Kata guest escape: runtime-rs guest-root to host-root escape via virtiofs

Summary

In the runtime-rs standalone virtio-fs path, verified here with QEMU (and verified with Cloud Hypervisor too), Kata Containers runs host virtiofsd as root with:

--sandbox none --seccomp none

If an attacker has root-equivalent execution inside the Kata guest VM, they can send raw FUSE requests directly to the host virtiofsd. With the tested runtime-rs virtio-fs configuration, a raw FUSE_SYMLINK request whose new symlink name is an absolute host path is honored outside the virtio-fs shared directory.

This lets guest root create host-root owned symlinks in sensitive host paths. The PoC created here will create symlinks in the host /etc/cron.d directory, causing host cron to execute a guest-controlled payload as host root.

Impact: guest root can execute code as host root.

Affected configuration

The verified host used:

/opt/kata/share/defaults/kata-containers/runtime-rs/configuration-qemu-runtime-rs.toml

rootless = false
shared_fs = "virtio-fs"
virtio_fs_daemon = "/opt/kata/libexec/virtiofsd"
hypervisor_name = "qemu"
debug_console_enabled = false

Pinned upstream references, using Kata Containers main commit 2ffd1538a296cff93a357bfba0dfca747480a1f8:

• runtime-rs standalone virtio-fs adds --sandbox none --seccomp none to the virtiofsd command line.
• runtime-rs QEMU leaves rootless mode disabled by default: rootless = false.
• The QEMU runtime-rs config template generates an installed config that uses standalone virtio-fs and points runtime-rs at the host virtiofsd binary: shared_fs and virtio_fs_daemon.
• The runtime-rs Makefile resolves those placeholders to virtio-fs and $(LIBEXECDIR)/virtiofsd.
• runtime-rs selects the same standalone virtio-fs implementation whenever shared_fs = "virtio-fs": ShareVirtioFsStandalone.

Details

The guest kernel normally owns the virtio-fs client. A normal guest process will use filesystem syscalls, and the guest kernel will validate the paths, and only then does the kernel send FUSE messages to the host backend.

An attacker with root-equivalent access inside the guest can bypass that guest virtio-fs client. They can access the virtio-fs PCI device, mmap the virtio PCI BAR, recover guest physical addresses from /proc/self/pagemap, and build their own virtqueue from userspace. That queue can submit attacker-built FUSE messages directly to host virtiofsd.

The relevant primitive is FUSE_SYMLINK. An attacker can send a request whose body contains:

new symlink name: /etc/cron.d/kata-go-escape-cron-<pid>
symlink target: /proc/<pid>/root/run/kata-containers/shared/sandboxes/<sid>/ro/passthrough/<sid>/rootfs/tmp/kata-go-escape-payload

The new symlink name is an absolute host path. virtiofsd should reject that request or force it to resolve below the configured --shared-dir. In the tested runtime-rs path, host-root unsandboxed virtiofsd accepts the absolute name, creating a real host symlink under /etc/cron.d.

The attacker can make the symlink target resolve through /proc/<pid>/root/... for a live Kata runtime process whose mount namespace can see the guest-created payload. One matching runtime PID is enough.

When the host cron reads /etc/cron.d, it follows the root-owned symlink, loads the guest-created crontab payload, and executes it as host root.

PoC

sudo timeout --foreground --kill-after=10s 600s ctr run --rm \
  --runtime /opt/kata/runtime-rs/bin/containerd-shim-kata-v2 \
  --runtime-config-path /opt/kata/share/defaults/kata-containers/runtime-rs/configuration-qemu-runtime-rs.toml \
  --privileged \
  --privileged-without-host-devices \
  docker.io/library/kata-go-escape:local \
  "$run_id"

The container is privileged only to model the post-escape condition where the attacker already has guest-root capabilities. It is not the vulnerability by itself.

Inside the guest, the PoC:

1. Writes a cron payload to guest /tmp/kata-go-escape-payload.
2. Finds the virtio-fs PCI device in guest /sys.
3. Takes over a virtio-fs queue from userspace.
4. Sends FUSE_INIT.
5. Discovers the current runtime-rs sandbox under passthrough/.
6. Looks up passthrough/<sid>/rootfs/tmp/kata-go-escape-payload.
7. Sends raw FUSE_SYMLINK requests where the new symlink names are absolute host paths under /etc/cron.d.
8. Keeps the guest alive while host cron scans.

Example log lines:

[guest] virtio-fs PCI device: /sys/devices/pci0000:00/0000:00:05.0
[res] sandbox_id=kata-go-escape-test-1778522686-1539
[res] lookup_path_error=0 path=passthrough/kata-go-escape-test-1778522686-1539/rootfs/tmp/kata-go-escape-payload nodeid=21
[spray] pid=1 err=-2 created_candidates=1

err=-2 is expected for the symlink spray. virtiofsd can return ENOENT after the side effect because its follow-up lookup is still relative to the export root. The host symlink creation has already happened.

Impact

The PoC proves guest-root to host-root command execution.

Verified host proof:

/run/kata-go-escape.proof

uid=0(root) gid=0(root) groups=0(root)
Mon May 11 18:05:01 UTC 2026

The proof file is written in host /run by host cron. It is not written by the guest process and not written by virtiofsd.

An attacker who reaches guest root can therefore cross the Kata isolation boundary and execute commands as host root on affected runtime-rs virtio-fs deployments.