CVE-2026-98685: smb: client: discard post-EOF pagecache when extending a file via clone range

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

smb: client: discard post-EOF pagecache when extending a file via clone range

cifsremapfilerange() discarded the pagecache only for the folios overlapping the destination region, so when a clone extends the file the folio straddling the old EOF - which may hold data dirtied past EOF through an mmap - was never dropped and became visible as file content once the file was extended.

Start the discard at the smaller of fstart and the old EOF. isize is still old here, so nothing past the old EOF is flushed to the server.