dm-verity: disable recursive forward error correction

There are two problems with the recursive correction:

1. It may cause denial-of-service. In fec_read_bufs, there is a loop that
has 253 iterations. For each iteration, we may call verity_hash_for_block
recursively. There is a limit of 4 nested recursions - that means that
there may be at most 253^4 (4 billion) iterations. Red Hat QE team
actually created an image that pushes dm-verity to this limit - and this
image just makes the udev-worker process get stuck in the 'D' state.

2. It doesn't work. In fec_read_bufs we store data into the variable
"fio->bufs", but fio bufs is shared between recursive invocations, if
"verity_hash_for_block" invoked correction recursively, it would
overwrite partially filled fio->bufs.

Signed-off-by: Mikulas Patocka <[e-mail]>
Reported-by: Guangwu Zhang <[e-mail]>
Reviewed-by: Sami Tolvanen <[e-mail]>
Reviewed-by: Eric Biggers <[e-mail]>

drivers/md/dm-verity-fec.c

···
 	if (!verity_fec_is_enabled(v))
 		return -EOPNOTSUPP;
 
-	if (fio->level >= DM_VERITY_FEC_MAX_RECURSION) {
-		DMWARN_LIMIT("%s: FEC: recursion too deep", v->data_dev->name);
+	if (fio->level)
 		return -EIO;
-	}
 
 	fio->level++;
 

drivers/md/dm-verity-fec.h

···
 #define DM_VERITY_FEC_BUF_MAX \
 	(1 << (PAGE_SHIFT - DM_VERITY_FEC_BUF_RS_BITS))
 
-/* maximum recursion level for verity_fec_decode */
-#define DM_VERITY_FEC_MAX_RECURSION	4
-
 #define DM_VERITY_OPT_FEC_DEV		"use_fec_from_device"
 #define DM_VERITY_OPT_FEC_BLOCKS	"fec_blocks"
 #define DM_VERITY_OPT_FEC_START		"fec_start"

drivers/md/dm-verity-target.c

···
 	.name		= "verity",
 /* Note: the LSMs depend on the singleton and immutable features */
 	.features	= DM_TARGET_SINGLETON | DM_TARGET_IMMUTABLE,
-	.version	= {1, 12, 0},
+	.version	= {1, 13, 0},
 	.module		= THIS_MODULE,
 	.ctr		= verity_ctr,
 	.dtr		= verity_dtr,