CVE-2025-21932: 
Linux Kernel vulnerability analysis and mitigation

CVE-2025-21932 is a vulnerability discovered in the Linux kernel related to memory management. The issue was identified in the vma_modify() function where an edge case scenario during memory merge operations could lead to invalid state handling. The vulnerability was reported by syzkaller and Brad Spengler, and was disclosed on April 1, 2025 (NVD, CVE).

The vulnerability occurs in the vmamodify() function where the vmg state is assumed to remain pristine after a merge attempt. In an edge case scenario, when a merge attempt fails due to an out-of-memory error during the commit phase, the vmg->start and end values become modified incorrectly. This results in subsequent VMA split attempts being performed with invalid start/end values. The issue manifests specifically during madvise() operations across multiple VMAs, triggering an assert: VMWARNONVMG(start >= end, vmg) in vmamergeexisting_range() (CVE).

While the vulnerability exists in the Linux kernel's memory management subsystem, its practical impact is considered limited. The scenario requires a specific maple tree node pre-allocation failure, which is described as 'practically impossible' to occur in real-world conditions since the kernel would typically keep retrying memory reclaim until successful (NVD).

The recommended mitigation is to abort the vma_modify() operation when a merge out-of-memory failure occurs. The fix involves giving up the operation early when memory allocation for merging fails, as attempting to split would also likely fail under such extreme memory pressure conditions. Additionally, the fix includes storing start and end values in local variables to prevent assuming VMG state stability after merge attempts (NVD).