CVE-2026-64159: netfs: Fix zeropoint update where i_size > remote_i_size

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

netfs: Fix zeropoint update where isize > remoteisize

Fix the update of the zero point[] by netfsreleasefolio() when there is uncommitted data in the pagecache beyond the folio being released but the on-server EOF is in this folio (ie. isize > remoteisize). The update needs to limit zeropoint to remoteisize, not isize as isize is a local phenomenon reflecting updates made locally to the pagecache, not stuff written to the server. remoteisize tracks the server's isize.

[] The zero point is the file position from which we can assume that the server will just return zeros, so we can avoid generating reads.

Note that netfsinvalidatefolio() probably doesn't need fixing as zeropoint should be updated by setattr after truncation or fallocate.

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 0x1bbae 0x82864 write 0x3ef2e 0xf9c8 0x1bbae write 0x67e05 0xcb5a 0x4e8f6 mapread 0x57781 0x85b6 0x7495f copyrange 0x5d3d 0x10329 0x54fac 0x7495f write 0x64710 0x1c2b 0x7495f mapread 0x64000 0x1000 0x7495f

on cifs with the default cache option.

It shows read-gaps on folio 0x64 failing with a short read (ie. it hits EOF) 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.