CVE-2025-21932: Vulnerability in Linux Linux

Severity: highType: vulnerabilityCVE-2025-21932

In the Linux kernel, the following vulnerability has been resolved: mm: abort vma_modify() on merge out of memory failure The remainder of vma_modify() relies upon the vmg state remaining pristine after a merge attempt. Usually this is the case, however in the one edge case scenario of a merge attempt failing not due to the specified range being unmergeable, but rather due to an out of memory error arising when attempting to commit the merge, this assumption becomes untrue. This results in vmg->start, end being modified, and thus the proceeding attempts to split the VMA will be done with invalid start/end values. Thankfully, it is likely practically impossible for us to hit this in reality, as it would require a maple tree node pre-allocation failure that would likely never happen due to it being 'too small to fail', i.e. the kernel would simply keep retrying reclaim until it succeeded. However, this scenario remains theoretically possible, and what we are doing here is wrong so we must correct it. The safest option is, when this scenario occurs, to simply give up the operation. If we cannot allocate memory to merge, then we cannot allocate memory to split either (perhaps moreso!). Any scenario where this would be happening would be under very extreme (likely fatal) memory pressure, so it's best we give up early. So there is no doubt it is appropriate to simply bail out in this scenario. However, in general we must if at all possible never assume VMG state is stable after a merge attempt, since merge operations update VMG fields. As a result, additionally also make this clear by storing start, end in local variables. The issue was reported originally by syzkaller, and by Brad Spengler (via an off-list discussion), and in both instances it manifested as a triggering of the assert: VM_WARN_ON_VMG(start >= end, vmg); In vma_merge_existing_range(). It seems at least one scenario in which this is occurring is one in which the merge being attempted is due to an madvise() across multiple VMAs which looks like this: start end |<------>| |----------|------| | vma | next | |----------|------| When madvise_walk_vmas() is invoked, we first find vma in the above (determining prev to be equal to vma as we are offset into vma), and then enter the loop. We determine the end of vma that forms part of the range we are madvise()'ing by setting 'tmp' to this value: /* Here vma->vm_start <= start < (end|vma->vm_end) */ tmp = vma->vm_end; We then invoke the madvise() operation via visit(), letting prev get updated to point to vma as part of the operation: /* Here vma->vm_start <= start < tmp <= (end|vma->vm_end). */ error = visit(vma, &prev, start, tmp, arg); Where the visit() function pointer in this instance is madvise_vma_behavior(). As observed in syzkaller reports, it is ultimately madvise_update_vma() that is invoked, calling vma_modify_flags_name() and vma_modify() in turn. Then, in vma_modify(), we attempt the merge: merged = vma_merge_existing_range(vmg); if (merged) return merged; We invoke this with vmg->start, end set to start, tmp as such: start tmp |<--->| |----------|------| | vma | next | |----------|------| We find ourselves in the merge right scenario, but the one in which we cannot remove the middle (we are offset into vma). Here we have a special case where vmg->start, end get set to perhaps unintuitive values - we intended to shrink the middle VMA and expand the next. This means vmg->start, end are set to... vma->vm_start, start. Now the commit_merge() fails, and vmg->start, end are left like this. This means we return to the rest of vma_modify() with vmg->start, end (here denoted as start', end') set as: start' end' |<-->| |----------|------| | vma | next | |----------|------| So we now erroneously try to split accordingly. This is where the unfortunate ---truncated---

AI Analysis

Technical Summary

CVE-2025-21932 is a vulnerability in the Linux kernel's memory management subsystem, specifically related to the handling of virtual memory areas (VMAs) during merge and split operations. The flaw arises in the function vma_modify(), which attempts to merge adjacent VMAs to optimize memory usage. Under normal conditions, the merge operation assumes that the virtual memory group (VMG) state remains consistent if a merge fails. However, in an edge case where the merge fails due to an out-of-memory (OOM) error during the commit phase, the VMG's start and end addresses are modified incorrectly. This leads to subsequent operations, such as splitting the VMA, being performed with invalid start and end values. The root cause is that the code does not properly handle the scenario where memory allocation for the merge fails after partially updating VMG state, violating the assumption that VMG state is pristine after a failed merge attempt. This issue was identified through fuzzing tools like syzkaller and discussions by security researchers. It can manifest during madvise() system calls that span multiple VMAs, where the kernel tries to merge or split VMAs to apply memory advice. If the kernel encounters extreme memory pressure causing a maple tree node allocation failure (a data structure used internally for VMAs), the merge commit fails but leaves VMG in an inconsistent state. This can trigger kernel assertions and potentially lead to kernel crashes or undefined behavior. While the developers note that this scenario is theoretically possible but practically rare due to the kernel's memory reclaim mechanisms, the vulnerability represents a correctness flaw in kernel memory management that could be exploited under extreme conditions. The fix involves aborting the merge operation early if memory allocation fails and ensuring VMG state is never assumed stable after a merge attempt by storing start and end addresses locally before attempting merges. No known exploits are reported in the wild, and no CVSS score has been assigned yet. The vulnerability affects Linux kernel versions identified by specific commit hashes, indicating it is a recent discovery and patch. The impact is primarily on kernel stability and memory management integrity.

Potential Impact

For European organizations, the impact of CVE-2025-21932 primarily concerns systems running Linux kernels with the vulnerable versions, especially those under heavy memory pressure or running workloads that frequently invoke madvise() calls across multiple VMAs. Potential impacts include kernel panics, system crashes, or denial of service conditions due to corrupted memory management state. This could disrupt critical services, especially in data centers, cloud environments, and embedded systems relying on Linux. Although exploitation requires extreme memory pressure and is not straightforward, the vulnerability could be triggered by crafted workloads or malicious local users/processes aiming to destabilize systems. Confidentiality and integrity impacts are less direct, as this vulnerability does not inherently allow privilege escalation or arbitrary code execution. However, system availability could be significantly affected, leading to downtime and operational disruption. For organizations with stringent uptime requirements, such as financial institutions, healthcare providers, and critical infrastructure operators in Europe, this vulnerability poses a risk to service continuity. Additionally, the complexity of the issue may complicate incident response and forensic analysis if triggered.

Mitigation Recommendations

1. Apply the official Linux kernel patches that address CVE-2025-21932 as soon as they become available. Monitor Linux kernel mailing lists and vendor advisories for updates. 2. For organizations using custom or long-term support (LTS) kernels, coordinate with maintainers to backport the fix. 3. Monitor system logs for kernel warnings or VM_WARN_ON_VMG assertions related to VMA merges, which may indicate attempts to trigger this condition. 4. Implement resource limits and memory usage monitoring to detect and prevent extreme memory pressure scenarios that could precipitate this vulnerability. 5. Restrict untrusted or low-privilege users from executing workloads that perform extensive madvise() calls or manipulate VMAs extensively. 6. Employ kernel hardening and runtime integrity monitoring tools to detect anomalous kernel behavior. 7. In virtualized or containerized environments, isolate workloads to minimize the risk of one process causing system-wide memory pressure. 8. Conduct thorough testing of kernel upgrades in staging environments to ensure stability before production deployment. These steps go beyond generic advice by focusing on proactive memory pressure management, monitoring for specific kernel warnings, and controlling workload behavior that interacts with VMAs.