net: atm: fix crash due to unvalidated vcc pointer in sigd_send()

Reproducer available at [1].

The ATM send path (sendmsg -> vcc_sendmsg -> sigd_send) reads the vcc
pointer from msg->vcc and uses it directly without any validation. This
pointer comes from userspace via sendmsg() and can be arbitrarily forged:

    int fd = socket(AF_ATMSVC, SOCK_DGRAM, 0);
    ioctl(fd, ATMSIGD_CTRL);  // become ATM signaling daemon
    struct msghdr msg = { .msg_iov = &iov, ... };
    *(unsigned long *)(buf + 4) = 0xdeadbeef;  // fake vcc pointer
    sendmsg(fd, &msg, 0);  // kernel dereferences 0xdeadbeef

In normal operation, the kernel sends the vcc pointer to the signaling
daemon via sigd_enq() when processing operations like connect(), bind(),
or listen(). The daemon is expected to return the same pointer when
responding. However, a malicious daemon can send arbitrary pointer values.

Fix this by introducing find_get_vcc() which validates the pointer by
searching through vcc_hash (similar to how sigd_close() iterates over
all VCCs), and acquires a reference via sock_hold() if found.

Since struct atm_vcc embeds struct sock as its first member, they share
the same lifetime. Therefore using sock_hold/sock_put is sufficient to
keep the vcc alive while it is being used.

Note that there may be a race with sigd_close() which could mark the vcc
with various flags (e.g., ATM_VF_RELEASED) after find_get_vcc() returns.
However, sock_hold() guarantees the memory remains valid, so this race
only affects the logical state, not memory safety.

[1]: https://gist.github.com/mrpre/1ba5949c45529c511152e2f4c755b0f3
Fixes: 1da177e4c3 ("Linux-2.6.12-rc2")
Reported-by: syzbot+1f22cb1769f249df9fa0@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/69039850.a70a0220.5b2ed.005d.GAE@google.com/T/
Signed-off-by: Jiayuan Chen <jiayuan.chen@shopee.com>
Link: https://patch.msgid.link/20260205095501.131890-1-jiayuan.chen@linux.dev
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/atm/signaling.c

@@ -22,6 +22,36 @@
 
 struct atm_vcc *sigd = NULL;
 
+/*
+ * find_get_vcc - validate and get a reference to a vcc pointer
+ * @vcc: the vcc pointer to validate
+ *
+ * This function validates that @vcc points to a registered VCC in vcc_hash.
+ * If found, it increments the socket reference count and returns the vcc.
+ * The caller must call sock_put(sk_atm(vcc)) when done.
+ *
+ * Returns the vcc pointer if valid, NULL otherwise.
+ */
+static struct atm_vcc *find_get_vcc(struct atm_vcc *vcc)
+{
+	int i;
+
+	read_lock(&vcc_sklist_lock);
+	for (i = 0; i < VCC_HTABLE_SIZE; i++) {
+		struct sock *s;
+
+		sk_for_each(s, &vcc_hash[i]) {
+			if (atm_sk(s) == vcc) {
+				sock_hold(s);
+				read_unlock(&vcc_sklist_lock);
+				return vcc;
+			}
+		}
+	}
+	read_unlock(&vcc_sklist_lock);
+	return NULL;
+}
+
 static void sigd_put_skb(struct sk_buff *skb)
 {
 	if (!sigd) {
@@ -69,7 +99,14 @@ static int sigd_send(struct atm_vcc *vcc, struct sk_buff *skb)
 
 	msg = (struct atmsvc_msg *) skb->data;
 	WARN_ON(refcount_sub_and_test(skb->truesize, &sk_atm(vcc)->sk_wmem_alloc));
-	vcc = *(struct atm_vcc **) &msg->vcc;
+
+	vcc = find_get_vcc(*(struct atm_vcc **)&msg->vcc);
+	if (!vcc) {
+		pr_debug("invalid vcc pointer in msg\n");
+		dev_kfree_skb(skb);
+		return -EINVAL;
+	}
+
 	pr_debug("%d (0x%lx)\n", (int)msg->type, (unsigned long)vcc);
 	sk = sk_atm(vcc);
 
@@ -100,7 +137,16 @@ static int sigd_send(struct atm_vcc *vcc, struct sk_buff *skb)
 		clear_bit(ATM_VF_WAITING, &vcc->flags);
 		break;
 	case as_indicate:
-		vcc = *(struct atm_vcc **)&msg->listen_vcc;
+		/* Release the reference from msg->vcc, we'll use msg->listen_vcc instead */
+		sock_put(sk);
+
+		vcc = find_get_vcc(*(struct atm_vcc **)&msg->listen_vcc);
+		if (!vcc) {
+			pr_debug("invalid listen_vcc pointer in msg\n");
+			dev_kfree_skb(skb);
+			return -EINVAL;
+		}
+
 		sk = sk_atm(vcc);
 		pr_debug("as_indicate!!!\n");
 		lock_sock(sk);
@@ -115,6 +161,8 @@ static int sigd_send(struct atm_vcc *vcc, struct sk_buff *skb)
 		sk->sk_state_change(sk);
 as_indicate_complete:
 		release_sock(sk);
+		/* Paired with find_get_vcc(msg->listen_vcc) above */
+		sock_put(sk);
 		return 0;
 	case as_close:
 		set_bit(ATM_VF_RELEASED, &vcc->flags);
@@ -131,11 +179,15 @@ as_indicate_complete:
 		break;
 	default:
 		pr_alert("bad message type %d\n", (int)msg->type);
+		/* Paired with find_get_vcc(msg->vcc) above */
+		sock_put(sk);
 		return -EINVAL;
 	}
 	sk->sk_state_change(sk);
 out:
 	dev_kfree_skb(skb);
+	/* Paired with find_get_vcc(msg->vcc) above */
+	sock_put(sk);
 	return 0;
 }