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](https://github.com/kata-containers/kata-containers/blob/2ffd1538a296cff93a357bfba0dfca747480a1f8/src/runtime-rs/crates/resource/src/share_fs/share_virtio_fs_standalone.rs#L82-L92) to the virtiofsd command line. - runtime-rs QEMU leaves rootless mode disabled by default: [
rootless = false](https://github.com/kata-containers/kata-containers/blob/2ffd1538a296cff93a357bfba0dfca747480a1f8/src/runtime-rs/config/configuration-qemu-runtime-rs.toml.in#L31-L34). - 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](https://github.com/kata-containers/kata-containers/blob/2ffd1538a296cff93a357bfba0dfca747480a1f8/src/runtime-rs/config/configuration-qemu-runtime-rs.toml.in#L164-L171). - The runtime-rs Makefile resolves those placeholders to [
virtio-fs](https://github.com/kata-containers/kata-containers/blob/2ffd1538a296cff93a357bfba0dfca747480a1f8/src/runtime-rs/Makefile#L496-L499) and [$(LIBEXECDIR)/virtiofsd](https://github.com/kata-containers/kata-containers/blob/2ffd1538a296cff93a357bfba0dfca747480a1f8/src/runtime-rs/Makefile#L184-L190). - runtime-rs selects the same standalone virtio-fs implementation whenever
shared_fs = "virtio-fs": [ShareVirtioFsStandalone](https://github.com/kata-containers/kata-containers/blob/2ffd1538a296cff93a357bfba0dfca747480a1f8/src/runtime-rs/crates/resource/src/share_fs/mod.rs#L158-L167).
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
```shell
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. Fin
