Re: [PATCH v6 24/26] powerpc/pseries: Implement secvars for dynamic secure boot

Stefan Berger
2023-02-10T21:23:26Z

On 2/10/23 03:03, Andrew Donnellan wrote:
> From: Russell Currey <ruscur@russell.cc>
>
> The pseries platform can support dynamic secure boot (i.e. secure boot
> using user-defined keys) using variables contained with the PowerVM LPAR
> Platform KeyStore (PLPKS). Using the powerpc secvar API, expose the
> relevant variables for pseries dynamic secure boot through the existing
> secvar filesystem layout.
>
> The relevant variables for dynamic secure boot are signed in the
> keystore, and can only be modified using the H_PKS_SIGNED_UPDATE hcall.
> Object labels in the keystore are encoded using ucs2 format. With our
> fixed variable names we don't have to care about encoding outside of the
> necessary byte padding.
>
> When a user writes to a variable, the first 8 bytes of data must contain
> the signed update flags as defined by the hypervisor.
>
> When a user reads a variable, the first 4 bytes of data contain the
> policies defined for the object.
>
> Limitations exist due to the underlying implementation of sysfs binary
> attributes, as is the case for the OPAL secvar implementation -
> partial writes are unsupported and writes cannot be larger than PAGE_SIZE.
> (Even when using bin_attributes, which can be larger than a single page,
> sysfs only gives us one page's worth of write buffer at a time, and the
> hypervisor does not expose an interface for partial writes.)
>
> Co-developed-by: Nayna Jain <nayna@linux.ibm.com>
> Signed-off-by: Nayna Jain <nayna@linux.ibm.com>
> Co-developed-by: Andrew Donnellan <ajd@linux.ibm.com>
> Signed-off-by: Andrew Donnellan <ajd@linux.ibm.com>
> Signed-off-by: Russell Currey <ruscur@russell.cc>
>
> ---
>
> v2: Remove unnecessary config vars from sysfs and document the others,
> thanks to review from Greg. If we end up needing to expose more, we
> can add them later and update the docs.
>
> Use sysfs_emit() instead of sprintf(), thanks to Greg.
>
> Change the size of the sysfs binary attributes to include the 8-byte
> flags header, preventing truncation of large writes.
>
> v3: plpks_set_variable(): pass var to plpks_signed_update_var() as a
> pointer (mpe)
>
> Update copyright date (ajd)
>
> Consistent comment style (ajd)
>
> Change device_initcall() to machine_arch_initcall(pseries...) so we
> don't try to load on powernv and kill the machine (mpe)
>
> Add config attributes into plpks_secvar_ops (mpe)
>
> Get rid of PLPKS_SECVAR_COUNT macro (mpe)
>
> Reworded descriptions in ABI documentation (mpe)
>
> Switch to using secvar_ops->var_names rather than
> secvar_ops->get_next() (ajd/mpe)
>
> Optimise allocation/copying of buffers (mpe)
>
> Elaborate the comment documenting the "format" string (mpe)
>
> Return -EIO on errors in the read case (mpe)
>
> Add "grubdbx" variable (Sudhakar Kuppusamy)
>
> Use utf8s_to_utf16s() rather than our own "UCS-2" conversion code (mpe)
>
> Change uint64_t to u64 (mpe)
>
> Fix SB_VERSION data length (ruscur)
>
> Stop prepending policy data on read (ruscur)
>
> Enforce max format length on format string (not strictly needed, but
> makes the length limit clear) (ajd)
>
> Update include of plpks.h to reflect new path (ruscur)
>
> Consistent constant naming scheme (ruscur)
>
> v4: Return set_secvar_ops() return code
>
> Pass buffer size to plpks_secvar_format() (stefanb, npiggin)
>
> Add missing null check (stefanb)
>
> Add comment to commit message explaining PAGE_SIZE write limit (joel)
>
> v5: Add comment explaining why we use "key_len - 1" (npiggin)
>
> Use strlen(var.name) instead of hardcoding 10 as length of
> "SB_VERSION" (npiggin)
>
> Improve comments about use of SB_VERSION and format string (npiggin)
>
> Change "+ 8" to "+ sizeof(u64)" when accounting for flags size in
> working out file's max size (npiggin)
>
> Compile plpks-secvar.c based on CONFIG_PPC_SECURE_BOOT, not
> CONFIG_PPC_SECVAR_SYSFS, as the secvar backend is needed for loading
> keys into keyrings even if the sysfs interface is disabled (ajd)
>
> v6: Update date in ABI docs (stefanb)
>
> Get rid of 1 byte kzalloc (npiggin)
> ---
> Documentation/ABI/testing/sysfs-secvar | 75 +++++-
> arch/powerpc/platforms/pseries/Makefile | 4 +-
> arch/powerpc/platforms/pseries/plpks-secvar.c | 218 ++++++++++++++++++
> 3 files changed, 294 insertions(+), 3 deletions(-)
> create mode 100644 arch/powerpc/platforms/pseries/plpks-secvar.c
>
