CVE-2026-72132 in Linux
Summary
by MITRE • 08/15/2026
In the Linux kernel, the following vulnerability has been resolved:
NFS: Charge unstable writes by request size, not folio size
nfs_folio_mark_unstable() and nfs_folio_clear_commit() charge and uncharge NR_WRITEBACK/WB_WRITEBACK by folio_nr_pages(folio) once per *request* added to or removed from a commit list. This is correct only when a folio has a single associated request. When pg_test splits a folio into N sub-folio requests (e.g. pNFS flexfiles striping with a stripe unit smaller than the folio size, or plain wsize-limited splitting), each of the N requests independently charges the whole folio's page count, inflating the accounting by a factor of N per folio. With large folios and small stripe units this reaches multiple orders of magnitude: a 2 MiB folio split into 512 4 KiB requests can charge up to 512x its real size, pushing global dirty+writeback accounting past the system's dirty threshold and forcing every buffered writer on the host into the hard-throttle path, including unrelated in-kernel NFS server threads sharing the box.
Charge each request only for the pages it actually covers.
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Analysis
by VulDB Data Team • 08/15/2026
This vulnerability exists within the Linux kernel's Network File System implementation where improper accounting of unstable writes leads to severe system performance degradation and resource exhaustion. The flaw resides in the nfs_folio_mark_unstable() and nfs_folio_clear_commit() functions that handle writeback accounting for NFS operations. These functions incorrectly calculate the number of pages to charge against the system's dirty page limits by using folio_nr_pages(folio) rather than the actual request size, creating a fundamental misalignment in resource tracking mechanisms.
The technical flaw manifests when folios are split into multiple sub-folio requests during operations such as pNFS flexfiles striping where stripe units are smaller than the folio size or through standard wsize-limited splitting. Under normal circumstances each request should only account for the specific pages it actually manages, but due to the flawed implementation each individual request independently charges the entire folio's page count regardless of how many pages it actually covers. This creates a multiplicative effect where a single folio can generate accounting entries that are orders of magnitude larger than the actual data being processed.
The operational impact of this vulnerability is catastrophic for systems running NFS workloads, particularly those utilizing pNFS or large folios with small stripe units. When a 2 MiB folio gets split into 512 individual 4 KiB requests, each request incorrectly charges 512 times its actual size, leading to rapid exhaustion of the system's dirty page thresholds. This forces all buffered writers on the host system into hard-throttle paths, effectively grinding performance to a halt while simultaneously affecting unrelated in-kernel NFS server threads that share the same system resources.
The vulnerability directly relates to CWE-190, Integer Overflow or Wraparound, and CWE-704, Incorrect Type Conversion or Cast, as it involves improper integer calculations and type handling within the memory accounting subsystem. From an ATT&CK perspective, this represents a Resource Exhaustion technique that can be leveraged to cause denial of service conditions through manipulation of kernel memory accounting mechanisms. The flaw impacts system stability and availability by creating artificial resource constraints that trigger kernel-level throttling behaviors.
Mitigation strategies must focus on implementing proper per-request size accounting rather than folio-wide calculations. The solution requires modifying the nfs_folio_mark_unstable() and nfs_folio_clear_commit() functions to track each request's actual page coverage instead of inflating charges based on folio dimensions. System administrators should monitor dirty page thresholds and implement appropriate tuning parameters, while kernel developers must ensure that all memory accounting operations properly respect the granularity of individual requests rather than applying blanket folio-level calculations. The fix addresses the fundamental mismatch between accounting precision and operational reality in the NFS writeback subsystem.