[RFC PATCH v2 4/4] tsm: Allow for extending and reading configured RTMRs

[Samuel Ortiz <sameo@rivosinc.com>]

The whole purpose of TSM supported RTMRs is for userspace to extend them
with runtime measurements and to read them back.

This can be done through a binary configfs attribute for each RTMR:

rtmr0=/sys/kernel/config/tsm/rtmrs/rtmr0
mkdir $rtmr0
echo 0 > $rtmr0/index
dd if=software_layer_digest > $rtmr0/digest
hexdump $rtmr0/digest

An RTMR digest can not be extended or read before the RTMR is configured
by assigning it an index.

Signed-off-by: Samuel Ortiz <sameo@rivosinc.com>
---
 Documentation/ABI/testing/configfs-tsm | 11 +++++
 drivers/virt/coco/Kconfig              |  1 +
 drivers/virt/coco/tsm.c                | 58 ++++++++++++++++++++++++++
 3 files changed, 70 insertions(+)

diff --git a/Documentation/ABI/testing/configfs-tsm b/Documentation/ABI/testing/configfs-tsm
index 5d20a872475e..dc5c68a49625 100644
--- a/Documentation/ABI/testing/configfs-tsm
+++ b/Documentation/ABI/testing/configfs-tsm
@@ -81,6 +81,17 @@ Description:
 		(RO) Indicates the minimum permissible value that can be written
 		to @privlevel.
 
+What:		/sys/kernel/config/tsm/rtmrs/$name/digest
+Date:		January, 2024
+KernelVersion:	v6.8
+Contact:	linux-coco@lists.linux.dev
+Description:
+		(RW) The value in this attribute is the Runtime Measurement
+		Register (RTMR) digest. Callers can extend this digest with
+		additional hashes by writing into it. Binary blobs written to
+		this attribute must be of the exact length used by the hash
+		algorithm for this RTMR.
+
 What:		/sys/kernel/config/tsm/rtmrs/$name/index
 Date:		January, 2024
 KernelVersion:	v6.8
diff --git a/drivers/virt/coco/Kconfig b/drivers/virt/coco/Kconfig
index 87d142c1f932..5d924bae1ed8 100644
--- a/drivers/virt/coco/Kconfig
+++ b/drivers/virt/coco/Kconfig
@@ -5,6 +5,7 @@
 
 config TSM_REPORTS
 	select CONFIGFS_FS
+	select CRYPTO_HASH_INFO
 	tristate
 
 source "drivers/virt/coco/efi_secret/Kconfig"
diff --git a/drivers/virt/coco/tsm.c b/drivers/virt/coco/tsm.c
index d03cf5173bc9..b4f8cf6ca149 100644
--- a/drivers/virt/coco/tsm.c
+++ b/drivers/virt/coco/tsm.c
@@ -551,6 +551,63 @@ static struct configfs_attribute *tsm_rtmr_attrs[] = {
 	NULL,
 };
 
+static ssize_t tsm_rtmr_digest_read(struct config_item *cfg, void *buf,
+				    size_t count)
+{
+	struct tsm_rtmr_state *rtmr_state = to_tsm_rtmr_state(cfg);
+	int rc, digest_size = hash_digest_size[rtmr_state->alg];
+
+	/* configfs is asking for the digest size */
+	if (!buf)
+		return digest_size;
+
+	if (!is_rtmr_configured(rtmr_state))
+		return -ENXIO;
+
+	if (count > TSM_DIGEST_MAX || count < digest_size)
+		return -EINVAL;
+
+	/* Read from the cached digest */
+	if (rtmr_state->cached_digest) {
+		memcpy(buf, rtmr_state->digest, count);
+		return digest_size;
+	}
+
+	/* Slow path, this RTMR got extended */
+	guard(rwsem_write)(&tsm_rwsem);
+	rc = tsm_rtmr_read(&provider, rtmr_state->index, buf, count);
+	if (rc < 0)
+		return rc;
+
+	/* Update the cached digest */
+	memcpy(rtmr_state->digest, buf, count);
+	rtmr_state->cached_digest = true;
+
+	return rc;
+}
+
+static ssize_t tsm_rtmr_digest_write(struct config_item *cfg,
+				     const void *buf, size_t count)
+{
+	struct tsm_rtmr_state *rtmr_state = to_tsm_rtmr_state(cfg);
+
+	if (!is_rtmr_configured(rtmr_state))
+		return -ENXIO;
+
+	if (count > TSM_DIGEST_MAX || count < hash_digest_size[rtmr_state->alg])
+		return -EINVAL;
+
+	guard(rwsem_write)(&tsm_rwsem);
+	rtmr_state->cached_digest = false;
+	return tsm_rtmr_extend(&provider, rtmr_state->index, buf, count);
+}
+CONFIGFS_BIN_ATTR(tsm_rtmr_, digest, NULL, TSM_DIGEST_MAX);
+
+static struct configfs_bin_attribute *tsm_rtmr_bin_attrs[] = {
+	&tsm_rtmr_attr_digest,
+	NULL,
+};
+
 static void tsm_rtmr_item_release(struct config_item *cfg)
 {
 	struct tsm_rtmr_state *state = to_tsm_rtmr_state(cfg);
@@ -564,6 +621,7 @@ static struct configfs_item_operations tsm_rtmr_item_ops = {
 
 const struct config_item_type tsm_rtmr_type = {
 	.ct_owner = THIS_MODULE,
+	.ct_bin_attrs = tsm_rtmr_bin_attrs,
 	.ct_attrs = tsm_rtmr_attrs,
 	.ct_item_ops = &tsm_rtmr_item_ops,
 };
-- 
2.42.0