
 static void cls_bpf_stop_offload(struct tcf_proto *tp,
@@ -217,7 +218,7 @@ static void cls_bpf_stop_offload(struct tcf_proto *tp,
 {
 	int err;
 
-	err = cls_bpf_offload_cmd(tp, NULL, prog, extack);
+	err = cls_bpf_offload_cmd(tp, NULL, prog, extack, false);
 	if (err)
 		pr_err("Stopping hardware offload failed: %d\n", err);
 }
-- 
2.43.0