dm verity: add ignore_zero_blocks feature

If ignore_zero_blocks is enabled dm-verity will return zeroes for blocks
matching a zero hash without validating the content.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

4 files changed, 93 insertions, 10 deletions

diff --git a/Documentation/device-mapper/verity.txt b/Documentation/device-mapper/verity.txt
index d602c801ff59..89fd8f9a259f 100644
--- a/Documentation/device-mapper/verity.txt
+++ b/Documentation/device-mapper/verity.txt
@@ -79,6 +79,11 @@ restart_on_corruption
     not compatible with ignore_corruption and requires user space support to
     avoid restart loops.
 
+ignore_zero_blocks
+    Do not verify blocks that are expected to contain zeroes and always return
+    zeroes instead. This may be useful if the partition contains unused blocks
+    that are not guaranteed to contain zeroes.
+
 use_fec_from_device <fec_dev>
     Use forward error correction (FEC) to recover from corruption if hash
     verification fails. Use encoding data from the specified device. This
diff --git a/drivers/md/dm-verity-fec.c b/drivers/md/dm-verity-fec.c
index 88143d36a1d2..1cc10c4de701 100644
--- a/drivers/md/dm-verity-fec.c
+++ b/drivers/md/dm-verity-fec.c
@@ -205,6 +205,7 @@ static int fec_read_bufs(struct dm_verity *v, struct dm_verity_io *io,
                          u64 rsb, u64 target, unsigned block_offset,
                          int *neras)
 {
+        bool is_zero;
         int i, j, target_index = -1;
         struct dm_buffer *buf;
         struct dm_bufio_client *bufio;
@@ -264,7 +265,12 @@ static int fec_read_bufs(struct dm_verity *v, struct dm_verity_io *io,
 
                 /* locate erasures if the block is on the data device */
                 if (bufio == v->fec->data_bufio &&
-                    verity_hash_for_block(v, io, block, want_digest) == 0) {
+                    verity_hash_for_block(v, io, block, want_digest,
+                                          &is_zero) == 0) {
+                        /* skip known zero blocks entirely */
+                        if (is_zero)
+                                continue;
+
                         /*
                          * skip if we have already found the theoretical
                          * maximum number (i.e. fec->roots) of erasures
diff --git a/drivers/md/dm-verity-target.c b/drivers/md/dm-verity-target.c
index 4f90ec2c6b7a..5c5d30cb6ec5 100644
--- a/drivers/md/dm-verity-target.c
+++ b/drivers/md/dm-verity-target.c
@@ -31,8 +31,9 @@
 
 #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_OPTS_MAX              (1 + DM_VERITY_OPTS_FEC)
+#define DM_VERITY_OPTS_MAX              (2 + DM_VERITY_OPTS_FEC)
 
 static unsigned dm_verity_prefetch_cluster = DM_VERITY_DEFAULT_PREFETCH_SIZE;
 
@@ -309,10 +310,9 @@ release_ret_r:
  * of the hash tree if necessary.
  */
 int verity_hash_for_block(struct dm_verity *v, struct dm_verity_io *io,
-                          sector_t block, u8 *digest)
+                          sector_t block, u8 *digest, bool *is_zero)
 {
-        int i;
-        int r;
+        int r = 0, i;
 
         if (likely(v->levels)) {
                 /*
@@ -324,7 +324,7 @@ int verity_hash_for_block(struct dm_verity *v, struct dm_verity_io *io,
                  */
                 r = verity_verify_level(v, io, block, 0, true, digest);
                 if (likely(r <= 0))
-                        return r;
+                        goto out;
         }
 
         memcpy(digest, v->root_digest, v->digest_size);
@@ -332,10 +332,15 @@ int verity_hash_for_block(struct dm_verity *v, struct dm_verity_io *io,
         for (i = v->levels - 1; i >= 0; i--) {
                 r = verity_verify_level(v, io, block, i, false, digest);
                 if (unlikely(r))
-                        return r;
+                        goto out;
         }
+out:
+        if (!r && v->zero_digest)
+                *is_zero = !memcmp(v->zero_digest, digest, v->digest_size);
+        else
+                *is_zero = false;
 
-        return 0;
+        return r;
 }
 
 /*
@@ -382,11 +387,19 @@ static int verity_bv_hash_update(struct dm_verity *v, struct dm_verity_io *io,
         return verity_hash_update(v, verity_io_hash_desc(v, io), data, len);
 }
 
