CVE-2019-15792 in Linux
Summary
by MITRE
In shiftfs, a non-upstream patch to the Linux kernel included in the Ubuntu 5.0 and 5.3 kernel series, shiftfs_btrfs_ioctl_fd_replace() calls fdget(oldfd), then without further checks passes the resulting file* into shiftfs_real_fdget(), which casts file->private_data, a void* that points to a filesystem-dependent type, to a "struct shiftfs_file_info *". As the private_data is not required to be a pointer, an attacker can use this to cause a denial of service or possibly execute arbitrary code.
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Analysis
by VulDB Data Team • 12/23/2024
The vulnerability described in CVE-2019-15792 resides within the shiftfs filesystem implementation in the Linux kernel, specifically affecting Ubuntu kernel versions 5.0 and 5.3. This issue stems from a non-upstream patch that introduces a critical flaw in the shiftfs_btrfs_ioctl_fd_replace() function. The vulnerability manifests through improper validation of file descriptor references during kernel operations, creating a potential pathway for both denial of service and arbitrary code execution attacks. The flaw is categorized under CWE-125 as an out-of-bounds read, though its exploitation extends beyond simple memory access violations.
The technical implementation of this vulnerability occurs in the shiftfs filesystem where the function shiftfs_btrfs_ioctl_fd_replace() performs a sequence of operations that bypass essential validation checks. When the function calls fdget(oldfd), it retrieves a file descriptor reference that should undergo rigorous validation before being processed. However, the subsequent call to shiftfs_real_fdget() directly casts the file->private_data field without verifying its type or contents. This casting operation assumes that private_data points to a struct shiftfs_file_info *, but the kernel documentation explicitly states that private_data is a filesystem-dependent type that may not conform to this expected structure.
The operational impact of this vulnerability extends across multiple attack vectors within the Linux kernel security model. An attacker with access to a process running under kernel privileges can exploit this flaw to trigger a kernel memory corruption condition through a carefully crafted file descriptor manipulation. The potential for privilege escalation exists because the vulnerability allows for arbitrary code execution within kernel space, which directly violates the fundamental security principles of kernel isolation. This vulnerability can be mapped to ATT&CK technique T1068, which covers "Exploitation for Privilege Escalation," as the flaw enables attackers to escalate their privileges from user-level to kernel-level execution. The denial of service aspect of this vulnerability can be classified under ATT&CK technique T1499, covering "Network Denial of Service," when the kernel memory corruption causes system instability or complete system crashes.
The mitigation strategies for this vulnerability require immediate kernel updates to address the flawed implementation in the shiftfs filesystem. System administrators should prioritize applying security patches that either remove the problematic non-upstream patch or correct the validation logic in shiftfs_real_fdget(). Additionally, monitoring for unusual file descriptor operations within kernel space can help detect potential exploitation attempts. The vulnerability highlights the importance of proper input validation and type checking in kernel space operations, as mandated by security standards such as the Kernel Security Architecture guidelines. Organizations should implement robust kernel hardening measures including stack canaries, kernel address space layout randomization, and strict kernel module signing requirements to reduce the attack surface. The flaw also demonstrates the risks associated with maintaining non-upstream kernel patches, which can introduce security vulnerabilities not present in the mainline kernel, emphasizing the need for careful evaluation of third-party kernel modifications and their security implications.