CVE-2026-98684

smb: client: flush and commit data before querying allocated ranges

Published Oct 18, 2026 CVSS 3.1 · 2.5 Low EPSS < 1% KEV · Not listed
Description
In the Linux kernel, the following vulnerability has been resolved:

smb: client: flush and commit data before querying allocated ranges

The mode 0 / FALLOC_FL_KEEP_SIZE fallocate emulation for small interior
ranges in smb3_simple_fallocate_range() issues
FSCTL_QUERY_ALLOCATED_RANGES and zero-fills any sub-range the server
reports as an unallocated hole, to force block allocation without
changing file contents.

This is unsafe against Windows servers. Per the Win32 documentation,
FSCTL_QUERY_ALLOCATED_RANGES only reports ranges that *may* contain
nonzero data and is explicitly not coherent with recently written data:

"a call to FSCTL_QUERY_ALLOCATED_RANGES [after writing to a network
file] would not necessarily return a correct list of allocated
regions. To ensure coherency [...] flush the data to the file"

fsx (generic/363) reproduces the resulting corruption against Windows
Server 2022: a range is written, an uncached read returns the data, yet
the next FSCTL_QUERY_ALLOCATED_RANGES on that range reports it as a whole
hole, so the emulation overwrites live data with zeroes. A network trace
confirmed the WRITE, the READ returning data and the query returning an
empty range list within microseconds. Samba does not exhibit this; its
query is coherent with the written data.

Write back the dirty pages, drain any outstanding I/O and issue an SMB2
FLUSH to force the server to commit the data before querying the
allocated ranges, so the query reflects the data actually present.

With this, generic/363 passes against both Windows Server 2022 and Samba
over 100000 fsx operations.

Remediation

No solution or workaround provided in the CVE record

Tracking

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Advisories

Source ID Title
Ubuntu USN Ubuntu USN USN-9018-1 Linux kernel vulnerabilities
Ubuntu USN Ubuntu USN USN-9018-2 Linux kernel (HWE) vulnerabilities
Ubuntu USN Ubuntu USN USN-9019-1 Linux kernel (Azure) vulnerabilities
Ubuntu USN Ubuntu USN USN-9020-1 Linux kernel vulnerabilities
History
Type Values Removed Values Added
Weaknesses CWE-750
Type Values Removed Values Added
Weaknesses CWE-820
References
Metrics threat_severity

None

cvssV3_1

{'score': 2.5, 'vector': 'CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:N'}

threat_severity

Low

Type Values Removed Values Added
Weaknesses CWE-750
Type Values Removed Values Added
Description In the Linux kernel, the following vulnerability has been resolved: smb: client: flush and commit data before querying allocated ranges The mode 0 / FALLOC_FL_KEEP_SIZE fallocate emulation for small interior ranges in smb3_simple_fallocate_range() issues FSCTL_QUERY_ALLOCATED_RANGES and zero-fills any sub-range the server reports as an unallocated hole, to force block allocation without changing file contents. This is unsafe against Windows servers. Per the Win32 documentation, FSCTL_QUERY_ALLOCATED_RANGES only reports ranges that *may* contain nonzero data and is explicitly not coherent with recently written data: "a call to FSCTL_QUERY_ALLOCATED_RANGES [after writing to a network file] would not necessarily return a correct list of allocated regions. To ensure coherency [...] flush the data to the file" fsx (generic/363) reproduces the resulting corruption against Windows Server 2022: a range is written, an uncached read returns the data, yet the next FSCTL_QUERY_ALLOCATED_RANGES on that range reports it as a whole hole, so the emulation overwrites live data with zeroes. A network trace confirmed the WRITE, the READ returning data and the query returning an empty range list within microseconds. Samba does not exhibit this; its query is coherent with the written data. Write back the dirty pages, drain any outstanding I/O and issue an SMB2 FLUSH to force the server to commit the data before querying the allocated ranges, so the query reflects the data actually present. With this, generic/363 passes against both Windows Server 2022 and Samba over 100000 fsx operations.
Title smb: client: flush and commit data before querying allocated ranges
First Time appeared Linux
Linux linux Kernel
CPEs cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*
Vendors & Products Linux
Linux linux Kernel
References
Weaknesses

Sources

Status
PUBLISHED
Assigner
Linux
Published
Updated
Nov 10, 2026, 6:15 PM
Reserved
Sep 25, 2026, 10:25 AM
Status
Received
Published
Oct 18, 2026, 4:17 PM
Modified
Nov 10, 2026, 7:22 PM
Severity
Low
Public date
Oct 18, 2026, 12:00 AM