[dm-devel] [PATCH v2 1/2] dm-integrity: disable recalculate due to secrurity reasons

[Mikulas Patocka]

Disable recalculating if we use internal_hash or journal_hash with a key
and we don't have the "legacy_recalculate" flag.

This may break activations of volumes that were created by an older kernel
and that are not yet fully recalculated - if this happens, the user should
add the "legacy_recalculate" flag to constructor parameters.

Signed-off-by: Mikulas Patocka <mpatocka at redhat.com>
Reported-by: Daniel Glockner <dg at emlix.com>

Index: linux-2.6/drivers/md/dm-integrity.c
===================================================================
--- linux-2.6.orig/drivers/md/dm-integrity.c
+++ linux-2.6/drivers/md/dm-integrity.c
@@ -257,8 +257,9 @@ struct dm_integrity_c {
 	bool journal_uptodate;
 	bool just_formatted;
 	bool recalculate_flag;
-	bool fix_padding;
 	bool discard;
+	bool fix_padding;
+	bool legacy_recalculate;
 
 	struct alg_spec internal_hash_alg;
 	struct alg_spec journal_crypt_alg;
@@ -386,6 +387,14 @@ static int dm_integrity_failed(struct dm
 	return READ_ONCE(ic->failed);
 }
 
+static bool dm_integrity_disable_recalculate(struct dm_integrity_c *ic)
+{
+	if ((ic->internal_hash_alg.key || ic->journal_mac_alg.key) &&
+	    !ic->legacy_recalculate)
+		return true;
+	return false;
+}
+
 static commit_id_t dm_integrity_commit_id(struct dm_integrity_c *ic, unsigned i,
 					  unsigned j, unsigned char seq)
 {
@@ -3140,6 +3149,7 @@ static void dm_integrity_status(struct d
 		arg_count += !!ic->journal_crypt_alg.alg_string;
 		arg_count += !!ic->journal_mac_alg.alg_string;
 		arg_count += (ic->sb->flags & cpu_to_le32(SB_FLAG_FIXED_PADDING)) != 0;
+		arg_count += ic->legacy_recalculate;
 		DMEMIT("%s %llu %u %c %u", ic->dev->name, ic->start,
 		       ic->tag_size, ic->mode, arg_count);
 		if (ic->meta_dev)
@@ -3163,6 +3173,8 @@ static void dm_integrity_status(struct d
 		}
 		if ((ic->sb->flags & cpu_to_le32(SB_FLAG_FIXED_PADDING)) != 0)
 			DMEMIT(" fix_padding");
+		if (ic->legacy_recalculate)
+			DMEMIT(" legacy_recalculate");
 
 #define EMIT_ALG(a, n)							\
 		do {							\
@@ -3792,7 +3804,7 @@ static int dm_integrity_ctr(struct dm_ta
 	unsigned extra_args;
 	struct dm_arg_set as;
 	static const struct dm_arg _args[] = {
-		{0, 15, "Invalid number of feature args"},
+		{0, 16, "Invalid number of feature args"},
 	};
 	unsigned journal_sectors, interleave_sectors, buffer_sectors, journal_watermark, sync_msec;
 	bool should_write_sb;
@@ -3940,6 +3952,8 @@ static int dm_integrity_ctr(struct dm_ta
 			ic->discard = true;
 		} else if (!strcmp(opt_string, "fix_padding")) {
 			ic->fix_padding = true;
+		} else if (!strcmp(opt_string, "legacy_recalculate")) {
+			ic->legacy_recalculate = true;
 		} else {
 			r = -EINVAL;
 			ti->error = "Invalid argument";
@@ -4243,6 +4257,14 @@ try_smaller_buffer:
 		}
 	}
 
+	if (ic->sb->flags & cpu_to_le32(SB_FLAG_RECALCULATING) &&
+	    le64_to_cpu(ic->sb->recalc_sector) < ic->provided_data_sectors &&
+	    dm_integrity_disable_recalculate(ic)) {
+		ti->error = "Recalculating with HMAC is disabled for security reasons - if you really need it, use the argument \"legacy_recalculate\"";
+		r = -EOPNOTSUPP;
+		goto bad;
+	}
+
 	ic->bufio = dm_bufio_client_create(ic->meta_dev ? ic->meta_dev->bdev : ic->dev->bdev,
 			1U << (SECTOR_SHIFT + ic->log2_buffer_sectors), 1, 0, NULL, NULL);
 	if (IS_ERR(ic->bufio)) {
Index: linux-2.6/Documentation/admin-guide/device-mapper/dm-integrity.rst
===================================================================
--- linux-2.6.orig/Documentation/admin-guide/device-mapper/dm-integrity.rst
+++ linux-2.6/Documentation/admin-guide/device-mapper/dm-integrity.rst
@@ -177,14 +177,20 @@ bitmap_flush_interval:number
 	The bitmap flush interval in milliseconds. The metadata buffers
 	are synchronized when this interval expires.
 
+allow_discards
+	Allow block discard requests (a.k.a. TRIM) for the integrity device.
+	Discards are only allowed to devices using internal hash.
+
 fix_padding
 	Use a smaller padding of the tag area that is more
 	space-efficient. If this option is not present, large padding is
 	used - that is for compatibility with older kernels.
 
-allow_discards
-	Allow block discard requests (a.k.a. TRIM) for the integrity device.
-	Discards are only allowed to devices using internal hash.
+legacy_recalculate
+	Allow recalculating of volumes with HMAC keys. This is disabled by
+	default for security reasons - an attacker could modify the volume,
+	set recalc_sector to zero, and the kernel would not detect the
+	modification.
 
 The journal mode (D/J), buffer_sectors, journal_watermark, commit_time and
 allow_discards can be changed when reloading the target (load an inactive
integrity-hmac-superblock.patch