CVE-2026-31411

Summary

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

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.

Affected Software

Weaknesses

References

• https://git.kernel.org/stable/c/c96549d07dfdd51aadf0722cfb40711574424840
• https://git.kernel.org/stable/c/1c8bda3df028d5e54134077dcd09f46ca8cfceb5
• https://git.kernel.org/stable/c/3e1a8b00095246a9a2b46b57f6d471c6d3c00ed2
• https://git.kernel.org/stable/c/e3f80666c2739296c3b69a127300455c43aa1067
• https://git.kernel.org/stable/c/21c303fec138c002f90ed33bce60e807d53072bb
• https://git.kernel.org/stable/c/69d3f9ee5489e6e8b66defcfa226e91d82393297
• https://git.kernel.org/stable/c/440c9a5fc477a8ee259d8bf669531250b8398651
• https://git.kernel.org/stable/c/ae88a5d2f29b69819dc7b04086734439d074a643

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