Support CopyBlocks= for Verity={hash,sig} (#41393)

This enables deriving the minimum size of the `Verity=hash` partition
using the `Verity=` logic when the size of the `Verity=data` partition
is bigger than the `CopyBlocks=` target.

This enables using `Minimize=true` for an "installer image" and later
using sd-repart to install to a system with reserve space for future
updates by specifying `Size{Min,Max}Bytes=` only in the `Verity=data`
partition, without needing to hardcode the corresponding size for the
`Verity=hash` partition.

While not strictly necessary for `Verity=signature` partitions (since
they have a fixed size) there isn't too much reason to not support it,
since then you can still specify `VerityMatchKey=` to indicate that the
partition is logically still part of that group of partitions.

---

Alternative to: https://github.com/systemd/systemd/pull/41156
Fixes https://github.com/systemd/systemd/issues/40995

src/repart/repart.c

@@ -3012,10 +3012,20 @@ static int partition_read_definition(
                                   "VerityMatchKey= can only be set if Verity= is not \"%s\".",
                                   verity_mode_to_string(p->verity));
 
-        if (IN_SET(p->verity, VERITY_HASH, VERITY_SIG) && (p->copy_blocks_path || p->copy_blocks_auto || p->format || partition_needs_populate(p)))
-                return log_syntax(NULL, LOG_ERR, path, 1, SYNTHETIC_ERRNO(EINVAL),
-                                  "CopyBlocks=/CopyFiles=/Format=/MakeDirectories=/MakeSymlinks= cannot be used with Verity=%s.",
-                                  verity_mode_to_string(p->verity));
+        if (IN_SET(p->verity, VERITY_HASH, VERITY_SIG)) {
+                if (p->format || partition_needs_populate(p))
+                        return log_syntax(NULL, LOG_ERR, path, 1, SYNTHETIC_ERRNO(EINVAL),
+                                          "CopyFiles=/Format=/MakeDirectories=/MakeSymlinks= cannot be used with Verity=%s.",
+                                          verity_mode_to_string(p->verity));
+
+                /* Later we check that the same CopyBlocks= type (auto vs path) is used for the entire verity set.
+                 * So we assume that CopyBlocks=auto is going to be correct and just path based blocks might result in
+                 * a broken setup */
+                if (p->copy_blocks_path)
+                        log_syntax(NULL, LOG_DEBUG, path, 1, 0,
+                                   "CopyBlocks= with Verity=%s bypasses dm-verity hash/signature computation; repart cannot verify the resulting setup is correct.",
+                                   verity_mode_to_string(p->verity));
+        }
 
         if (p->verity != VERITY_OFF && p->encrypt != ENCRYPT_OFF)
                 return log_syntax(NULL, LOG_ERR, path, 1, SYNTHETIC_ERRNO(EINVAL),
@@ -3516,6 +3526,27 @@ static int context_read_definitions(Context *context) {
                 }
         }
 
+        LIST_FOREACH(partitions, p, context->partitions) {
+                if (!IN_SET(p->verity, VERITY_HASH, VERITY_SIG))
+                        continue;
+
+                /* We check all verity siblings up until our current type and ensure that if we are using CopyBlocks=
+                 * the previous ones are using the same type of CopyBlocks=. */
+                for (VerityMode mode = VERITY_DATA; mode < p->verity; mode++) {
+                        Partition *q = ASSERT_PTR(p->siblings[mode]);
+
+                        if (p->copy_blocks_auto && !q->copy_blocks_auto)
+                                return log_syntax(NULL, LOG_ERR, p->definition_path, 1, SYNTHETIC_ERRNO(EINVAL),
+                                                  "CopyBlocks=auto set with Verity=%s but Verity=%s partition does not set CopyBlocks=auto.",
+                                                  verity_mode_to_string(p->verity), verity_mode_to_string(mode));
+
+                        if (p->copy_blocks_path && !q->copy_blocks_path)
+                                return log_syntax(NULL, LOG_ERR, p->definition_path, 1, SYNTHETIC_ERRNO(EINVAL),
+                                                  "CopyBlocks= set with Verity=%s but Verity=%s partition does not set CopyBlocks=.",
+                                                  verity_mode_to_string(p->verity), verity_mode_to_string(mode));
+                }
+        }
+
         LIST_FOREACH(partitions, p, context->partitions) {
                 Partition *dp;
 
