configfs-tsm: Introduce a shared ABI for attestation reports

One of the common operations of a TSM (Trusted Security Module) is to
provide a way for a TVM (confidential computing guest execution
environment) to take a measurement of its launch state, sign it and
submit it to a verifying party. Upon successful attestation that
verifies the integrity of the TVM additional secrets may be deployed.
The concept is common across TSMs, but the implementations are
unfortunately vendor specific. While the industry grapples with a common
definition of this attestation format [1], Linux need not make this
problem worse by defining a new ABI per TSM that wants to perform a
similar operation. The current momentum has been to invent new ioctl-ABI
per TSM per function which at best is an abdication of the kernel's
responsibility to make common infrastructure concepts share common ABI.

The proposal, targeted to conceptually work with TDX, SEV-SNP, COVE if
not more, is to define a configfs interface to retrieve the TSM-specific
blob.

    report=/sys/kernel/config/tsm/report/report0
    mkdir $report
    dd if=binary_userdata_plus_nonce > $report/inblob
    hexdump $report/outblob

This approach later allows for the standardization of the attestation
blob format without needing to invent a new ABI. Once standardization
happens the standard format can be emitted by $report/outblob and
indicated by $report/provider, or a new attribute like
"$report/tcg_coco_report" can emit the standard format alongside the
vendor format.

Review of previous iterations of this interface identified that there is
a need to scale report generation for multiple container environments
[2]. Configfs enables a model where each container can bind mount one or
more report generation item instances. Still, within a container only a
single thread can be manipulating a given configuration instance at a
time. A 'generation' count is provided to detect conflicts between
multiple threads racing to configure a report instance.

The SEV-SNP concepts of "extended reports" and "privilege levels" are
optionally enabled by selecting 'tsm_report_ext_type' at register_tsm()
time. The expectation is that those concepts are generic enough that
they may be adopted by other TSM implementations. In other words,
configfs-tsm aims to address a superset of TSM specific functionality
with a common ABI where attributes may appear, or not appear, based on
the set of concepts the implementation supports.

Link: http://lore.kernel.org/r/64961c3baf8ce_142af829436@dwillia2-xfh.jf.intel.com.notmuch [1]
Link: http://lore.kernel.org/r/57f3a05e-8fcd-4656-beea-56bb8365ae64@linux.microsoft.com [2]
Cc: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Cc: Dionna Amalie Glaze <dionnaglaze@google.com>
Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: Peter Gonda <pgonda@google.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Samuel Ortiz <sameo@rivosinc.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Tested-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

3 files changed, 431 insertions, 0 deletions

diff --git a/drivers/virt/coco/Kconfig b/drivers/virt/coco/Kconfig
index fc5c64f04c4a..87d142c1f932 100644
--- a/drivers/virt/coco/Kconfig
+++ b/drivers/virt/coco/Kconfig
@@ -2,6 +2,11 @@
 #
 # Confidential computing related collateral
 #
+
+config TSM_REPORTS
+	select CONFIGFS_FS
+	tristate
+
 source "drivers/virt/coco/efi_secret/Kconfig"
 
