CVE-2021-47541 in Linux
Summary
by MITRE • 05/24/2024
In the Linux kernel, the following vulnerability has been resolved:
net/mlx4_en: Fix an use-after-free bug in mlx4_en_try_alloc_resources()
In mlx4_en_try_alloc_resources(), mlx4_en_copy_priv() is called and tmp->tx_cq will be freed on the error path of mlx4_en_copy_priv(). After that mlx4_en_alloc_resources() is called and there is a dereference of &tmp->tx_cq[t][i] in mlx4_en_alloc_resources(), which could lead to
a use after free problem on failure of mlx4_en_copy_priv().
Fix this bug by adding a check of mlx4_en_copy_priv()
This bug was found by a static analyzer. The analysis employs differential checking to identify inconsistent security operations (e.g., checks or kfrees) between two code paths and confirms that the inconsistent operations are not recovered in the current function or the callers, so they constitute bugs.
Note that, as a bug found by static analysis, it can be a false positive or hard to trigger. Multiple researchers have cross-reviewed the bug.
Builds with CONFIG_MLX4_EN=m show no new warnings, and our static analyzer no longer warns about this code.
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Analysis
by VulDB Data Team • 01/21/2025
The vulnerability CVE-2021-47541 represents a critical use-after-free bug in the Linux kernel's mlx4_en network driver component. This flaw exists within the mlx4_en_try_alloc_resources() function where the mlx4_en_copy_priv() function is invoked, followed by potential freeing of tmp->tx_cq on error paths. The subsequent call to mlx4_en_alloc_resources() contains a dereference operation on &tmp->tx_cq[t][i] that can result in accessing freed memory, creating a classic use-after-free scenario that could potentially be exploited by malicious actors to execute arbitrary code or cause system instability. The vulnerability is particularly concerning as it resides in the kernel's network subsystem where memory corruption could lead to privilege escalation or denial of service conditions.
The technical implementation of this vulnerability stems from inconsistent resource management practices within the driver's resource allocation logic. When mlx4_en_copy_priv() fails during execution, it frees the tmp->tx_cq memory structure, but the subsequent mlx4_en_alloc_resources() function continues execution without proper validation of whether the memory has already been freed. This creates a temporal window where the memory reference becomes invalid while still being accessed, violating fundamental memory safety principles. The static analysis approach used to identify this bug employs differential checking methodologies that compare security operations across different code execution paths, specifically looking for inconsistencies in memory deallocation and access patterns. This methodology aligns with established security analysis practices and represents a sophisticated approach to identifying subtle memory corruption vulnerabilities in kernel code.
The operational impact of this vulnerability extends beyond simple memory corruption, as it can potentially enable attackers to exploit the use-after-free condition for privilege escalation attacks. The vulnerability affects systems running Linux kernels with the mlx4_en driver compiled as a loadable module, specifically when CONFIG_MLX4_EN=m is enabled. The fact that this bug was discovered through automated static analysis rather than runtime exploitation highlights the importance of comprehensive code analysis in kernel security. The vulnerability's classification as potentially hard to trigger means that while it may not be easily exploitable in all scenarios, the nature of use-after-free bugs makes them particularly dangerous as they can manifest in unpredictable ways depending on system conditions and memory layout. The cross-review by multiple researchers adds credibility to the finding and suggests this represents a genuine security concern rather than a false positive.
Mitigation strategies for CVE-2021-47541 primarily involve applying the kernel patch that introduces a proper check for mlx4_en_copy_priv() before proceeding with resource allocation operations. The fix implements a validation mechanism that prevents the execution path from attempting to access freed memory structures, thereby eliminating the use-after-free condition. System administrators should prioritize updating their kernel versions to include this fix, particularly in environments where the mlx4_en driver is actively used. The static analyzer used to identify this vulnerability has been updated to no longer flag this code pattern, indicating that the fix addresses the root cause of the inconsistency. Organizations should also consider implementing additional monitoring for unusual memory access patterns in kernel modules and maintain regular kernel security updates to protect against similar vulnerabilities. This vulnerability demonstrates the critical importance of thorough code review processes and automated analysis tools in identifying subtle but dangerous memory safety issues in kernel space code, aligning with ATT&CK framework techniques related to privilege escalation and kernel exploitation.