CVE-2026-64063

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 0x63fb8 0x53c8 0

copy_range 0xb704 0x19b9 0x24429 0x79380

write 0x2402b 0x144a2 0x90660 *

write 0x204d5 0x140a0 0x927c0 *

copy_range 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 FMODE_READ check is commented out in

netfs_perform_write():

if (//(file->f_mode & FMODE_READ) ||

netfs_is_cache_enabled(ctx)) {

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