 source "drivers/virt/coco/sev-guest/Kconfig"
diff --git a/drivers/virt/coco/Makefile b/drivers/virt/coco/Makefile
index 55302ef719ad..18c1aba5edb7 100644
--- a/drivers/virt/coco/Makefile
+++ b/drivers/virt/coco/Makefile
@@ -2,6 +2,7 @@
 #
 # Confidential computing related collateral
 #
+obj-$(CONFIG_TSM_REPORTS)	+= tsm.o
 obj-$(CONFIG_EFI_SECRET)	+= efi_secret/
 obj-$(CONFIG_SEV_GUEST)		+= sev-guest/
 obj-$(CONFIG_INTEL_TDX_GUEST)	+= tdx-guest/
diff --git a/drivers/virt/coco/tsm.c b/drivers/virt/coco/tsm.c
new file mode 100644
index 000000000000..d1c2db83a8ca
--- /dev/null
+++ b/drivers/virt/coco/tsm.c
@@ -0,0 +1,425 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/* Copyright(c) 2023 Intel Corporation. All rights reserved. */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/tsm.h>
+#include <linux/err.h>
+#include <linux/slab.h>
+#include <linux/rwsem.h>
+#include <linux/string.h>
+#include <linux/module.h>
+#include <linux/cleanup.h>
+#include <linux/configfs.h>
+
+static struct tsm_provider {
+	const struct tsm_ops *ops;
+	const struct config_item_type *type;
+	void *data;
+} provider;
+static DECLARE_RWSEM(tsm_rwsem);
+
+/**
+ * DOC: Trusted Security Module (TSM) Attestation Report Interface
+ *
+ * The TSM report interface is a common provider of blobs that facilitate
+ * attestation of a TVM (confidential computing guest) by an attestation
+ * service. A TSM report combines a user-defined blob (likely a public-key with
+ * a nonce for a key-exchange protocol) with a signed attestation report. That
+ * combined blob is then used to obtain secrets provided by an agent that can
+ * validate the attestation report. The expectation is that this interface is
+ * invoked infrequently, however configfs allows for multiple agents to
+ * own their own report generation instances to generate reports as
+ * often as needed.
+ *
+ * The attestation report format is TSM provider specific, when / if a standard
+ * materializes that can be published instead of the vendor layout. Until then
+ * the 'provider' attribute indicates the format of 'outblob', and optionally
+ * 'auxblob'.
+ */
+
+struct tsm_report_state {
+	struct tsm_report report;
+	unsigned long write_generation;
+	unsigned long read_generation;
+	struct config_item cfg;
+};
+
+enum tsm_data_select {
+	TSM_REPORT,
+	TSM_CERTS,
+};
+
+static struct tsm_report *to_tsm_report(struct config_item *cfg)
+{
+	struct tsm_report_state *state =
+		container_of(cfg, struct tsm_report_state, cfg);
+
+	return &state->report;
+}
+
+static struct tsm_report_state *to_state(struct tsm_report *report)
+{
+	return container_of(report, struct tsm_report_state, report);
+}
+
+static int try_advance_write_generation(struct tsm_report *report)
+{
+	struct tsm_report_state *state = to_state(report);
+
+	lockdep_assert_held_write(&tsm_rwsem);
+
+	/*
+	 * Malicious or broken userspace has written enough times for
+	 * read_generation == write_generation by modular arithmetic without an
+	 * interim read. Stop accepting updates until the current report
+	 * configuration is read.
+	 */
+	if (state->write_generation == state->read_generation - 1)
+		return -EBUSY;
+	state->write_generation++;
+	return 0;
+}
+
+static ssize_t tsm_report_privlevel_store(struct config_item *cfg,
+					  const char *buf, size_t len)
+{
+	struct tsm_report *report = to_tsm_report(cfg);
+	unsigned int val;
+	int rc;
+
+	rc = kstrtouint(buf, 0, &val);
+	if (rc)
+		return rc;
+
+	/*
+	 * The valid privilege levels that a TSM might accept, if it accepts a
+	 * privilege level setting at all, are a max of TSM_PRIVLEVEL_MAX (see
+	 * SEV-SNP GHCB) and a minimum of a TSM selected floor value no less
+	 * than 0.
+	 */
+	if (provider.ops->privlevel_floor > val || val > TSM_PRIVLEVEL_MAX)
+		return -EINVAL;
+
+	guard(rwsem_write)(&tsm_rwsem);
+	rc = try_advance_write_generation(report);
+	if (rc)
+		return rc;
+	report->desc.privlevel = val;
+
+	return len;
+}
+CONFIGFS_ATTR_WO(tsm_report_, privlevel);
+
+static ssize_t tsm_report_privlevel_floor_show(struct config_item *cfg,
