CVE-2017-8246 in Android
Summary
by MITRE
In function msm_pcm_playback_close() in all Android releases from CAF using the Linux kernel, prtd is assigned substream->runtime->private_data. Later, prtd is freed. However, prtd is not sanitized and set to NULL, resulting in a dangling pointer. There are other functions that access the same memory (substream->runtime->private_data) with a NULL check, such as msm_pcm_volume_ctl_put(), which means this freed memory could be used.
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Analysis
by VulDB Data Team • 12/30/2019
This vulnerability exists in the msm_pcm_playback_close() function within Android's Linux kernel implementation from CAF (Code Aurora Forum) versions, representing a classic dangling pointer issue that can lead to arbitrary code execution. The flaw occurs when the prtd variable is assigned from substream->runtime->private_data and subsequently freed without proper nullification of the pointer reference. This creates a scenario where the freed memory location remains accessible through other function calls that may attempt to access the same memory location through the substream->runtime->private_data field, establishing a potential vector for memory corruption attacks. The vulnerability is particularly concerning because it involves audio subsystem components that are frequently accessed during normal device operation, making exploitation more likely.
The technical implementation of this vulnerability follows a well-documented pattern of improper memory management that aligns with CWE-415, which describes improper deallocation of memory, and CWE-416, which covers use after free conditions. The memory management error occurs in the audio driver layer where the prtd structure is allocated during PCM playback initialization and later freed during the close operation. However, the failure to set prtd to NULL after freeing creates a dangling pointer that other functions can still reference. The msm_pcm_volume_ctl_put() function serves as a critical example of how this dangling pointer can be exploited since it performs a NULL check before accessing the memory, but this check becomes ineffective after the memory has been freed and potentially reallocated.
The operational impact of this vulnerability extends beyond simple memory corruption, as it provides attackers with potential pathways for privilege escalation and system compromise. The audio subsystem is typically accessible to unprivileged users and applications, making this a particularly attractive target for exploitation. Attackers could potentially leverage the dangling pointer to execute arbitrary code with kernel privileges, effectively bypassing standard security boundaries. The vulnerability's presence in all Android releases from CAF using the Linux kernel indicates a widespread exposure across multiple device manufacturers and product lines, amplifying the potential attack surface. Additionally, the fact that the memory can be accessed through multiple functions means that the attack surface is not limited to a single code path, increasing the likelihood of successful exploitation.
Mitigation strategies for this vulnerability should focus on implementing proper nullification of freed pointers and strengthening the kernel's memory management practices. The primary fix involves ensuring that prtd is set to NULL immediately after freeing the memory, preventing subsequent access to the freed memory location. Security patches should also implement stricter validation checks in functions that access private_data fields, including additional bounds checking and memory integrity verification. Organizations should prioritize immediate deployment of vendor-provided security updates and consider implementing kernel lockdown mechanisms that restrict access to critical kernel memory areas. From an ATT&CK perspective, this vulnerability maps to techniques involving privilege escalation through kernel exploits and memory corruption, with the potential to enable lateral movement and persistent access to affected systems. Regular security auditing of kernel components and implementation of automated memory safety checking tools can help identify similar patterns and prevent future occurrences of this class of vulnerability.