[PATCH v2] Add an option to dm-verity to validate hashes at most once

[Patrik Torstensson <totte@google.com>]

Add an option to dm-verity to validate hashes at most once
to allow platforms that is CPU/memory contraint to be
protected by dm-verity against offline attacks.

The 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.

This patch has been developed and tested on entry-level
Android Go devices.

Signed-off-by: Patrik Torstensson <totte@google.com>
---
Changes since version 1:
   - added documentation about the new option
   - added the new option into DM_TABLE_STATUS ioctl
   - replaced 'io->block + b' with 'cur_block'
   - increase minor version
 
 Documentation/device-mapper/verity.txt | 11 +++++
 drivers/md/dm-verity-target.c          | 66 ++++++++++++++++++++++++--
 drivers/md/dm-verity.h                 |  1 +
 3 files changed, 73 insertions(+), 5 deletions(-)

diff --git a/Documentation/device-mapper/verity.txt b/Documentation/device-mapper/verity.txt
index 89fd8f9a259f..b3d2e4a42255 100644
--- a/Documentation/device-mapper/verity.txt
+++ b/Documentation/device-mapper/verity.txt
@@ -109,6 +109,17 @@ fec_start <offset>
     This is the offset, in <data_block_size> blocks, from the start of the
     FEC device to the beginning of the encoding data.
 
+check_at_most_once
+    Verify data blocks only the first time they are read from the data device,
+    rather than every time.  This reduces the overhead of dm-verity so that it
+    can be used on systems that are memory and/or CPU constrained.  However, 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.
 
 Theory of operation
 ===================
diff --git a/drivers/md/dm-verity-target.c b/drivers/md/dm-verity-target.c
index aedb8222836b..195a899e7466 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)
 
@@ -432,6 +433,19 @@ static int verity_bv_zero(struct dm_verity *v, struct dm_verity_io *io,
 	return 0;
 }
 
+/*
+ * 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.
  */
@@ -445,9 +459,16 @@ 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 +502,17 @@ 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 +698,8 @@ 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 +718,8 @@ 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 +769,7 @@ 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 +790,26 @@ 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 +879,12 @@ 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 +1152,7 @@ static int verity_ctr(struct dm_target *ti, unsigned argc, char **argv)
 
 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,
diff --git a/drivers/md/dm-verity.h b/drivers/md/dm-verity.h
index b675bc015512..ace5ec781b5f 100644
--- a/drivers/md/dm-verity.h
+++ b/drivers/md/dm-verity.h
@@ -63,6 +63,7 @@ struct dm_verity {
 	sector_t hash_level_block[DM_VERITY_MAX_LEVELS];
 
 	struct dm_verity_fec *fec;	/* forward error correction */
+	unsigned long *validated_blocks; /* bitset blocks validated */
 };
 
 struct dm_verity_io {
-- 
2.17.0.rc0.231.g781580f067-goog