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CVE-2026-74356
vhost: fix vhost_get_avail_idx for a non empty ring
Published 2026-08-15 (2 months ago) by the Linux kernel CNA. Subsystem drivers/vhost. Debian urgency: not yet assigned.
Debian does not build this code. The per-release build detail is under "Where it lives" below.
What to do, per Debian release
Debian's own status for CVE-2026-74356 in every release this site tracks, and what each one means in practice. Every status below is the one the Debian Security Team publishes for this CVE. source
| Release | Status | Version | What this means |
|---|---|---|---|
| Debian 12bookworm | vulnerable | 6.1.190-1 | No fix published anywhere yet, neither in Debian nor upstream for the series bookworm tracks. Watch the Debian tracker page for this CVE. Advisories for bookworm have historically landed a median of 23 days after publication. |
| Debian 13trixie | fixed | 6.12.100-1 | Update the kernel and reboot. Fixed in 6.12.100-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 7.1.5-1 | Update the kernel and reboot. Fixed in 7.1.5-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.1.5-1 | Update the kernel and reboot. Fixed in 7.1.5-1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
vhost_get_avail_idx is supposed to report whether it has updated vq->avail_idx. Instead, it returns whether all entries have been consumed, which is usually the same. But not always - in drivers/vhost/net.c and when mergeable buffers have been enabled, the driver checks whether the combined entries are big enough to store an incoming packet. If not, the driver re-enables notifications with available entries still in the ring. The incorrect return value from vhost_get_avail_idx propagates through vhost_enable_notify and causes the host to livelock if the guest is not making progress, as vhost will immediately disable notifications and retry using the available entries. This goes back to commit d3bb267bbdcb ("vhost: cache avail index in vhost_enable_notify()") which changed vhost_enable_notify() to compare the freshly read avail index against vq->last_avail_idx instead of the previously cached vq->avail_idx. Commit 7ad472397667 ("vhost: move smp_rmb() into vhost_get_avail_idx()") then carried over the same comparison when refactoring vhost_enable_notify() to call the unified vhost_get_avail_idx(). The obvious fix is to make vhost_get_avail_idx do what the comment says it does and report whether new entries have been added.
Where it lives
The fix touches these files. If the subsystem is one you do not use, the practical exposure is lower, though the package is still the vulnerable one.
drivers/vhost/vhost.c
- What this is
- VIRTIO HOST (VHOST)MAINTAINERS section matched on drivers/vhost/ · kvm@vger.kernel.org · Maintained
- Kernel module
- none to look for. The file would have gone into vhost, and Debian does not compile it into that module, so finding vhost in lsmod would say nothing about this CVE.
Check it on the machine
grep -wE 'CONFIG_VHOST|CONFIG_VHOST_IOTLB|CONFIG_VHOST_RING' /boot/config-$(uname -r)Reads the booted kernel's own configuration, on the machine and nowhere else. =y is built in, =m is a module, and an is not set line means the code is not there.
Show whether Debian builds this, per release
Debian does not build this code. In 4 releases below, every kernel flavour Debian publishes has the switch off, so the vulnerable file is compiled into no kernel binary Debian ships.
- bookworm, trixie, forky, sidn
- n
- not enabled on any flavour - this release does not build the code
- Build switches
- CONFIG_VHOST, CONFIG_VHOST_IOTLB, CONFIG_VHOST_RING
- Reads as
- all on means the file is compiled; any one off means it is not.
From the configuration Debian publishes for every kernel flavour it builds. It covers only the architectures Debian ships a linux-config package for, and nothing about a kernel you built yourself. Debian's own status above stays authoritative about the source package.MAINTAINERS kbuild Makefiles Debian linux-config packages
Signals
- Published
- 2026-08-15 (2 months ago)source
- CVSS
- 7.4 high
CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:Hsource recompute it - Attack vector
- adjacent network (AV:A)source
- EPSS
- 0.23%, higher than 13.1% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- Subsystem
- drivers/vhost
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 5.18 | 6.12.97 | e11547100811 |
| 5.18 | 6.18.40 | 7f229d27bf27 |
| 5.18 | 7.1.5 | a9326b652bc7 |
| 5.18 | 7.2 | 09861858a683 |
Why the CNA scored it this way
Quoted from the kernel CNA's own scoring file, word for word. A highlighted metric carries the most severe value CVSS v3.1 defines for it, which is what pushes the score up. source
- AV:AAttack vector, adjacent network
- Exploitation requires a malicious VM guest driving the virtio/vhost-net RX virtqueue on a KVM host; inbound packets cross the adjacent virtual-network boundary into host vhost-net processing, matching kernel CNA precedent for guest-to-host vhost-net bugs.
- AC:LAttack complexity, low
- A guest can deterministically enable mergeable RX buffers, post undersized descriptors, stop refilling, and sustain inbound traffic so handle_rx() repeatedly hits the faulty vhost_enable_notify()/vhost_get_avail_idx() loop without races or rare configs.
- PR:NPrivileges required, none
- A cloud VM tenant or any attacker with code execution inside a vhost-net guest needs no host root, capabilities, or /dev/vhost-net access—only control of the guest virtio RX ring and normal VM networking, which QEMU/KVM provides by default.
- UI:NUser interaction, none
- No host administrator or victim user action is required beyond standard VM operation; the guest attacker directly manipulates virtio descriptors and can generate or receive the RX traffic that triggers host-side vhost processing.
- S:CScope, changed
- The livelock executes in host kernel vhost worker context while processing guest-controlled virtio ring state, crossing the VM/host isolation boundary and degrading availability for the hypervisor and co-hosted workloads beyond the guest's own scope.
- C:NConfidentiality impact, none
- This is a pure logic error in available-index comparison causing a notification retry loop; there is no memory corruption, out-of-bounds access, use-after-free, or other primitive that could disclose kernel memory.
- I:NIntegrity impact, none
- No kernel memory is corrupted or written beyond the virtio notification state machine; the bug cannot be leveraged for arbitrary write, control-flow hijacking, or code execution on the host.
- A:HAvailability impact, high
- The faulty vhost_enable_notify() loop causes host CPU livelock when the guest stops making progress, pinning a vhost worker at 100% and denying service to the host and other VMs until the offending guest is stopped or killed.