Commit ae88a5d2 authored by Jiayuan Chen's avatar Jiayuan Chen Committed by Paolo Abeni
Browse files

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: 1da177e4 ("Linux-2.6.12-rc2")
Reported-by: default avatar <syzbot+1f22cb1769f249df9fa0@syzkaller.appspotmail.com>
Closes: https://lore.kernel.org/all/69039850.a70a0220.5b2ed.005d.GAE@google.com/T/


Signed-off-by: default avatarJiayuan Chen <jiayuan.chen@shopee.com>
Link: https://patch.msgid.link/20260205095501.131890-1-jiayuan.chen@linux.dev


Signed-off-by: default avatarPaolo Abeni <pabeni@redhat.com>
parent 6d2f142b

net/atm/signaling.c

+54 −2
Original line number Diff line number Diff line
@@ -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 @@ static int sigd_send(struct atm_vcc *vcc, struct sk_buff *skb) 
		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;

}