Linux Kernel Vulnerability Explained
Researchers have discovered a significant vulnerability in the Linux kernel's dmam_free_coherent() function. This vulnerability arises due to a race condition, a situation where the sequence of operations is not properly managed. Specifically, the issue occurs when freeing DMA (Direct Memory Access) allocations and handling related resources.
Understanding DMA and Its Importance
DMA is an essential mechanism that allows hardware devices to transfer data directly to and from the system's memory without involving the CPU. This process optimizes performance by reducing the CPU's workload. However, if there is a flaw in this process, it could lead to incorrect memory access, which might result in data corruption, unexpected behavior, or system crashes.
The Role of the dmam_free_coherent Function
The dmam_free_coherent function is tasked with freeing a DMA allocation and eliminating the associated data structure that monitors such allocations. An attacker could exploit the race condition by precisely timing their actions to coincide with the freeing and reallocation of DMA memory.
Potential Consequences of Exploitation
If an attacker succeeds, the devres_destroy function might mistakenly free the wrong entry, prompting a WARN_ON assertion in the dmam_match function. This function is a part of the DMA management subsystem within the Linux kernel.
How the Vulnerability Could Be Exploited
The vulnerability could create a scenario where two entries in the devres list share the same virtual address, leading devres_destroy() to free the incorrect entry. This circumstance arises when a concurrent task makes an allocation using the same virtual address before the original entry is removed from the list.
According to the original report, "dmam_free_coherent() frees a DMA allocation, making the freed virtual address available for reuse. It then calls devres_destroy() to remove and free the data structure tracking the DMA allocation. Between these two actions, a concurrent task could allocate memory with the same virtual address and add it to the devres list."
The Patch – CVE-2024-43856
Greg Kroah-Hartman committed a new patch to address this Linux kernel vulnerability, identified as CVE-2024-43856. The patch targets a potential problem in managing DMA allocations.
How the Patch Works
The patch, crafted by Lance Richardson from Google, modifies the dmam_free_coherent function. The aim is to prevent issues during the freeing and reallocation of DMA memory. By swapping the order of function calls, the patch ensures that the tracking data structure is destroyed using devres_destroy before the DMA allocation is freed with dma_free_coherent. This change blocks any concurrent task from interfering with the cleanup process.
Testing and Implementation
The patch has undergone testing on Google's internal "kokonut" network encryption project. It received sign-offs from Christoph Hellwig and Sasha Levin, signaling its readiness for integration into the mainline Linux kernel.
Conclusion
While the dmam_free_coherent() vulnerability involves a race condition in resource management, exploiting it to write arbitrary data into CPU memory would be complex. Attackers would need specific system configurations and additional vulnerabilities. Nonetheless, this patch is a testament to the ongoing efforts of the developer community to enhance the stability and reliability of the Linux operating system across the globe.
