CVE-2022-48771 in Linux
Summary
by MITRE • 06/20/2024
In the Linux kernel, the following vulnerability has been resolved:
drm/vmwgfx: Fix stale file descriptors on failed usercopy
A failing usercopy of the fence_rep object will lead to a stale entry in the file descriptor table as put_unused_fd() won't release it. This enables userland to refer to a dangling 'file' object through that still valid file descriptor, leading to all kinds of use-after-free exploitation scenarios.
Fix this by deferring the call to fd_install() until after the usercopy has succeeded.
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Analysis
by VulDB Data Team • 01/07/2025
The vulnerability described in CVE-2022-48771 resides within the Linux kernel's graphics subsystem, specifically affecting the vmwgfx driver used for VMware graphics acceleration. This issue represents a critical flaw in the kernel's handling of file descriptor management during user copy operations, creating a persistent security risk that could be exploited by malicious userspace processes. The vulnerability manifests when the kernel attempts to copy data from user space to kernel space, specifically involving fence_rep objects that are part of the graphics driver's synchronization mechanisms.
The technical root cause of this vulnerability stems from improper ordering of operations within the kernel's file descriptor management system. When a usercopy operation fails during the processing of fence_rep objects, the kernel's cleanup mechanism fails to properly release the file descriptor that was allocated for this operation. This occurs because put_unused_fd() is called before the usercopy operation completes, leaving a stale file descriptor entry in the file descriptor table. The fundamental flaw lies in the sequence of function calls where fd_install() is invoked before the usercopy succeeds, creating a window where a dangling file object remains accessible through a valid file descriptor.
The operational impact of this vulnerability extends beyond simple resource management issues to create serious exploitation opportunities for privilege escalation attacks. Attackers can leverage this stale file descriptor to maintain access to a freed kernel object, enabling use-after-free exploitation patterns that can be used to execute arbitrary code with kernel privileges. This vulnerability directly maps to CWE-415, which describes improper cleanup of resources leading to use-after-free conditions, and aligns with ATT&CK technique T1068 by providing a path for privilege escalation through kernel memory corruption. The persistent nature of the stale file descriptor means that malicious processes can repeatedly access the dangling object, potentially allowing for multiple exploitation attempts or the construction of more sophisticated attack chains.
The fix implemented for this vulnerability addresses the core issue by reordering the sequence of operations to ensure that file descriptor installation occurs only after successful usercopy completion. This defensive programming approach prevents the creation of stale file descriptors by deferring fd_install() until after the usercopy operation has been validated as successful. This solution follows established kernel security practices for preventing use-after-free vulnerabilities and aligns with the principle of least privilege by ensuring that kernel resources are only made accessible when they are guaranteed to be valid and properly initialized. The resolution demonstrates proper error handling and resource management, preventing the conditions that would otherwise allow userland processes to maintain references to freed kernel objects and thereby compromising system security.