CVE-2020-36788 in Linux
Summary
by MITRE • 05/21/2024
In the Linux kernel, the following vulnerability has been resolved:
drm/nouveau: avoid a use-after-free when BO init fails
nouveau_bo_init() is backed by ttm_bo_init() and ferries its return code back to the caller. On failures, ttm_bo_init() invokes the provided destructor which should de-initialize and free the memory.
Thus, when nouveau_bo_init() returns an error the gem object has already been released and the memory freed by nouveau_bo_del_ttm().
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Analysis
by VulDB Data Team • 12/12/2024
The vulnerability described in CVE-2020-36788 represents a critical use-after-free condition within the nouveau graphics driver component of the Linux kernel. This flaw exists in the direct rendering manager subsystem where the nouveau driver handles buffer object initialization. The issue manifests when the nouveau_bo_init() function encounters a failure during the initialization process, creating a scenario where memory is freed before subsequent operations attempt to access it. The root cause stems from the interaction between nouveau_bo_init() and ttm_bo_init() functions, where the underlying TTM (Tiling and Memory) subsystem's destructor is invoked upon initialization failure, leading to premature memory deallocation.
The technical implementation of this vulnerability involves the improper handling of memory lifecycle management within the graphics driver's buffer object management system. When nouveau_bo_init() fails to initialize a buffer object, it relies on the TTM subsystem to clean up resources through its destructor mechanism. However, the current implementation does not properly account for the fact that the destructor function nouveau_bo_del_ttm() has already been executed, resulting in the freeing of memory that subsequent code paths may still attempt to reference. This creates a classic use-after-free scenario where an attacker could potentially exploit the freed memory location for arbitrary code execution or system instability. The vulnerability specifically affects the nouveau graphics driver's implementation of the drm/nouveau subsystem and operates at the kernel level where privilege escalation opportunities exist.
The operational impact of CVE-2020-36788 extends beyond simple system instability to potentially enable privilege escalation attacks within the Linux kernel environment. When exploited, this vulnerability could allow a malicious user to execute arbitrary code with kernel privileges, effectively compromising the entire system. The attack surface is primarily limited to systems running the nouveau graphics driver and utilizing the direct rendering manager subsystem for graphics operations. The vulnerability's exploitation requires specific conditions where buffer object initialization fails, making it somewhat situational but still dangerous given the kernel-level privileges involved. This flaw directly relates to CWE-416 which describes the use of freed memory vulnerability, and can be categorized under ATT&CK technique T1068 for bypassing user execution restrictions through kernel exploits.
Mitigation strategies for this vulnerability focus on both immediate patch application and system hardening measures. The primary and most effective solution involves applying the official kernel patch that resolves the memory management issue by ensuring proper handling of the destructor invocation sequence. System administrators should prioritize updating their Linux kernel installations to versions containing the fix, particularly in production environments where graphics-intensive operations are common. Additional defensive measures include monitoring for unusual graphics driver behavior, implementing proper access controls to limit graphics driver usage, and maintaining regular system updates to prevent exploitation of similar vulnerabilities. Organizations should also consider implementing kernel module signing and integrity checking mechanisms to prevent unauthorized modifications that could exacerbate the vulnerability's impact. The fix specifically addresses the improper memory management pattern by ensuring that error paths properly handle the state transitions between buffer object initialization and cleanup operations, preventing the premature freeing of memory resources that leads to the use-after-free condition.