CVE-2026-89775 KVM: arm64: Handle negative S1 walk levels in VNCR TLB size evaluation
A flaw in Linux kernel KVM arm64 TLB invalidation logic causes incorrect size calculations when the MMU is disabled, potentially leading to denial of service.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-89775?
CVE-2026-89775 is a critical denial of service vulnerability in Linux, published on 16 Sep 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
KVM: arm64: Handle negative S1 walk levels in VNCR TLB size evaluation
Computing the effects of a TLB invalidation involves looking at the size of the mapping cached by the TLB. For S1 mappings such as VNCR, this is deducted from the combination of the base granule size and the mapping level.
However, this implies that the S1 MMU is *on*. When the MMU is off, we indicate this with the level being set to a "creative" value of -127 (S1_MMU_DISABLED).
This ends-up being misinterpreted by pgshift_level_to_ttl() as it doesn't handle negative levels at all (the level is immediately cast to a u8 and only the bottom two bits considered), leading to an invalidation size of 0. Not helpful.
Tidy-up pgshift_level_to_ttl() to handle these negative levels, and ttl_to_size() to always return SZ_1G when no valid TTL is present. This allows the removal of open-coded checks for similar situations.
Note that the check for a negative value not explicitely checking for S1_MMU_DISABLED is deliberate, so that actual negative levels introduced with LVA2 and D128 can take the same path if we ever support them.
Which versions are affected by CVE-2026-89775?
How do I fix CVE-2026-89775?
Apply the vendor's patch.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-89775 being exploited?
There is no known exploitation of CVE-2026-89775 in the wild and no public exploit code.
Sources
Also known as BELL-CVE-2026-89775, DEBIAN-CVE-2026-89775, GHSA-4h2c-gw77-f478, UBUNTU-CVE-2026-89775.