[PATCH bpf] bpf, sockmap: fix page_counter underflow in strparser SK_PASS

[Junseo Lim <zirajs7@gmail.com>]

tcp_bpf_strp_read_sock() delays cleanup of SK_PASS bytes by
subtracting psock->ingress_bytes from the amount passed to
__tcp_cleanup_rbuf(). But when sk_psock_verdict_apply() queues the skb
directly through sk_psock_skb_ingress_self(), skb_set_owner_r() is called
unconditionally and charges the skb again.

The duplicated charge is later released independently and can trigger a
page_counter underflow.

Add a charge_skb argument to sk_psock_skb_ingress_self() and skip
skb_set_owner_r() only for the direct strparser SK_PASS path. Keep existing
accounting for the other self-ingress caller and for non-strparser SK_PASS.

Fixes: 36b62df5683c ("bpf: Fix wrong copied_seq calculation")
Signed-off-by: Junseo Lim <zirajs7@gmail.com>
---
Crash reproduced on Linux tree 94515f3a7d4256a5062176b7d6ed0471938cd51a
with KASAN, MEMCG, panic_on_warn=1, and oops=panic.

Reproducer/log/config: https://gist.github.com/ZirAjs/16c95c89972ace73910d9b5ac78a5807

The reproducer drives the strparser SK_PASS path until teardown reports:

  page_counter underflow
  Workqueue: events sk_psock_destroy
  Kernel panic - not syncing: kernel: panic_on_warn set ...

 net/core/skmsg.c | 29 +++++++++++++++++------------
 1 file changed, 17 insertions(+), 12 deletions(-)

diff --git a/net/core/skmsg.c b/net/core/skmsg.c
index 2521b643fa05..17260681479a 100644
--- a/net/core/skmsg.c
+++ b/net/core/skmsg.c
@@ -586,7 +586,8 @@ static int sk_psock_skb_ingress_enqueue(struct sk_buff *skb,
 }
 
 static int sk_psock_skb_ingress_self(struct sk_psock *psock, struct sk_buff *skb,
-				     u32 off, u32 len, bool take_ref);
+				     u32 off, u32 len, bool take_ref,
+				     bool charge_skb);
 
 static int sk_psock_skb_ingress(struct sk_psock *psock, struct sk_buff *skb,
 				u32 off, u32 len)
@@ -595,12 +596,9 @@ static int sk_psock_skb_ingress(struct sk_psock *psock, struct sk_buff *skb,
 	struct sk_msg *msg;
 	int err;
 
-	/* If we are receiving on the same sock skb->sk is already assigned,
-	 * skip memory accounting and owner transition seeing it already set
-	 * correctly.
-	 */
 	if (unlikely(skb->sk == sk))
-		return sk_psock_skb_ingress_self(psock, skb, off, len, true);
+		return sk_psock_skb_ingress_self(psock, skb, off, len, true,
+						 true);
 	msg = sk_psock_create_ingress_msg(sk, skb);
 	if (!msg)
 		return -EAGAIN;
@@ -618,12 +616,14 @@ static int sk_psock_skb_ingress(struct sk_psock *psock, struct sk_buff *skb,
 	return err;
 }
 
-/* Puts an skb on the ingress queue of the socket already assigned to the
- * skb. In this case we do not need to check memory limits or skb_set_owner_r
- * because the skb is already accounted for here.
+/* Puts an skb on the ingress queue for psock->sk.
+ *
+ * When charge_skb is false, the direct strparser SK_PASS path keeps the TCP
+ * receive queue accounting in place and must not call skb_set_owner_r().
  */
 static int sk_psock_skb_ingress_self(struct sk_psock *psock, struct sk_buff *skb,
-				     u32 off, u32 len, bool take_ref)
+				     u32 off, u32 len, bool take_ref,
+				     bool charge_skb)
 {
 	struct sk_msg *msg = alloc_sk_msg(GFP_ATOMIC);
 	struct sock *sk = psock->sk;
@@ -631,7 +631,8 @@ static int sk_psock_skb_ingress_self(struct sk_psock *psock, struct sk_buff *skb
 
 	if (unlikely(!msg))
 		return -EAGAIN;
-	skb_set_owner_r(skb, sk);
+	if (charge_skb)
+		skb_set_owner_r(skb, sk);
 
 	/* This is used in tcp_bpf_recvmsg_parser() to determine whether the
 	 * data originates from the socket's own protocol stack. No need to
@@ -1017,6 +1018,8 @@ static int sk_psock_verdict_apply(struct sk_psock *psock, struct sk_buff *skb,
 		 * retrying later from workqueue.
 		 */
 		if (skb_queue_empty(&psock->ingress_skb)) {
+			bool charge_skb = true;
+
 			len = skb->len;
 			off = 0;
 			if (skb_bpf_strparser(skb)) {
@@ -1024,8 +1027,10 @@ static int sk_psock_verdict_apply(struct sk_psock *psock, struct sk_buff *skb,
 
 				off = stm->offset;
 				len = stm->full_len;
+				charge_skb = false;
 			}
-			err = sk_psock_skb_ingress_self(psock, skb, off, len, false);
+			err = sk_psock_skb_ingress_self(psock, skb, off, len,
+							false, charge_skb);
 		}
 		if (err < 0) {
 			spin_lock_bh(&psock->ingress_lock);
-- 
2.55.0