CVE-2021-47039 in Linux
Summary
by MITRE • 02/28/2024
In the Linux kernel, the following vulnerability has been resolved:
ataflop: potential out of bounds in do_format()
The function uses "type" as an array index:
q = unit[drive].disk[type]->queue;
Unfortunately the bounds check on "type" isn't done until later in the function. Fix this by moving the bounds check to the start.
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Analysis
by VulDB Data Team • 01/09/2025
The vulnerability identified as CVE-2021-47039 resides within the Linux kernel's ataflop driver, which handles floppy disk operations for idefloppy devices. This flaw represents a classic out-of-bounds memory access issue that could potentially lead to system instability or privilege escalation. The ataflop driver is part of the broader IDE subsystem that manages legacy floppy disk controllers, making it a component that remains relevant in systems supporting older hardware configurations. The vulnerability manifests specifically within the do_format() function where the driver processes formatting operations for floppy disk units.
The technical flaw occurs due to improper input validation within the do_format() function where the variable "type" is used as an array index without prior bounds checking. The problematic code line q = unit[drive].disk[type]->queue; directly accesses memory using "type" as an index without verifying that the value falls within acceptable range for the array. This array access pattern creates a potential for memory corruption when the "type" parameter contains values that exceed the array boundaries. The vulnerability stems from a logical flow issue where the bounds validation occurs later in the function execution rather than at the beginning where such checks are conventionally and most effectively performed.
The operational impact of this vulnerability extends beyond simple memory corruption as it could potentially be exploited to achieve privilege escalation within the kernel context. When an attacker can control the "type" parameter through malformed input or by manipulating the driver interface, they could cause the kernel to access memory locations outside the intended array boundaries. This out-of-bounds access could result in kernel crashes, data corruption, or potentially allow an attacker to execute arbitrary code with kernel privileges. The vulnerability is particularly concerning because it affects the kernel's core storage subsystem, which handles critical I/O operations for legacy hardware. According to CWE classification, this vulnerability maps to CWE-129: Improper Validation of Array Index, which is a well-established weakness in software security practices. The flaw also aligns with ATT&CK technique T1068: Exploitation for Privilege Escalation, as kernel-level memory corruption can be leveraged to gain elevated system privileges.
The fix for this vulnerability involves implementing the bounds check at the beginning of the function before any array access occurs, rather than deferring it to a later stage in execution. This defensive programming approach ensures that all input parameters are validated before being used in memory operations, preventing the out-of-bounds access that could lead to system instability or security compromise. The resolution addresses the root cause by moving the validation logic to the appropriate location in the code flow, which is a standard security practice recommended in secure coding guidelines. This mitigation strategy aligns with industry best practices for preventing buffer overflow conditions and maintains the integrity of kernel memory management operations. The fix demonstrates proper input validation methodology that should be applied to similar code patterns throughout the kernel to prevent analogous vulnerabilities from emerging in other subsystems.