CVE-2026-53392: NFSv4/flexfiles: reject zero filehandle version count
In the Linux kernel, the following vulnerability has been resolved:
NFSv4/flexfiles: reject zero filehandle version count
fflayoutalloclseg() decodes the filehandle-version array count from the flexfiles layout body. The value is used as the count for kzallocobjs(), and the current code only rejects NULL.
A zero count yields ZEROSIZEPTR, which can be stored in dssinfo->fhversions even though later flexfiles paths assume that at least one filehandle version exists.
Reject fhcount == 0 before the allocation, matching the existing zero versioncount validation in the flexfiles GETDEVICEINFO parser.
A QEMU/KASAN run with a malformed flexfiles layout hit:
KASAN: null-ptr-deref in range [0x0000000000000010-0x0000000000000017] RIP: 0010:fflayoutencodefflayoutupdate.isra.0+0x15f/0x750 fflayoutencodelayoutreturn+0x683/0x970 nfs4xdrenclayoutreturn+0x278/0x3a0 Kernel panic - not syncing: Fatal exception
The patched kernel rejects the malformed layout without KASAN/oops/panic, and a valid fhcount=1 regression still opens, reads, and unmounts cleanly.
Affected Software
Remediation
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Fixed in 6.6.145.2-1 - Upgrade
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debian/linux-6.12to a version that resolves this vulnerability.Fixed in 6.12.113-1~deb12u1 - Compensating control
In ff_layout_alloc_lseg(), reject fh_count == 0 before the filehandle-version array allocation, matching the existing zero version_count validation in the flexfiles GETDEVICEINFO parser.
Event History
Peer vulnerabilities
Found alongside the following vulnerabilities.
Frequently Asked Questions
Who can trigger this issue?
An unauthenticated network attacker can trigger it by supplying a malformed NFSv4 flexfiles layout with a zero filehandle-version count. The CVSS vector indicates no privileges or user interaction are required.
What is the practical impact?
The malformed layout can lead to a null-pointer dereference in Linux kernel flexfiles processing, causing a kernel oops or panic. The reported impact is denial of service; no confidentiality or integrity impact is listed.
What configurations are affected?
Systems using the Linux kernel's NFSv4 flexfiles functionality are implicated. The provided data does not state whether flexfiles is enabled or used by default.
How can I tell whether a system has encountered exploitation or a triggering condition?
Affected systems may log a KASAN null-pointer dereference involving ff_layout_encode_ff_layoutupdate or ff_layout_encode_layoutreturn, followed by a kernel panic or fatal exception. The triggering input is a flexfiles layout containing a zero filehandle-version count.