> diff --git a/Documentation/ABI/testing/sysfs-secvar b/Documentation/ABI/testing/sysfs-secvar
> index feebb8c57294..857cf12b0904 100644
> --- a/Documentation/ABI/testing/sysfs-secvar
> +++ b/Documentation/ABI/testing/sysfs-secvar
> @@ -18,6 +18,14 @@ Description: A string indicating which backend is in use by the firmware.
> This determines the format of the variable and the accepted
> format of variable updates.
>
> + On powernv/OPAL, this value is provided by the OPAL firmware
> + and is expected to be "ibm,edk2-compat-v1".
> +
> + On pseries/PLPKS, this is generated by the kernel based on the
> + version number in the SB_VERSION variable in the keystore, and
> + has the form "ibm,plpks-sb-v<version>", or
> + "ibm,plpks-sb-unknown" if there is no SB_VERSION variable.
> +
> What: /sys/firmware/secvar/vars/<variable name>
> Date: August 2019
> Contact: Nayna Jain <nayna@linux.ibm.com>
> @@ -34,7 +42,7 @@ Description: An integer representation of the size of the content of the
>
> What: /sys/firmware/secvar/vars/<variable_name>/data
> Date: August 2019
> -Contact: Nayna Jain h<nayna@linux.ibm.com>
> +Contact: Nayna Jain <nayna@linux.ibm.com>
> Description: A read-only file containing the value of the variable. The size
> of the file represents the maximum size of the variable data.
>
> @@ -44,3 +52,68 @@ Contact: Nayna Jain <nayna@linux.ibm.com>
> Description: A write-only file that is used to submit the new value for the
> variable. The size of the file represents the maximum size of
> the variable data that can be written.
> +
> +What: /sys/firmware/secvar/config
> +Date: February 2023
> +Contact: Nayna Jain <nayna@linux.ibm.com>
> +Description: This optional directory contains read-only config attributes as
> + defined by the secure variable implementation. All data is in
> + ASCII format. The directory is only created if the backing
> + implementation provides variables to populate it, which at
> + present is only PLPKS on the pseries platform.
> +
> +What: /sys/firmware/secvar/config/version
> +Date: February 2023
> +Contact: Nayna Jain <nayna@linux.ibm.com>
> +Description: Config version as reported by the hypervisor in ASCII decimal
> + format.
> +
> + Currently only provided by PLPKS on the pseries platform.
> +
> +What: /sys/firmware/secvar/config/max_object_size
> +Date: February 2023
> +Contact: Nayna Jain <nayna@linux.ibm.com>
> +Description: Maximum allowed size of objects in the keystore in bytes,
> + represented in ASCII decimal format.
> +
> + This is not necessarily the same as the max size that can be
> + written to an update file as writes can contain more than
> + object data, you should use the size of the update file for
> + that purpose.
> +
> + Currently only provided by PLPKS on the pseries platform.
> +
> +What: /sys/firmware/secvar/config/total_size
> +Date: February 2023
> +Contact: Nayna Jain <nayna@linux.ibm.com>
> +Description: Total size of the PLPKS in bytes, represented in ASCII decimal
> + format.
> +
> + Currently only provided by PLPKS on the pseries platform.
> +
> +What: /sys/firmware/secvar/config/used_space
> +Date: February 2023
> +Contact: Nayna Jain <nayna@linux.ibm.com>
> +Description: Current space consumed by the key store, in bytes, represented
> + in ASCII decimal format.
> +
> + Currently only provided by PLPKS on the pseries platform.
> +
> +What: /sys/firmware/secvar/config/supported_policies
> +Date: February 2023
> +Contact: Nayna Jain <nayna@linux.ibm.com>
> +Description: Bitmask of supported policy flags by the hypervisor,
> + represented as an 8 byte hexadecimal ASCII string. Consult the
> + hypervisor documentation for what these flags are.
> +
> + Currently only provided by PLPKS on the pseries platform.
> +
> +What: /sys/firmware/secvar/config/signed_update_algorithms
> +Date: February 2023
> +Contact: Nayna Jain <nayna@linux.ibm.com>
> +Description: Bitmask of flags indicating which algorithms the hypervisor
> + supports for signed update of objects, represented as a 16 byte
> + hexadecimal ASCII string. Consult the hypervisor documentation
> + for what these flags mean.
> +
> + Currently only provided by PLPKS on the pseries platform.
> diff --git a/arch/powerpc/platforms/pseries/Makefile b/arch/powerpc/platforms/pseries/Makefile
> index 92310202bdd7..20a0f3c3fe04 100644
> --- a/arch/powerpc/platforms/pseries/Makefile
> +++ b/arch/powerpc/platforms/pseries/Makefile
> @@ -27,8 +27,8 @@ obj-$(CONFIG_PAPR_SCM) += papr_scm.o
> obj-$(CONFIG_PPC_SPLPAR) += vphn.o
