dm verity: add 'check_at_most_once' option to only validate hashes once

This allows platforms that are CPU/memory contrained to verify data
blocks only the first time they are read from the data device, rather
than every time.  As such, it provides a reduced level of security
because only offline tampering of the data device's content will be
detected, not online tampering.

Hash blocks are still verified each time they are read from the hash
device, since verification of hash blocks is less performance critical
than data blocks, and a hash block will not be verified any more after
all the data blocks it covers have been verified anyway.

This option introduces a bitset that is used to check if a block has
been validated before or not.  A block can be validated more than once
as there is no thread protection for the bitset.

These changes were developed and tested on entry-level Android Go
devices.

Signed-off-by: Patrik Torstensson <totte@google.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

1 files changed, 59 insertions, 5 deletions

diff --git a/drivers/md/dm-verity-target.c b/drivers/md/dm-verity-target.c
index aedb8222836b..14c620992794 100644
--- a/drivers/md/dm-verity-target.c
+++ b/drivers/md/dm-verity-target.c
@@ -32,6 +32,7 @@
 #define DM_VERITY_OPT_LOGGING		"ignore_corruption"
 #define DM_VERITY_OPT_RESTART		"restart_on_corruption"
 #define DM_VERITY_OPT_IGN_ZEROES	"ignore_zero_blocks"
+#define DM_VERITY_OPT_AT_MOST_ONCE	"check_at_most_once"
 
 #define DM_VERITY_OPTS_MAX		(2 + DM_VERITY_OPTS_FEC)
 
@@ -433,6 +434,18 @@ static int verity_bv_zero(struct dm_verity *v, struct dm_verity_io *io,
 }
 
 /*
+ * Moves the bio iter one data block forward.
+ */
+static inline void verity_bv_skip_block(struct dm_verity *v,
+					struct dm_verity_io *io,
+					struct bvec_iter *iter)
+{
+	struct bio *bio = dm_bio_from_per_bio_data(io, v->ti->per_io_data_size);
+
+	bio_advance_iter(bio, iter, 1 << v->data_dev_block_bits);
+}
+
+/*
  * Verify one "dm_verity_io" structure.
  */
 static int verity_verify_io(struct dm_verity_io *io)
@@ -445,9 +458,16 @@ static int verity_verify_io(struct dm_verity_io *io)
 
 	for (b = 0; b < io->n_blocks; b++) {
 		int r;
+		sector_t cur_block = io->block + b;
 		struct ahash_request *req = verity_io_hash_req(v, io);
 
-		r = verity_hash_for_block(v, io, io->block + b,
+		if (v->validated_blocks &&
+		    likely(test_bit(cur_block, v->validated_blocks))) {
+			verity_bv_skip_block(v, io, &io->iter);
+			continue;
+		}
+
+		r = verity_hash_for_block(v, io, cur_block,
 					  verity_io_want_digest(v, io),
 					  &is_zero);
 		if (unlikely(r < 0))
@@ -481,13 +501,16 @@ static int verity_verify_io(struct dm_verity_io *io)
 			return r;
 
 		if (likely(memcmp(verity_io_real_digest(v, io),
-				  verity_io_want_digest(v, io), v->digest_size) == 0))
+				  verity_io_want_digest(v, io), v->digest_size) == 0)) {
+			if (v->validated_blocks)
+				set_bit(cur_block, v->validated_blocks);
 			continue;
+		}
 		else if (verity_fec_decode(v, io, DM_VERITY_BLOCK_TYPE_DATA,
-					   io->block + b, NULL, &start) == 0)
+					   cur_block, NULL, &start) == 0)
 			continue;
 		else if (verity_handle_err(v, DM_VERITY_BLOCK_TYPE_DATA,
-					   io->block + b))
+					   cur_block))
 			return -EIO;
 	}
 
@@ -673,6 +696,8 @@ static void verity_status(struct dm_target *ti, status_type_t type,
 			args += DM_VERITY_OPTS_FEC;
 		if (v->zero_digest)
 			args++;
+		if (v->validated_blocks)
+			args++;
 		if (!args)
 			return;
 		DMEMIT(" %u", args);
@@ -691,6 +716,8 @@ static void verity_status(struct dm_target *ti, status_type_t type,
 		}
 		if (v->zero_digest)
 			DMEMIT(" " DM_VERITY_OPT_IGN_ZEROES);
+		if (v->validated_blocks)
+			DMEMIT(" " DM_VERITY_OPT_AT_MOST_ONCE);
 		sz = verity_fec_status_table(v, sz, result, maxlen);
 		break;
 	}
@@ -740,6 +767,7 @@ static void verity_dtr(struct dm_target *ti)
 	if (v->bufio)
 		dm_bufio_client_destroy(v->bufio);
 
+	kvfree(v->validated_blocks);
 	kfree(v->salt);
 	kfree(v->root_digest);
 	kfree(v->zero_digest);
@@ -760,6 +788,26 @@ static void verity_dtr(struct dm_target *ti)
 	kfree(v);
 }
 
+static int verity_alloc_most_once(struct dm_verity *v)
+{
+	struct dm_target *ti = v->ti;
+
+	/* the bitset can only handle INT_MAX blocks */
+	if (v->data_blocks > INT_MAX) {
+		ti->error = "device too large to use check_at_most_once";
+		return -E2BIG;
+	}
+
+	v->validated_blocks = kvzalloc(BITS_TO_LONGS(v->data_blocks) *
+				       sizeof(unsigned long), GFP_KERNEL);
+	if (!v->validated_blocks) {
+		ti->error = "failed to allocate bitset for check_at_most_once";
+		return -ENOMEM;
+	}
+
+	return 0;
+}
+
 static int verity_alloc_zero_digest(struct dm_verity *v)
 {
 	int r = -ENOMEM;
@@ -829,6 +877,12 @@ static int verity_parse_opt_args(struct dm_arg_set *as, struct dm_verity *v)
 			}
 			continue;
 
+		} else if (!strcasecmp(arg_name, DM_VERITY_OPT_AT_MOST_ONCE)) {
+			r = verity_alloc_most_once(v);
+			if (r)
+				return r;
+			continue;
+
 		} else if (verity_is_fec_opt_arg(arg_name)) {
 			r = verity_fec_parse_opt_args(as, v, &argc, arg_name);
 			if (r)
@@ -1096,7 +1150,7 @@ bad:
 
 static struct target_type verity_target = {
 	.name		= "verity",
-	.version	= {1, 3, 0},
+	.version	= {1, 4, 0},
 	.module		= THIS_MODULE,
 	.ctr		= verity_ctr,
 	.dtr		= verity_dtr,