[PATCH net/bpf] bpf: add bpf_prog_run_data_pointers()

[Eric Dumazet <edumazet@google.com>]

syzbot found that cls_bpf_classify() is able to change
tc_skb_cb(skb)->drop_reason triggering a warning in sk_skb_reason_drop().

WARNING: CPU: 0 PID: 5965 at net/core/skbuff.c:1192 __sk_skb_reason_drop net/core/skbuff.c:1189 [inline]
WARNING: CPU: 0 PID: 5965 at net/core/skbuff.c:1192 sk_skb_reason_drop+0x76/0x170 net/core/skbuff.c:1214

struct tc_skb_cb has been added in commit ec624fe740b4 ("net/sched:
Extend qdisc control block with tc control block"), which added a wrong
interaction with db58ba459202 ("bpf: wire in data and data_end for
cls_act_bpf").

drop_reason was added later.

Add bpf_prog_run_data_pointers() helper to save/restore the net_sched
storage colliding with BPF data_meta/data_end.

Fixes: ec624fe740b4 ("net/sched: Extend qdisc control block with tc control block")
Reported-by: syzbot <syzkaller@googlegroups.com>
Closes: https://lore.kernel.org/netdev/6913437c.a70a0220.22f260.013b.GAE@google.com/
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Paul Blakey <paulb@nvidia.com>
---
 include/linux/filter.h | 20 ++++++++++++++++++++
 net/sched/act_bpf.c    |  7 +++----
 net/sched/cls_bpf.c    |  6 ++----
 3 files changed, 25 insertions(+), 8 deletions(-)

diff --git a/include/linux/filter.h b/include/linux/filter.h
index f5c859b8131a3e5fa5111b60cc291cedd44f096d..973233b82dc1fd422f26ac221eeb46c66c47767a 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -901,6 +901,26 @@ static inline void bpf_compute_data_pointers(struct sk_buff *skb)
 	cb->data_end  = skb->data + skb_headlen(skb);
 }
 
+static inline int bpf_prog_run_data_pointers(
+	const struct bpf_prog *prog,
+	struct sk_buff *skb)
+{
+	struct bpf_skb_data_end *cb = (struct bpf_skb_data_end *)skb->cb;
+	void *save_data_meta, *save_data_end;
+	int res;
+
+	save_data_meta = cb->data_meta;
+	save_data_end = cb->data_end;
+
+	bpf_compute_data_pointers(skb);
+	res = bpf_prog_run(prog, skb);
+
+	cb->data_meta = save_data_meta;
+	cb->data_end = save_data_end;
+
+	return res;
+}
+
 /* Similar to bpf_compute_data_pointers(), except that save orginal
  * data in cb->data and cb->meta_data for restore.
  */
diff --git a/net/sched/act_bpf.c b/net/sched/act_bpf.c
index 396b576390d00aad56bca6a18b7796e5324c0aef..3f5a5dc55c29433525b319f1307725d7feb015c6 100644
--- a/net/sched/act_bpf.c
+++ b/net/sched/act_bpf.c
@@ -47,13 +47,12 @@ TC_INDIRECT_SCOPE int tcf_bpf_act(struct sk_buff *skb,
 	filter = rcu_dereference(prog->filter);
 	if (at_ingress) {
 		__skb_push(skb, skb->mac_len);
-		bpf_compute_data_pointers(skb);
-		filter_res = bpf_prog_run(filter, skb);
+		filter_res = bpf_prog_run_data_pointers(filter, skb);
 		__skb_pull(skb, skb->mac_len);
 	} else {
-		bpf_compute_data_pointers(skb);
-		filter_res = bpf_prog_run(filter, skb);
+		filter_res = bpf_prog_run_data_pointers(filter, skb);
 	}
+
 	if (unlikely(!skb->tstamp && skb->tstamp_type))
 		skb->tstamp_type = SKB_CLOCK_REALTIME;
 	if (skb_sk_is_prefetched(skb) && filter_res != TC_ACT_OK)
diff --git a/net/sched/cls_bpf.c b/net/sched/cls_bpf.c
index 7fbe42f0e5c2b7aca0a28c34cd801c3a767c804e..a32754a2658bb7d21e8ceb62c67d6684ed4f9fcc 100644
--- a/net/sched/cls_bpf.c
+++ b/net/sched/cls_bpf.c
@@ -97,12 +97,10 @@ TC_INDIRECT_SCOPE int cls_bpf_classify(struct sk_buff *skb,
 		} else if (at_ingress) {
 			/* It is safe to push/pull even if skb_shared() */
 			__skb_push(skb, skb->mac_len);
-			bpf_compute_data_pointers(skb);
-			filter_res = bpf_prog_run(prog->filter, skb);
+			filter_res = bpf_prog_run_data_pointers(prog->filter, skb);
 			__skb_pull(skb, skb->mac_len);
 		} else {
-			bpf_compute_data_pointers(skb);
-			filter_res = bpf_prog_run(prog->filter, skb);
+			filter_res = bpf_prog_run_data_pointers(prog->filter, skb);
 		}
 		if (unlikely(!skb->tstamp && skb->tstamp_type))
 			skb->tstamp_type = SKB_CLOCK_REALTIME;
-- 
2.51.2.1041.gc1ab5b90ca-goog