Commit d9f3e47d authored by Mikulas Patocka's avatar Mikulas Patocka
Browse files

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: default avatarMikulas Patocka <mpatocka@redhat.com>
Reported-by: default avatarGuangwu Zhang <guazhang@redhat.com>
Reviewed-by: default avatarSami Tolvanen <samitolvanen@google.com>
Reviewed-by: default avatarEric Biggers <ebiggers@kernel.org>
parent 7fa3e7d1

drivers/md/dm-verity-fec.c

+1 −3
Original line number Diff line number Diff line
@@ -418,10 +418,8 @@ int verity_fec_decode(struct dm_verity *v, struct dm_verity_io *io, 
	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

+0 −3
Original line number Diff line number Diff line
@@ -23,9 +23,6 @@ 
#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

+1 −1
Original line number Diff line number Diff line
@@ -1789,7 +1789,7 @@ static struct target_type verity_target = { 
	.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,