CVE-2025-21932 mm: abort vma_modify() on merge out of memory failure

What is CVE-2025-21932?

CVE-2025-21932 is a high-severity vulnerability in Linux kernel, published on 1 Apr 2025. It can only be exploited from the same machine or local network, with no action from the victim.

Official description

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---

Which versions are affected by CVE-2025-21932?

It affects 1 product.

Products include Linux kernel.

How do I fix CVE-2025-21932?

Apply the vendor's patch.

Is CVE-2025-21932 being exploited?

There is no known exploitation of CVE-2025-21932 in the wild and no public exploit code.

Other vulnerabilities in Linux kernel

• CVE-2026-53266 Linux Kernel Out-of-Bounds Write Exploited High 8.8
• CVE-2013-6282 Linux Kernel Improper Input Validation Exploited High 8.8
• CVE-2022-0185 Linux Kernel Heap-Based Buffer Overflow Exploited High 8.4
• CVE-2013-2094 Linux Kernel Privilege Escalation Exploited High 8.4
• CVE-2026-53362 Linux Kernel Unspecified Exploited High 7.8

All Linux kernel vulnerabilities

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as BELL-CVE-2025-21932, DEBIAN-CVE-2025-21932, GHSA-fcgh-gjcg-cmc2, UBUNTU-CVE-2025-21932.