CVE-2024-27036: Out-of-bounds Write

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

cifs: Fix writeback data corruption

cifs writeback doesn't correctly handle the case where cifs_extend_writeback() hits a point where it is considering an additional folio, but this would overrun the wsize - at which point it drops out of the xarray scanning loop and calls xas_pause(). The problem is that xas_pause() advances the loop counter - thereby skipping that page.

What needs to happen is for xas_reset() to be called any time we decide we don't want to process the page we're looking at, but rather send the request we are building and start a new one.

Fix this by copying and adapting the netfslib writepages code as a temporary measure, with cifs writeback intending to be offloaded to netfslib in the near future.

This also fixes the issue with the use of filemap_get_folios_tag() causing retry of a bunch of pages which the extender already dealt with.

This can be tested by creating, say, a 64K file somewhere not on cifs (otherwise copy-offload may get underfoot), mounting a cifs share with a wsize of 64000, copying the file to it and then comparing the original file and the copy:

dd if=/dev/urandom of=/tmp/64K bs=64k count=1 mount //192.168.6.1/test /mnt -o user=...,pass=...,wsize=64000 cp /tmp/64K /mnt/64K cmp /tmp/64K /mnt/64K

Without the fix, the cmp fails at position 64000 (or shortly thereafter).