CVE-2025-21860 mm/zswap: fix inconsistency when zswap_store_page() fails
A logic error in Linux kernel zswap handling causes accounting inconsistencies and kernel warnings, potentially leading to system instability or denial of service.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2025-21860?
CVE-2025-21860 is a low-severity denial of service vulnerability in Linux kernel, published on 12 Mar 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/zswap: fix inconsistency when zswap_store_page() fails
Commit b7c0ccdfbafd ("mm: zswap: support large folios in zswap_store()") skips charging any zswap entries when it failed to zswap the entire folio.
However, when some base pages are zswapped but it failed to zswap the entire folio, the zswap operation is rolled back. When freeing zswap entries for those pages, zswap_entry_free() uncharges the zswap entries that were not previously charged, causing zswap charging to become inconsistent.
This inconsistency triggers two warnings with following steps: # On a machine with 64GiB of RAM and 36GiB of zswap $ stress-ng --bigheap 2 # wait until the OOM-killer kills stress-ng $ sudo reboot
The two warnings are: in mm/memcontrol.c:163, function obj_cgroup_release(): WARN_ON_ONCE(nr_bytes & (PAGE_SIZE - 1));
in mm/page_counter.c:60, function page_counter_cancel(): if (WARN_ONCE(new < 0, "page_counter underflow: %ld nr_pages=%lu\n", new, nr_pages))
zswap_stored_pages also becomes inconsistent in the same way.
As suggested by Kanchana, increment zswap_stored_pages and charge zswap entries within zswap_store_page() when it succeeds. This way, zswap_entry_free() will decrement the counter and uncharge the entries when it failed to zswap the entire folio.
While this could potentially be optimized by batching objcg charging and incrementing the counter, let's focus on fixing the bug this time and leave the optimization for later after some evaluation.
After resolving the inconsistency, the warnings disappear.
[[email protected]: refactor zswap_store_page()]
Which versions are affected by CVE-2025-21860?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2025-21860?
Apply the vendor's patch.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2025-21860 being exploited?
There is no known exploitation of CVE-2025-21860 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as BELL-CVE-2025-21860, GHSA-gp26-q8h6-xxgj, UBUNTU-CVE-2025-21860.