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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>
This commit is contained in:
Jiayuan Chen 2026-02-05 17:54:51 +08:00 committed by Paolo Abeni
parent 6d2f142b1e
commit ae88a5d2f2
1 changed files with 54 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

56
net/atm/signaling.c
View File

@ -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;
}