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

Overview

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.

Affected

• Linux >= 966a3cb7c7db786452a87afdc3b48858fc4d4d6b < f82242fe3109a3ef72f429540b5bdde08fbfb0e4
• Linux >= 966a3cb7c7db786452a87afdc3b48858fc4d4d6b < bf84e44ff0ed802778eafd7ecb623635379cb506
• Linux >= 966a3cb7c7db786452a87afdc3b48858fc4d4d6b < aa994489bd835547c191827acc460192e147032d
• Linux c406bb9ece6ef63721daab106f132ff4b4234e81
• Linux 2f9f4a2d0e6fcf0673ed51195e06e47abe966900
• Linux 5e397c943424de94879830e72c95f2679e297a76
• Linux >= 5.10.50 < 5.11
• Linux >= 5.12.17 < 5.13
• Linux >= 5.13.2 < 5.14
• Linux 5.14

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.