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

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Date: Sun, 28 Jan 2024 22:25:23 +0100
From: Samuel Ortiz <sameo@...osinc.com>
To: Dan Williams <dan.j.williams@...el.com>
Cc: Samuel Ortiz <sameo@...osinc.com>,
	Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>,
	Qinkun Bao <qinkun@...gle.com>,
	"Yao, Jiewen" <jiewen.yao@...el.com>,
	"Xing, Cedric" <cedric.xing@...el.com>,
	Dionna Amalie Glaze <dionnaglaze@...gle.com>,
	biao.lu@...el.com,
	linux-coco@...ts.linux.dev,
	linux-integrity@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [RFC PATCH v2 4/4] tsm: Allow for extending and reading configured RTMRs

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@...osinc.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/$name/digest
+Date:		January, 2024
+KernelVersion:	v6.8
+Contact:	linux-coco@...ts.linux.dev
+Description:
+		(RW) The value in this attribute is the Runtime Measurement
+		Register (RTMR) digest. Callers can extend this digest with
+		additional hashes by writing into it. Binary blobs written to
+		this attribute must be of the exact length used by the hash
+		algorithm for this RTMR.
+
 What:		/sys/kernel/config/tsm/rtmrs/$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