@@ -5658,7 +5689,7 @@ static int partition_format_verity_hash(
         if (PARTITION_EXISTS(p)) /* Never format existing partitions */
                 return 0;
 
-        /* Minimized partitions will use the copy blocks logic so skip those here. */
+        /* Either we are minimizing the partition or we were instructed to copy an existing hash block directly. */
         if (p->copy_blocks_fd >= 0)
                 return 0;
 
@@ -5795,21 +5826,41 @@ static int sign_verity_roothash(
 #endif
 }
 
-static const VeritySettings *lookup_verity_settings_by_uuid_pair(sd_id128_t data_uuid, sd_id128_t hash_uuid) {
-        uint8_t root_hash_key[sizeof(sd_id128_t) * 2];
+static int iovec_roothash_from_uuid_pair(
+                sd_id128_t data_uuid,
+                sd_id128_t hash_uuid,
+                struct iovec *ret_roothash) {
+
+        uint8_t roothash_bytes[sizeof(sd_id128_t) * 2];
+
+        assert(ret_roothash);
 
         if (sd_id128_is_null(data_uuid) || sd_id128_is_null(hash_uuid))
-                return NULL;
+                return -EINVAL;
 
         /* As per the https://uapi-group.org/specifications/specs/discoverable_partitions_specification/ the
          * UUIDs of the data and verity partitions are respectively the first and second halves of the
          * dm-verity roothash, so we can use them to match the signature to the right partition. */
 
-        memcpy(root_hash_key, data_uuid.bytes, sizeof(sd_id128_t));
-        memcpy(root_hash_key + sizeof(sd_id128_t), hash_uuid.bytes, sizeof(sd_id128_t));
+        memcpy(roothash_bytes, data_uuid.bytes, sizeof(sd_id128_t));
+        memcpy(roothash_bytes + sizeof(sd_id128_t), hash_uuid.bytes, sizeof(sd_id128_t));
+
+        if (!iovec_memdup(&IOVEC_MAKE(roothash_bytes, sizeof(roothash_bytes)), ret_roothash))
+                return -ENOMEM;
+
+        return 0;
+}
+
+static const VeritySettings *lookup_verity_settings_by_uuid_pair(sd_id128_t data_uuid, sd_id128_t hash_uuid) {
+        _cleanup_(iovec_done) struct iovec roothash = {};
+        int r;
+
+        r = iovec_roothash_from_uuid_pair(data_uuid, hash_uuid, &roothash);
+        if (r < 0)
+                return NULL;
 
         VeritySettings key = {
-                .root_hash = IOVEC_MAKE(root_hash_key, sizeof(root_hash_key)),
+                .root_hash = roothash,
         };
 
         return set_get(arg_verity_settings, &key);
@@ -5832,11 +5883,23 @@ static int partition_format_verity_sig(Context *context, Partition *p) {
         if (PARTITION_EXISTS(p))
                 return 0;
 
+        /* We were instructed to copy an existing signature block directly */
+        if (p->copy_blocks_fd >= 0)
+                return 0;
+
         assert_se(hp = p->siblings[VERITY_HASH]);
         assert(!hp->dropped);
         assert_se(rp = p->siblings[VERITY_DATA]);
         assert(!rp->dropped);
 
+        /* Currently only set while formatting the hash partition. But if this is skipped via CopyBlocks=
+         * we just derive the roothash from the UUIDs from the data + hash partition. */
+        if (!iovec_is_set(&hp->roothash)) {
+                r = iovec_roothash_from_uuid_pair(rp->new_uuid, hp->new_uuid, &hp->roothash);
+                if (r < 0)
+                        return log_error_errno(r, "Unable to derive roothash: %m");
+        }
+
         verity_settings = lookup_verity_settings_by_uuid_pair(rp->current_uuid, hp->current_uuid);
 
         if (!verity_settings) {
@@ -9259,6 +9322,9 @@ static int context_minimize(Context *context) {
                 if (PARTITION_EXISTS(p)) /* Never format existing partitions */
                         continue;
 
+                if (p->copy_blocks_fd >= 0)
+                        continue;
+
                 if (p->minimize == MINIMIZE_OFF)
                         continue;