CVE-2026-64063 — netfs: Fix streaming write being overwritten

Affected: Linux

In the Linux kernel, the following vulnerability has been resolved:

netfs: Fix streaming write being overwritten

In order to avoid reading whilst writing, netfslib will allow "streaming writes" in which dirty data is stored directly into folios without reading them first. Such folios are marked dirty but may not be marked uptodate. If a folio is entirely written by a streaming write, uptodate will be set, otherwise it will have a netfs_folio struct attached to ->private recording the dirty region.

In the event that a partially written streaming write page is to be overwritten entirely by a single write(), netfs_perform_write() will try to copy over it, but doesn't discard the netfs_folio if it succeeds; further, it doesn't correctly handle a partial copy that overwrites some of the dirty data.

Fix this by the following:

(1) If the folio is successfully overwritten, free the netfs_folio struct before marking the page uptodate.

(2) If the copy to the folio partially fails, but short of the dirty data, just ignore the copy.

(3) If the copy partially fails and overwrites some of the dirty data, accept the copy, update the netfs_folio struct to record the new data. If the folio is now filled, free the netfs_folio and set uptodate, otherwise return a partial write.

Found with:

fsx -q -N 1000000 -p 10000 -o 128000 -l 600000 \ /xfstest.test/junk --replay-ops=junk.fsxops

using the following as junk.fsxops:

truncate 0x0 0 0x927c0 write 0x6…

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CVE-2026-64063 — netfs: Fix streaming write being overwritten