+static int verity_bv_zero(struct dm_verity *v, struct dm_verity_io *io,
+                          u8 *data, size_t len)
+{
+        memset(data, 0, len);
+        return 0;
+}
+
 /*
  * Verify one "dm_verity_io" structure.
  */
 static int verity_verify_io(struct dm_verity_io *io)
 {
+        bool is_zero;
         struct dm_verity *v = io->v;
         struct bvec_iter start;
         unsigned b;
@@ -396,10 +409,24 @@ static int verity_verify_io(struct dm_verity_io *io)
                 struct shash_desc *desc = verity_io_hash_desc(v, io);
 
                 r = verity_hash_for_block(v, io, io->block + b,
-                                          verity_io_want_digest(v, io));
+                                          verity_io_want_digest(v, io),
+                                          &is_zero);
                 if (unlikely(r < 0))
                         return r;
 
+                if (is_zero) {
+                        /*
+                         * If we expect a zero block, don't validate, just
+                         * return zeros.
+                         */
+                        r = verity_for_bv_block(v, io, &io->iter,
+                                                verity_bv_zero);
+                        if (unlikely(r < 0))
+                                return r;
+
+                        continue;
+                }
+
                 r = verity_hash_init(v, desc);
                 if (unlikely(r < 0))
                         return r;
@@ -604,6 +631,8 @@ static void verity_status(struct dm_target *ti, status_type_t type,
                         args++;
                 if (verity_fec_is_enabled(v))
                         args += DM_VERITY_OPTS_FEC;
+                if (v->zero_digest)
+                        args++;
                 if (!args)
                         return;
                 DMEMIT(" %u", args);
@@ -620,6 +649,8 @@ static void verity_status(struct dm_target *ti, status_type_t type,
                                 BUG();
                         }
                 }
+                if (v->zero_digest)
+                        DMEMIT(" " DM_VERITY_OPT_IGN_ZEROES);
                 sz = verity_fec_status_table(v, sz, result, maxlen);
                 break;
         }
@@ -671,6 +702,7 @@ static void verity_dtr(struct dm_target *ti)
 
         kfree(v->salt);
         kfree(v->root_digest);
+        kfree(v->zero_digest);
 
         if (v->tfm)
                 crypto_free_shash(v->tfm);
@@ -688,6 +720,37 @@ static void verity_dtr(struct dm_target *ti)
         kfree(v);
 }
 
+static int verity_alloc_zero_digest(struct dm_verity *v)
+{
+        int r = -ENOMEM;
+        struct shash_desc *desc;
+        u8 *zero_data;
+
+        v->zero_digest = kmalloc(v->digest_size, GFP_KERNEL);
+
+        if (!v->zero_digest)
+                return r;
+
+        desc = kmalloc(v->shash_descsize, GFP_KERNEL);
+
+        if (!desc)
+                return r; /* verity_dtr will free zero_digest */
+
+        zero_data = kzalloc(1 << v->data_dev_block_bits, GFP_KERNEL);
+
+        if (!zero_data)
+                goto out;
+
+        r = verity_hash(v, desc, zero_data, 1 << v->data_dev_block_bits,
+                        v->zero_digest);
+
+out:
+        kfree(desc);
+        kfree(zero_data);
+
+        return r;
+}
+
 static int verity_parse_opt_args(struct dm_arg_set *as, struct dm_verity *v)
 {
         int r;
@@ -718,6 +781,14 @@ static int verity_parse_opt_args(struct dm_arg_set *as, struct dm_verity *v)
                         v->mode = DM_VERITY_MODE_RESTART;
                         continue;
 
+                } else if (!strcasecmp(arg_name, DM_VERITY_OPT_IGN_ZEROES)) {
+                        r = verity_alloc_zero_digest(v);
+                        if (r) {
+                                ti->error = "Cannot allocate zero digest";
+                                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)
diff --git a/drivers/md/dm-verity.h b/drivers/md/dm-verity.h
index 8e853722f6c6..fb419f422d73 100644
--- a/drivers/md/dm-verity.h
+++ b/drivers/md/dm-verity.h
@@ -40,6 +40,7 @@ struct dm_verity {
         struct crypto_shash *tfm;
         u8 *root_digest;        /* digest of the root block */
         u8 *salt;               /* salt: its size is salt_size */
+        u8 *zero_digest;        /* digest for a zero block */
         unsigned salt_size;
         sector_t data_start;    /* data offset in 512-byte sectors */
         sector_t hash_start;    /* hash start in blocks */
@@ -123,6 +124,6 @@ extern int verity_hash(struct dm_verity *v, struct shash_desc *desc,
                        const u8 *data, size_t len, u8 *digest);
 
 extern int verity_hash_for_block(struct dm_verity *v, struct dm_verity_io *io,
-                                 sector_t block, u8 *digest);
+                                 sector_t block, u8 *digest, bool *is_zero);
 
 #endif /* DM_VERITY_H */