CVE-2019-19927 in Linux
Summary
by MITRE
In the Linux kernel 5.0.0-rc7 (as distributed in ubuntu/linux.git on kernel.ubuntu.com), mounting a crafted f2fs filesystem image and performing some operations can lead to slab-out-of-bounds read access in ttm_put_pages in drivers/gpu/drm/ttm/ttm_page_alloc.c. This is related to the vmwgfx or ttm module.
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Analysis
by VulDB Data Team • 03/18/2024
The vulnerability CVE-2019-19927 represents a critical memory safety issue within the Linux kernel's graphics subsystem that affects versions including 5.0.0-rc7. This flaw exists in the ttm (Translation Table Manager) module which is part of the DRM (Direct Rendering Manager) framework responsible for managing GPU memory allocations. The vulnerability specifically manifests when a maliciously crafted f2fs filesystem image is mounted and certain operations are performed, creating a scenario where out-of-bounds memory access can occur. The technical implementation involves the ttm_put_pages function located in drivers/gpu/drm/ttm/ttm_page_alloc.c, which fails to properly validate memory boundaries during page deallocation operations. This issue directly impacts the vmwgfx virtual machine graphics driver that relies on the ttm subsystem for memory management, creating a potential attack vector through filesystem manipulation that could be exploited by local or unprivileged users.
The core technical flaw stems from improper bounds checking in the memory management routines of the ttm subsystem, specifically when handling page deallocation operations. When a crafted f2fs filesystem is mounted, the kernel's memory allocator processes the filesystem metadata in a way that causes the ttm_put_pages function to access memory locations beyond the allocated slab boundaries. This slab-out-of-bounds read vulnerability allows an attacker to potentially read sensitive kernel memory contents, which could expose kernel stack data, heap contents, or other critical system information. The vulnerability is categorized under CWE-125 as an out-of-bounds read, and its exploitation can lead to information disclosure, privilege escalation, or system instability. The attack requires mounting a specifically crafted filesystem image which makes this vulnerability somewhat more constrained but still potentially dangerous in environments where untrusted filesystems might be mounted.
The operational impact of this vulnerability extends beyond simple information disclosure, as it can potentially enable more sophisticated attacks within the kernel context. An attacker who successfully exploits this vulnerability could gain access to kernel memory contents that might reveal sensitive information such as cryptographic keys, memory layout details, or other confidential data. The vulnerability affects systems running Linux kernel versions up to 5.0.0-rc7, particularly those utilizing the vmwgfx driver in virtualized environments or systems with GPU acceleration. This issue poses significant risks in cloud environments where untrusted users might mount malicious filesystems, or in scenarios where users have the ability to mount filesystems with elevated privileges. The vulnerability can be exploited through local privilege escalation or by manipulating filesystem mounting operations in virtualized environments, making it particularly concerning for server and cloud deployments where multiple users or untrusted code might interact with the system.
Mitigation strategies for CVE-2019-19927 should focus on both immediate patching and operational security measures. The primary solution involves updating to a patched kernel version that contains the fix for the ttm subsystem memory management routines, specifically addressing the slab-out-of-bounds read condition in ttm_put_pages. System administrators should ensure that all systems running affected kernel versions are updated promptly, particularly in environments where untrusted filesystems might be mounted. Additional mitigations include implementing strict filesystem mounting policies that prevent untrusted users from mounting arbitrary filesystem images, disabling unnecessary GPU drivers in environments where they are not required, and implementing monitoring for unusual filesystem mounting activities. The vulnerability demonstrates the importance of proper bounds checking in kernel memory management subsystems and highlights the need for comprehensive testing of filesystem drivers and memory allocators in graphics subsystems. Organizations should also consider implementing kernel hardening measures such as stack canaries, memory protection mechanisms, and runtime monitoring to detect potential exploitation attempts, as this vulnerability aligns with ATT&CK technique T1068 which covers privilege escalation through kernel exploits.