CVE-2025-21908 NFS: fix nfs_release_folio() to not deadlock via kcompactd writeback
A deadlock in the Linux kernel's NFS module can cause system hangs when kcompactd triggers writeback, leading to 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-2025-21908?
CVE-2025-21908 is a medium-severity denial of service 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:
NFS: fix nfs_release_folio() to not deadlock via kcompactd writeback
Add PF_KCOMPACTD flag and current_is_kcompactd() helper to check for it so nfs_release_folio() can skip calling nfs_wb_folio() from kcompactd.
Otherwise NFS can deadlock waiting for kcompactd enduced writeback which recurses back to NFS (which triggers writeback to NFSD via NFS loopback mount on the same host, NFSD blocks waiting for XFS's call to __filemap_get_folio):
6070.550357] INFO: task kcompactd0:58 blocked for more than 4435 seconds.
{--- [58] "kcompactd0" [<0>] folio_wait_bit+0xe8/0x200 [<0>] folio_wait_writeback+0x2b/0x80 [<0>] nfs_wb_folio+0x80/0x1b0 [nfs] [<0>] nfs_release_folio+0x68/0x130 [nfs] [<0>] split_huge_page_to_list_to_order+0x362/0x840 [<0>] migrate_pages_batch+0x43d/0xb90 [<0>] migrate_pages_sync+0x9a/0x240 [<0>] migrate_pages+0x93c/0x9f0 [<0>] compact_zone+0x8e2/0x1030 [<0>] compact_node+0xdb/0x120 [<0>] kcompactd+0x121/0x2e0 [<0>] kthread+0xcf/0x100 [<0>] ret_from_fork+0x31/0x40 [<0>] ret_from_fork_asm+0x1a/0x30 ---}
[[email protected]: fix build]
Weakness: CWE-667
Which versions are affected by CVE-2025-21908?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2025-21908?
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-2025-21908 being exploited?
There is no known exploitation of CVE-2025-21908 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as AZL-60352, BELL-CVE-2025-21908, DEBIAN-CVE-2025-21908, GHSA-6m8x-cvmh-fpwm, ROOT-OS-UBUNTU-2204-CVE-2025-21908, ROOT-OS-UBUNTU-2404-CVE-2025-21908 and 1 more.