dm-verity FEC: Fix RS FEC repair for roots unaligned to block size (take 2)

commit 6df90c02 upstream.

This patch fixes an issue that was fixed in the commit
  df7b59ba ("dm verity: fix FEC for RS roots unaligned to block size")
but later broken again in the commit
  8ca7cab8 ("dm verity fec: fix misaligned RS roots IO")

If the Reed-Solomon roots setting spans multiple blocks, the code does not
use proper parity bytes and randomly fails to repair even trivial errors.

This bug cannot happen if the sector size is multiple of RS roots
setting (Android case with roots 2).

The previous solution was to find a dm-bufio block size that is multiple
of the device sector size and roots size. Unfortunately, the optimization
in commit 8ca7cab8 ("dm verity fec: fix misaligned RS roots IO")
is incorrect and uses data block size for some roots (for example, it uses
4096 block size for roots = 20).

This patch uses a different approach:

 - It always uses a configured data block size for dm-bufio to avoid
 possible misaligned IOs.

 - and it caches the processed parity bytes, so it can join it
 if it spans two blocks.

As the RS calculation is called only if an error is detected and
the process is computationally intensive, copying a few more bytes
should not introduce performance issues.

The issue was reported to cryptsetup with trivial reproducer
  https://gitlab.com/cryptsetup/cryptsetup/-/issues/923



Reproducer (with roots=20):

 # create verity device with RS FEC
 dd if=/dev/urandom of=data.img bs=4096 count=8 status=none
 veritysetup format data.img hash.img --fec-device=fec.img --fec-roots=20 | \
 awk '/^Root hash/{ print $3 }' >roothash

 # create an erasure that should always be repairable with this roots setting
 dd if=/dev/zero of=data.img conv=notrunc bs=1 count=4 seek=4 status=none

 # try to read it through dm-verity
 veritysetup open data.img test hash.img --fec-device=fec.img --fec-roots=20 $(cat roothash)
 dd if=/dev/mapper/test of=/dev/null bs=4096 status=noxfer

 Even now the log says it cannot repair it:
   : verity-fec: 7:1: FEC 0: failed to correct: -74
   : device-mapper: verity: 7:1: data block 0 is corrupted
   ...

With this fix, errors are properly repaired.
   : verity-fec: 7:1: FEC 0: corrected 4 errors

Signed-off-by: Milan Broz <gmazyland@gmail.com>
Fixes: 8ca7cab8 ("dm verity fec: fix misaligned RS roots IO")
Cc: stable@vger.kernel.org
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Milan Broz <gmazyland@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>