CVE-2025-21977 in Linux
Summary
by MITRE • 04/01/2025
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---
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Analysis
by VulDB Data Team • 02/01/2026
The vulnerability CVE-2025-21977 affects the Linux kernel's handling of framebuffer devices in Hyper-V Generation 2 virtual machines, specifically when utilizing kdump for crash analysis. This issue arises from a race condition and improper state management during the transition from the primary kernel to a kdump kernel, leading to system hangs or severe performance degradation. The core problem lies in how the hyperv_fb driver manages framebuffer memory addresses, particularly when conflicts with other framebuffer drivers like efifb or simplefb occur. When a Gen 2 Hyper-V VM boots via EFI, it initializes with a standard EFI framebuffer device, but the hyperv_fb driver may relocate this framebuffer to a different MMIO address due to conflicts with already-running framebuffer drivers. This relocation is communicated to the Hyper-V host through the VMBus protocol, but the kdump kernel loading process fails to account for this change, resulting in a mismatch between the expected framebuffer address and the actual memory location. The issue stems from the kexec_file_load system call not being aware of the framebuffer relocation, causing it to initialize the kdump screen_info table with the original framebuffer address instead of the updated one. This mismatch triggers repeated memory access attempts to a non-existent address, which Hyper-V interprets as malicious behavior, leading to guest throttling or system hangs. The vulnerability is classified under CWE-129, which deals with improper validation of array indices, and relates to ATT&CK technique T1490, specifically "Inhibit System Recovery" through resource exhaustion or system degradation. The problem was first identified in 2020 and partially addressed with commit 3cb73bc3fa2a, which set orig_video_isVGA to 0 to prevent efifb from loading. However, this fix was effectively undone in 2024 by commit c25a19afb81c, which removed the screen_info clearing mechanism without considering the implications. The hyperv_drm driver avoids this issue because it removes conflicting framebuffers before allocating MMIO addresses, whereas hyperv_fb performs these actions in the reverse order, creating potential conflicts that force address relocation. This specific scenario demonstrates a classic case of improper state management in virtualized environments, where the transition between kernel states does not properly synchronize device state information. The fix involves reordering the steps in the hyperv_fb driver to ensure conflicting framebuffers are removed before MMIO address allocation, thereby maintaining a consistent framebuffer address throughout the system lifecycle. This approach aligns with established kernel development practices and the module's own usage guidelines, particularly the function aperture_remove_conflicting_devices(), which is already employed by hyperv_drm. The solution ensures that the kdump kernel always uses the original framebuffer address provided by Hyper-V, eliminating the possibility of address mismatches during crash analysis. This fix also resolves a related issue where kexec_load() could cause framebuffer address changes that might conflict with the kdump kernel's expectations, since the reordering ensures a stable address throughout the system's operation. The vulnerability represents a critical security concern in virtualized environments where system recovery mechanisms are essential for maintaining availability and reliability. The problem affects Linux systems running on Hyper-V Gen 2 VMs, particularly those configured for crash analysis using kdump, and could potentially be exploited to cause denial-of-service conditions through resource exhaustion or system degradation. The fix ensures proper synchronization between the primary kernel and kdump kernel regarding framebuffer state, thereby maintaining system stability and preventing malicious behavior detection by the hypervisor. This vulnerability highlights the importance of careful state management in virtualized environments and the need for comprehensive testing of kernel transitions, especially in systems where multiple drivers interact with shared hardware resources. The solution's approach of removing conflicts before allocation aligns with established best practices for device driver development and resource management in kernel space.