Описание
Kata Container to Guest micro VM privilege escalation
Summary
An issue in Kata with Cloud Hypervisor allows a user of the container to modify the file system used by the Guest micro VM ultimately achieving arbitrary code execution as root in said VM. The current understinding is this doesn’t impact the security of the Host or of other containers / VMs running on that Host (note that arm64 QEMU lacks NVDIMM read-only support: It is believed that until the upstream QEMU gains this capability, a guest write could reach the image file).
Details
Linux virtio-pmem
The virtio-pmem probe path always registers the region as a generic pagemap that supports asynchronous flushes, but it never marks the region as read-only. Only the ND_REGION_PAGEMAP and ND_REGION_ASYNC bits are set before the region is created, so nd_region->ro always stays cleared and the block device is left writable.
Later, pmem_attach_disk() wires the region into the block layer with full read/write semantics – the block device operations call pmem_do_write() which performs cache-flushed memcpy operations directly into the host-provided shared memory window. nvdimm_check_and_set_ro() would set the disk read-only if the region had been flagged as such, but because virtio_pmem never sets that flag, the helper becomes a no-op.
Cloud-Hypervisor virtio_pmem
discard_writes=on causes the file backing the virtio-pmem device to be opened read-only and mapped with MAP_PRIVATE rather than MAP_SHARED. That combination means the guest can modify the private copy of the mapped pages, but those modifications never propagate back to the underlying file. The guest (and Cloud Hypervisor process) will still read the modified data because it lives in the private copy of the mapping, so write-then-read sequences appear to succeed even though nothing is persisted. Once the mapping is dropped or the VM is restarted, those copy-on-write changes disappear, leaving the backing file unchanged.
Kata /dev/pmem0
Kata boots each pod/VM by DAX-mapping a read-only guest image from the host into the VM and telling the guest kernel to mount the resulting /dev/pmem* device as its root filesystem.
Since DAX maps the backing file directly into guest memory, there is no way for the hypervisor to intercept or reject individual stores, so a container with sufficient permissions can open /dev/pmem0 and observe its own writes until the VM is rebooted or the cache is dropped.
PoC
When putting all this together, this means that a user of a Container (not necessarily privileged, we don’t need CAP_SYS_ADMIN, but we need CAP_MKNOD) can modify the Guest OS filesystem, replacing libraries or binaries to achieve arbitrary code execution outside of the Container. This requires computing offsets of files within the device, which requires information like the partition start sector, sector size in bytes, the filesystem block size, and the physical block index of the file.
To achieve execution on the Guest, I replaced /usr/bin/systemd-tmpfiles with a connect-back shell to localhost: timers end up executing 15min after boot.
I use debugfs to not require mounting privileges and work directly with the filesystem on /dev/pmem0p1 to get the absolute offset of the file to modify in the device.
If you want a simpler PoC, just dd write something into /dev/pmem0 and observe it's dd readable until discarded.
Impact
Container to Guest micro VM Escape (no escape to Host, no persistence of the overwritten image)
Ссылки
- https://github.com/kata-containers/kata-containers/security/advisories/GHSA-wwj6-vghv-5p64
- https://nvd.nist.gov/vuln/detail/CVE-2026-24834
- https://github.com/kata-containers/kata-containers/commit/6a672503973bf7c687053e459bfff8a9652e16bf
- https://github.com/kata-containers/kata-containers/releases/tag/3.27.0
Пакеты
github.com/kata-containers/kata-containers/src/runtime
< 0.0.0-20260219090056-6a672503973b
0.0.0-20260219090056-6a672503973b
Связанные уязвимости
Kata Containers is an open source project focusing on a standard implementation of lightweight Virtual Machines (VMs) that perform like containers. In versions prior to 3.27.0, an issue in Kata with Cloud Hypervisor allows a user of the container to modify the file system used by the Guest micro VM ultimately achieving arbitrary code execution as root in said VM. The current understanding is this doesn’t impact the security of the Host or of other containers / VMs running on that Host (note that arm64 QEMU lacks NVDIMM read-only support: It is believed that until the upstream QEMU gains this capability, a guest write could reach the image file). Version 3.27.0 patches the issue.
Kata Containers is an open source project focusing on a standard implementation of lightweight Virtual Machines (VMs) that perform like containers. In versions prior to 3.27.0, an issue in Kata with Cloud Hypervisor allows a user of the container to modify the file system used by the Guest micro VM ultimately achieving arbitrary code execution as root in said VM. The current understanding is this doesn’t impact the security of the Host or of other containers / VMs running on that Host (note that arm64 QEMU lacks NVDIMM read-only support: It is believed that until the upstream QEMU gains this capability, a guest write could reach the image file). Version 3.27.0 patches the issue.
