cifs: Fix writeback data corruption

Vulnerability Description

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).

References

• https://git.kernel.org/stable/c/e45deec35bf7f1f4f992a707b2d04a8c162f2240
• https://git.kernel.org/stable/c/65f2ced695982ccd516196d0a9447d85dbe2eed5
• https://git.kernel.org/stable/c/844b4e132f57f1333dc79feaa035075a096762e4
• https://git.kernel.org/stable/c/f3dc1bdb6b0b0693562c7c54a6c28bafa608ba3c