+					       char *buf)
+{
+	guard(rwsem_read)(&tsm_rwsem);
+	return sysfs_emit(buf, "%u\n", provider.ops->privlevel_floor);
+}
+CONFIGFS_ATTR_RO(tsm_report_, privlevel_floor);
+
+static ssize_t tsm_report_inblob_write(struct config_item *cfg,
+				       const void *buf, size_t count)
+{
+	struct tsm_report *report = to_tsm_report(cfg);
+	int rc;
+
+	guard(rwsem_write)(&tsm_rwsem);
+	rc = try_advance_write_generation(report);
+	if (rc)
+		return rc;
+
+	report->desc.inblob_len = count;
+	memcpy(report->desc.inblob, buf, count);
+	return count;
+}
+CONFIGFS_BIN_ATTR_WO(tsm_report_, inblob, NULL, TSM_INBLOB_MAX);
+
+static ssize_t tsm_report_generation_show(struct config_item *cfg, char *buf)
+{
+	struct tsm_report *report = to_tsm_report(cfg);
+	struct tsm_report_state *state = to_state(report);
+
+	guard(rwsem_read)(&tsm_rwsem);
+	return sysfs_emit(buf, "%lu\n", state->write_generation);
+}
+CONFIGFS_ATTR_RO(tsm_report_, generation);
+
+static ssize_t tsm_report_provider_show(struct config_item *cfg, char *buf)
+{
+	guard(rwsem_read)(&tsm_rwsem);
+	return sysfs_emit(buf, "%s\n", provider.ops->name);
+}
+CONFIGFS_ATTR_RO(tsm_report_, provider);
+
+static ssize_t __read_report(struct tsm_report *report, void *buf, size_t count,
+			     enum tsm_data_select select)
+{
+	loff_t offset = 0;
+	ssize_t len;
+	u8 *out;
+
+	if (select == TSM_REPORT) {
+		out = report->outblob;
+		len = report->outblob_len;
+	} else {
+		out = report->auxblob;
+		len = report->auxblob_len;
+	}
+
+	/*
+	 * Recall that a NULL @buf is configfs requesting the size of
+	 * the buffer.
+	 */
+	if (!buf)
+		return len;
+	return memory_read_from_buffer(buf, count, &offset, out, len);
+}
+
+static ssize_t read_cached_report(struct tsm_report *report, void *buf,
+				  size_t count, enum tsm_data_select select)
+{
+	struct tsm_report_state *state = to_state(report);
+
+	guard(rwsem_read)(&tsm_rwsem);
+	if (!report->desc.inblob_len)
+		return -EINVAL;
+
+	/*
+	 * A given TSM backend always fills in ->outblob regardless of
+	 * whether the report includes an auxblob or not.
+	 */
+	if (!report->outblob ||
+	    state->read_generation != state->write_generation)
+		return -EWOULDBLOCK;
+
+	return __read_report(report, buf, count, select);
+}
+
+static ssize_t tsm_report_read(struct tsm_report *report, void *buf,
+			       size_t count, enum tsm_data_select select)
+{
+	struct tsm_report_state *state = to_state(report);
+	const struct tsm_ops *ops;
+	ssize_t rc;
+
+	/* try to read from the existing report if present and valid... */
+	rc = read_cached_report(report, buf, count, select);
+	if (rc >= 0 || rc != -EWOULDBLOCK)
+		return rc;
+
+	/* slow path, report may need to be regenerated... */
+	guard(rwsem_write)(&tsm_rwsem);
+	ops = provider.ops;
+	if (!ops)
+		return -ENOTTY;
+	if (!report->desc.inblob_len)
+		return -EINVAL;
+
+	/* did another thread already generate this report? */
+	if (report->outblob &&
+	    state->read_generation == state->write_generation)
+		goto out;
+
+	kvfree(report->outblob);
+	kvfree(report->auxblob);
+	report->outblob = NULL;
+	report->auxblob = NULL;
+	rc = ops->report_new(report, provider.data);
+	if (rc < 0)
+		return rc;
+	state->read_generation = state->write_generation;
+out:
+	return __read_report(report, buf, count, select);
+}
+
+static ssize_t tsm_report_outblob_read(struct config_item *cfg, void *buf,
+				       size_t count)
+{
+	struct tsm_report *report = to_tsm_report(cfg);
+
+	return tsm_report_read(report, buf, count, TSM_REPORT);
+}
+CONFIGFS_BIN_ATTR_RO(tsm_report_, outblob, NULL, TSM_OUTBLOB_MAX);
+
+static ssize_t tsm_report_auxblob_read(struct config_item *cfg, void *buf,
+				       size_t count)
+{
+	struct tsm_report *report = to_tsm_report(cfg);
+
+	return tsm_report_read(report, buf, count, TSM_CERTS);
+}
+CONFIGFS_BIN_ATTR_RO(tsm_report_, auxblob, NULL, TSM_OUTBLOB_MAX);
+
+#define TSM_DEFAULT_ATTRS() \
+	&tsm_report_attr_generation, \
+	&tsm_report_attr_provider
+
+static struct configfs_attribute *tsm_report_attrs[] = {
