CVE-2026-31411: net: atm: fix crash due to unvalidated vcc pointer in sigd_send()

In the Linux kernel, the following vulnerability has been resolved:

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

Reproducer available at [1].

The ATM send path (sendmsg -> vccsendmsg -> sigdsend) 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(AFATMSVC, SOCKDGRAM, 0); ioctl(fd, ATMSIGDCTRL); // become ATM signaling daemon struct msghdr msg = { .msgiov = &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 sigdenq() 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 findgetvcc() which validates the pointer by searching through vcchash (similar to how sigdclose() iterates over all VCCs), and acquires a reference via sockhold() if found.

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

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

[1]: https://gist.github.com/mrpre/1ba5949c45529c511152e2f4c755b0f3