# CVE-2025-21977: fbdev: hyperv_fb: Fix hang in kdump kernel when on Hyper-V Gen 2 VMs > A flaw in the Linux kernel hyperv_fb driver causes a hang in kdump kernels on Hyper-V Gen 2 VMs due to incorrect framebuffer address handling. - **Severity:** Medium 5.5 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H`) - **Exploited in the wild:** Not known - **Login needed:** Any signed-in user - **Attackable:** Local or same network only - **Victim action needed:** Not needed - **Fix available:** Yes - **Published:** 1 Apr 2025 - **Updated:** 11 May 2026 - **Also known as:** BELL-CVE-2025-21977, DEBIAN-CVE-2025-21977, GHSA-fcxj-8496-8247, ROOT-OS-UBUNTU-2404-CVE-2025-21977, UBUNTU-CVE-2025-21977 ## What is CVE-2025-21977? CVE-2025-21977 is a medium-severity denial of service vulnerability in Linux kernel, published on 1 Apr 2025. 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: 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 reset the framebuffer address like it would do on a reboot. When efifb tries to run, it accesses a non-existent framebuffer address, which traps to the Hyper-V host. After many such accesses, the Hyper-V host thinks the guest is being malicious, and throttles the guest to the point that it runs very slowly or appears to have hung. When the kdump kernel is loaded into crash memory via the kexec_load() system call, the problem does not occur. In this case, the kexec command builds the screen_info table itself in user space from data returned by the FBIOGET_FSCREENINFO ioctl against /dev/fb0, which gives it the new framebuffer location. This problem was originally reported in 2020 [1], resulting in commit 3cb73bc3fa2a ("hyperv_fb: Update screen_info after removing old framebuffer"). This commit solved the problem by setting orig_video_isVGA to 0, so the kdump kernel was unaware of the EFI framebuffer. The efifb driver did not try to load, and no hang occurred. But in 2024, commit c25a19afb81c ("fbdev/hyperv_fb: Do not clear global screen_info") effectively reverted 3cb73bc3fa2a. Commit c25a19afb81c has no reference to 3cb73bc3fa2a, so perhaps it was done without knowing the implications that were reported with 3cb73bc3fa2a. In any case, as of commit c25a19afb81c, the original problem came back again. Interestingly, the hyperv_drm driver does not have this problem because it never moves the framebuffer. The difference is that the hyperv_drm driver removes any conflicting framebuffers *before* allocating an MMIO address, while the hyperv_fb drivers removes conflicting framebuffers *after* allocating an MMIO address. With the "after" ordering, hyperv_fb may encounter a conflict and move the framebuffer to a different MMIO address. But the conflict is essentially bogus because it is removed a few lines of code later. Rather than fix the problem with the approach from 2020 in commit 3cb73bc3fa2a, instead slightly reorder the steps in hyperv_fb so conflicting framebuffers are removed before allocating an MMIO address. Then the default framebuffer MMIO address should always be available, and there's never any confusion about which framebuffer address the kdump kernel should use -- it's always the original address provided by the Hyper-V host. This approach is already used by the hyperv_drm driver, and is consistent with the usage guidelines at the head of the module with the function aperture_remove_conflicting_devices(). This approach also solves a related minor problem when kexec_load() is used to load the kdump kernel. With current code, unbinding and rebinding the hyperv_fb driver could result in the framebuffer moving back to the default framebuffer address, because on the rebind there are no conflicts. If such a move is done after the kdump kernel is loaded with the new framebuffer address, at kdump time it could again have the wrong address. This problem and fix are described in terms of the kdump kernel, but it can also occur ---truncated--- ## Which versions are affected by CVE-2025-21977? It affects 1 product. Products: Linux kernel. ## How do I fix CVE-2025-21977? Apply the vendor's patch. ## Is CVE-2025-21977 being exploited? There is no known exploitation of CVE-2025-21977 in the wild and no public exploit code. ## Other vulnerabilities in Linux kernel - [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild) - [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild) - [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild) - [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild) - [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild) ## Fix prompt for AI coding agents ```text Check this codebase for CVE-2025-21977 and fix it if it is affected. What it is: CVE-2025-21977 is a medium-severity denial of service vulnerability in Linux kernel, published on 1 Apr 2025. It can only be exploited from the same machine or local network, with no action from the victim. Exploitation: There is no known exploitation of CVE-2025-21977 in the wild and no public exploit code. Affected product: Linux kernel. No package-level version data is available, so check the vendor advisory linked from the reference page. Task: 1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies. 2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it. 3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead. 4. Run the tests and summarise what changed and why. 5. If nothing in the project is affected, say so and change nothing. Reference: https://insights.sbomify.com/CVE-2025-21977/ (Markdown: https://insights.sbomify.com/CVE-2025-21977/index.md) ``` ## Sources - CVE record: https://www.cve.org/CVERecord?id=CVE-2025-21977 - NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-21977 - OSV: https://osv.dev/vulnerability/CVE-2025-21977 --- From sbomify Insights: https://insights.sbomify.com/CVE-2025-21977/ Summaries are generated and may contain mistakes; the official description above is the record.