+	TSM_DEFAULT_ATTRS(),
+	NULL,
+};
+
+static struct configfs_attribute *tsm_report_extra_attrs[] = {
+	TSM_DEFAULT_ATTRS(),
+	&tsm_report_attr_privlevel,
+	&tsm_report_attr_privlevel_floor,
+	NULL,
+};
+
+#define TSM_DEFAULT_BIN_ATTRS() \
+	&tsm_report_attr_inblob, \
+	&tsm_report_attr_outblob
+
+static struct configfs_bin_attribute *tsm_report_bin_attrs[] = {
+	TSM_DEFAULT_BIN_ATTRS(),
+	NULL,
+};
+
+static struct configfs_bin_attribute *tsm_report_bin_extra_attrs[] = {
+	TSM_DEFAULT_BIN_ATTRS(),
+	&tsm_report_attr_auxblob,
+	NULL,
+};
+
+static void tsm_report_item_release(struct config_item *cfg)
+{
+	struct tsm_report *report = to_tsm_report(cfg);
+	struct tsm_report_state *state = to_state(report);
+
+	kvfree(report->auxblob);
+	kvfree(report->outblob);
+	kfree(state);
+}
+
+static struct configfs_item_operations tsm_report_item_ops = {
+	.release = tsm_report_item_release,
+};
+
+const struct config_item_type tsm_report_default_type = {
+	.ct_owner = THIS_MODULE,
+	.ct_bin_attrs = tsm_report_bin_attrs,
+	.ct_attrs = tsm_report_attrs,
+	.ct_item_ops = &tsm_report_item_ops,
+};
+EXPORT_SYMBOL_GPL(tsm_report_default_type);
+
+const struct config_item_type tsm_report_extra_type = {
+	.ct_owner = THIS_MODULE,
+	.ct_bin_attrs = tsm_report_bin_extra_attrs,
+	.ct_attrs = tsm_report_extra_attrs,
+	.ct_item_ops = &tsm_report_item_ops,
+};
+EXPORT_SYMBOL_GPL(tsm_report_extra_type);
+
+static struct config_item *tsm_report_make_item(struct config_group *group,
+						const char *name)
+{
+	struct tsm_report_state *state;
+
+	guard(rwsem_read)(&tsm_rwsem);
+	if (!provider.ops)
+		return ERR_PTR(-ENXIO);
+
+	state = kzalloc(sizeof(*state), GFP_KERNEL);
+	if (!state)
+		return ERR_PTR(-ENOMEM);
+
+	config_item_init_type_name(&state->cfg, name, provider.type);
+	return &state->cfg;
+}
+
+static struct configfs_group_operations tsm_report_group_ops = {
+	.make_item = tsm_report_make_item,
+};
+
+static const struct config_item_type tsm_reports_type = {
+	.ct_owner = THIS_MODULE,
+	.ct_group_ops = &tsm_report_group_ops,
+};
+
+static const struct config_item_type tsm_root_group_type = {
+	.ct_owner = THIS_MODULE,
+};
+
+static struct configfs_subsystem tsm_configfs = {
+	.su_group = {
+		.cg_item = {
+			.ci_namebuf = "tsm",
+			.ci_type = &tsm_root_group_type,
+		},
+	},
+	.su_mutex = __MUTEX_INITIALIZER(tsm_configfs.su_mutex),
+};
+
+int tsm_register(const struct tsm_ops *ops, void *priv,
+		 const struct config_item_type *type)
+{
+	const struct tsm_ops *conflict;
+
+	if (!type)
+		type = &tsm_report_default_type;
+	if (!(type == &tsm_report_default_type || type == &tsm_report_extra_type))
+		return -EINVAL;
+
+	guard(rwsem_write)(&tsm_rwsem);
+	conflict = provider.ops;
+	if (conflict) {
+		pr_err("\"%s\" ops already registered\n", conflict->name);
+		return -EBUSY;
+	}
+
+	provider.ops = ops;
+	provider.data = priv;
+	provider.type = type;
+	return 0;
+}
+EXPORT_SYMBOL_GPL(tsm_register);
+
+int tsm_unregister(const struct tsm_ops *ops)
+{
+	guard(rwsem_write)(&tsm_rwsem);
+	if (ops != provider.ops)
+		return -EBUSY;
+	provider.ops = NULL;
+	provider.data = NULL;
+	provider.type = NULL;
+	return 0;
+}
+EXPORT_SYMBOL_GPL(tsm_unregister);
+
+static struct config_group *tsm_report_group;
+
+static int __init tsm_init(void)
+{
+	struct config_group *root = &tsm_configfs.su_group;
+	struct config_group *tsm;
+	int rc;
+
+	config_group_init(root);
+	rc = configfs_register_subsystem(&tsm_configfs);
+	if (rc)
+		return rc;
+
+	tsm = configfs_register_default_group(root, "report",
+					      &tsm_reports_type);
+	if (IS_ERR(tsm)) {
+		configfs_unregister_subsystem(&tsm_configfs);
+		return PTR_ERR(tsm);
+	}
+	tsm_report_group = tsm;
+
+	return 0;
+}
+module_init(tsm_init);
+
+static void __exit tsm_exit(void)
+{
+	configfs_unregister_default_group(tsm_report_group);
+	configfs_unregister_subsystem(&tsm_configfs);
+}
+module_exit(tsm_exit);
+
+MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION("Provide Trusted Security Module attestation reports via configfs");