CVE-2021-47531: 
Linux Kernel vulnerability analysis and mitigation

CVE-2021-47531 is a vulnerability in the Linux kernel's DRM (Direct Rendering Manager) subsystem, specifically affecting the MSM (Qualcomm Mobile Station Modem) graphics driver. The issue was discovered when a change in the memory mapping implementation for GEM (Graphics Execution Manager) objects omitted crucial virtual memory flags, leading to potential system crashes. The vulnerability was identified in November 2021 and particularly affected Chromebooks using ARC++ during logout operations (Kernel Git).

The vulnerability arose from a change in commit 510410bfc034 that implemented a new method for memory mapping in the MSM graphics driver. The implementation failed to set important virtual memory flags (VM_IO and VM_DONTDUMP) that were previously set by default through drm_gem_mmap_obj(). This omission caused memory handling issues, particularly when obj->funcs->mmap() was no longer NULL, preventing the execution of default memory protection code (Kernel Git).

The vulnerability resulted in kernel paging request failures and system crashes, particularly affecting Chromebooks using ARC++ during logout operations. The issue manifested as memory abort errors with specific error signatures (ESR = 0x96000006) and was observed on both 5.10 and 5.15 kernel versions (Kernel Git).

The vulnerability primarily manifested as a stability issue rather than a security exploit. It was triggered during normal system operations, specifically during user logout processes on affected systems, and required no special privileges to encounter (Kernel Git).

The issue was resolved by adding back the VM_IO and VM_DONTDUMP flags to the virtual memory area flags (vma->vm_flags) in the msm_gem_object_mmap function. Additionally, the unnecessary clearing of VM_PFNMAP was removed to optimize the code. The fix was confirmed to resolve the crashes, with VM_IO being the critical flag for addressing the immediate issue (Kernel Git).