CVE-2025-21977

/// Technical description

In the Linux kernel, the following vulnerability has been resolved:

fbdev: hyperv_fb: Fix hang in kdump kernel when on Hyper-V Gen 2 VMs

Gen 2 Hyper-V VMs boot via EFI and have a standard EFI framebuffer

device. When the kdump kernel runs in such a VM, loading the efifb

driver may hang because of accessing the framebuffer at the wrong

memory address.

The scenario occurs when the hyperv_fb driver in the original kernel

moves the framebuffer to a different MMIO address because of conflicts

with an already-running efifb or simplefb driver. The hyperv_fb driver

then informs Hyper-V of the change, which is allowed by the Hyper-V FB

VMBus device protocol. However, when the kexec command loads the kdump

kernel into crash memory via the kexec_file_load() system call, the

system call doesn't know the framebuffer has moved, and it sets up the

kdump screen_info using the original framebuffer address. The transition

to the kdump kernel does not go through the Hyper-V host, so Hyper-V

does not r

/// Remediation

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

/// References