> obj-$(CONFIG_PPC_SVM) += svm.o
> obj-$(CONFIG_FA_DUMP) += rtas-fadump.o
> -obj-$(CONFIG_PSERIES_PLPKS) += plpks.o
> -
> +obj-$(CONFIG_PSERIES_PLPKS) += plpks.o
> +obj-$(CONFIG_PPC_SECURE_BOOT) += plpks-secvar.o
> obj-$(CONFIG_SUSPEND) += suspend.o
> obj-$(CONFIG_PPC_VAS) += vas.o vas-sysfs.o
>
> diff --git a/arch/powerpc/platforms/pseries/plpks-secvar.c b/arch/powerpc/platforms/pseries/plpks-secvar.c
> new file mode 100644
> index 000000000000..98d0f2b31e0d
> --- /dev/null
> +++ b/arch/powerpc/platforms/pseries/plpks-secvar.c
> @@ -0,0 +1,218 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +
> +// Secure variable implementation using the PowerVM LPAR Platform KeyStore (PLPKS)
> +//
> +// Copyright 2022, 2023 IBM Corporation
> +// Authors: Russell Currey
> +// Andrew Donnellan
> +// Nayna Jain
> +
> +#define pr_fmt(fmt) "secvar: "fmt
> +
> +#include <linux/printk.h>
> +#include <linux/init.h>
> +#include <linux/types.h>
> +#include <linux/slab.h>
> +#include <linux/string.h>
> +#include <linux/kobject.h>
> +#include <linux/nls.h>
> +#include <asm/machdep.h>
> +#include <asm/secvar.h>
> +#include <asm/plpks.h>
> +
> +// Config attributes for sysfs
> +#define PLPKS_CONFIG_ATTR(name, fmt, func) \
> + static ssize_t name##_show(struct kobject *kobj, \
> + struct kobj_attribute *attr, \
> + char *buf) \
> + { \
> + return sysfs_emit(buf, fmt, func()); \
> + } \
> + static struct kobj_attribute attr_##name = __ATTR_RO(name)
> +
> +PLPKS_CONFIG_ATTR(version, "%u\n", plpks_get_version);
> +PLPKS_CONFIG_ATTR(max_object_size, "%u\n", plpks_get_maxobjectsize);
> +PLPKS_CONFIG_ATTR(total_size, "%u\n", plpks_get_totalsize);
> +PLPKS_CONFIG_ATTR(used_space, "%u\n", plpks_get_usedspace);
> +PLPKS_CONFIG_ATTR(supported_policies, "%08x\n", plpks_get_supportedpolicies);
> +PLPKS_CONFIG_ATTR(signed_update_algorithms, "%016llx\n", plpks_get_signedupdatealgorithms);
> +
> +static const struct attribute *config_attrs[] = {
> + &attr_version.attr,
> + &attr_max_object_size.attr,
> + &attr_total_size.attr,
> + &attr_used_space.attr,
> + &attr_supported_policies.attr,
> + &attr_signed_update_algorithms.attr,
> + NULL,
> +};
> +
> +static u32 get_policy(const char *name)
> +{
> + if ((strcmp(name, "db") == 0) ||
> + (strcmp(name, "dbx") == 0) ||
> + (strcmp(name, "grubdb") == 0) ||
> + (strcmp(name, "grubdbx") == 0) ||
> + (strcmp(name, "sbat") == 0))
> + return (PLPKS_WORLDREADABLE | PLPKS_SIGNEDUPDATE);
> + else
> + return PLPKS_SIGNEDUPDATE;
> +}
> +
> +static const char * const plpks_var_names[] = {
> + "PK",
> + "KEK",
> + "db",
> + "dbx",
> + "grubdb",
> + "grubdbx",
> + "sbat",
> + "moduledb",
> + "trustedcadb",
> + NULL,
> +};
> +
> +static int plpks_get_variable(const char *key, u64 key_len, u8 *data,
> + u64 *data_size)
> +{
> + struct plpks_var var = {0};
> + int rc = 0;
> +
> + // We subtract 1 from key_len because we don't need to include the
> + // null terminator at the end of the string
> + var.name = kcalloc(key_len - 1, sizeof(wchar_t), GFP_KERNEL);
> + if (!var.name)
> + return -ENOMEM;
> + rc = utf8s_to_utf16s(key, key_len - 1, UTF16_LITTLE_ENDIAN, (wchar_t *)var.name,
> + key_len - 1);
> + if (rc < 0)
> + goto err;
> + var.namelen = rc * 2;
> +
> + var.os = PLPKS_VAR_LINUX;
> + if (data) {
> + var.data = data;
> + var.datalen = *data_size;
> + }
> + rc = plpks_read_os_var(&var);
> +
> + if (rc)
> + goto err;
> +
> + *data_size = var.datalen;
> +
> +err:
> + kfree(var.name);
> + if (rc && rc != -ENOENT) {
> + pr_err("Failed to read variable '%s': %d\n", key, rc);
> + // Return -EIO since userspace probably doesn't care about the
> + // specific error
> + rc = -EIO;
> + }
> + return rc;
> +}
> +
> +static int plpks_set_variable(const char *key, u64 key_len, u8 *data,
> + u64 data_size)
> +{
> + struct plpks_var var = {0};
> + int rc = 0;
> + u64 flags;
> +
> + // Secure variables need to be prefixed with 8 bytes of flags.
> + // We only want to perform the write if we have at least one byte of data.
> + if (data_size <= sizeof(flags))
> + return -EINVAL;
> +
> + // We subtract 1 from key_len because we don't need to include the
> + // null terminator at the end of the string
> + var.name = kcalloc(key_len - 1, sizeof(wchar_t), GFP_KERNEL);
> + if (!var.name)
> + return -ENOMEM;
> + rc = utf8s_to_utf16s(key, key_len - 1, UTF16_LITTLE_ENDIAN, (wchar_t *)var.name,
> + key_len - 1);
> + if (rc < 0)
> + goto err;
> + var.namelen = rc * 2;
> +
> + memcpy(&flags, data, sizeof(flags));

