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

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 netfsfolio 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(), netfsperformwrite() will try to copy over it, but doesn't discard the netfsfolio 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 netfsfolio 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 netfsfolio struct to record the new data. If the folio is now filled, free the netfsfolio 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 0x63fb8 0x53c8 0 copyrange 0xb704 0x19b9 0x24429 0x79380 write 0x2402b 0x144a2 0x90660 write 0x204d5 0x140a0 0x927c0 copyrange 0x1f72c 0x137d0 0x7a906 0x927c0 read 0x00000 0x20000 0x9157c read 0x20000 0x20000 0x9157c read 0x40000 0x20000 0x9157c read 0x60000 0x20000 0x9157c read 0x7e1a0 0xcfb9 0x9157c

on cifs with the default cache option.

It shows folio 0x24 misbehaving if the FMODEREAD check is commented out in netfsperformwrite():

if (//(file->fmode & FMODEREAD) || netfsiscacheenabled(ctx)) {

and no fscache. This was initially found with the generic/522 xfstest.