conversion from bytestream to integer: I think in this case it would be better to use

flags = cpu_to_be64p((__u64*)data);

so that the flags always in hypervisor/big endian format

> +
> + var.datalen = data_size - sizeof(flags);
> + var.data = data + sizeof(flags);
> + var.os = PLPKS_VAR_LINUX;
> + var.policy = get_policy(key);
> +
> + // Unlike in the read case, the plpks error code can be useful to
> + // userspace on write, so we return it rather than just -EIO
> + rc = plpks_signed_update_var(&var, flags);
> +
> +err:
> + kfree(var.name);
> + return rc;
> +}
> +
> +// PLPKS dynamic secure boot doesn't give us a format string in the same way OPAL does.
> +// Instead, report the format using the SB_VERSION variable in the keystore.
> +// The string is made up by us, and takes the form "ibm,plpks-sb-v<n>" (or "ibm,plpks-sb-unknown"
> +// if the SB_VERSION variable doesn't exist). Hypervisor defines the SB_VERSION variable as a
> +// "1 byte unsigned integer value".
> +static ssize_t plpks_secvar_format(char *buf, size_t bufsize)
> +{
> + struct plpks_var var = {0};
> + ssize_t ret;
> + u8 version;
> +
> + var.component = NULL;

Since it's initialized with {0} this is not necessary.

> + // Only the signed variables have null bytes in their names, this one doesn't
> + var.name = "SB_VERSION";
> + var.namelen = strlen(var.name);
> + var.datalen = 1;
> + var.data = &version;
> +
> + // Unlike the other vars, SB_VERSION is owned by firmware instead of the OS
> + ret = plpks_read_fw_var(&var);
> + if (ret) {
> + if (ret == -ENOENT) {
> + ret = snprintf(buf, bufsize, "ibm,plpks-sb-unknown");
> + } else {
> + pr_err("Error %ld reading SB_VERSION from firmware\n", ret);
> + ret = -EIO;
> + }
> + goto err;
> + }
> +
> + ret = snprintf(buf, bufsize, "ibm,plpks-sb-v%hhu", version);
> +
> +err:
> + kfree(var.data);

remove the kfree()

> + return ret;
> +}
> +
> +static int plpks_max_size(u64 *max_size)
> +{
> + // The max object size reported by the hypervisor is accurate for the
> + // object itself, but we use the first 8 bytes of data on write as the
> + // signed update flags, so the max size a user can write is larger.
> + *max_size = (u64)plpks_get_maxobjectsize() + sizeof(u64);
> +
> + return 0;
> +}
> +
> +
> +static const struct secvar_operations plpks_secvar_ops = {
> + .get = plpks_get_variable,
> + .set = plpks_set_variable,
> + .format = plpks_secvar_format,
> + .max_size = plpks_max_size,
> + .config_attrs = config_attrs,
> + .var_names = plpks_var_names,
> +};
> +
> +static int plpks_secvar_init(void)
> +{
> + if (!plpks_is_available())
> + return -ENODEV;
> +
> + return set_secvar_ops(&plpks_secvar_ops);
> +}
> +machine_device_initcall(pseries, plpks_secvar_init);