Re: [PATCH v2 7/7] powerpc/pseries: Implement secvars for dynamic secure boot

Michael Ellerman
2023-01-06T10:49:35Z

Russell Currey <ruscur@russell.cc> writes:
> 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.
>
> 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.
>
> Documentation/ABI/testing/sysfs-secvar | 67 ++++-
> arch/powerpc/platforms/pseries/Kconfig | 13 +
> arch/powerpc/platforms/pseries/Makefile | 4 +-
> arch/powerpc/platforms/pseries/plpks-secvar.c | 245 ++++++++++++++++++
> 4 files changed, 326 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..466a8cb92b92 100644
> --- a/Documentation/ABI/testing/sysfs-secvar
> +++ b/Documentation/ABI/testing/sysfs-secvar
> @@ -34,7 +34,7 @@ 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 +44,68 @@ 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: December 2022
> +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.

I think it's OK to mention that currently this only exists for PLPKS ...

> +What: /sys/firmware/secvar/config/version
> +Date: December 2022
> +Contact: Nayna Jain <nayna@linux.ibm.com>
> +Description: RO file, only present if the secvar implementation is PLPKS.

... but I don't think we want to specify that files are only present for PLPKS.

Because if another backend wanted to create them in future, that would
technically be an ABI change.

> + Contains the config version as reported by the hypervisor in
> + ASCII decimal format.
> +
> +What: /sys/firmware/secvar/config/max_object_size
> +Date: December 2022
> +Contact: Nayna Jain <nayna@linux.ibm.com>
> +Description: RO file, only present if the secvar implementation is PLPKS.
> +
> + Contains the 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.
> +
> +What: /sys/firmware/secvar/config/total_size
> +Date: December 2022
> +Contact: Nayna Jain <nayna@linux.ibm.com>
> +Description: RO file, only present if the secvar implementation is PLPKS.
> +
> + Contains the total size of the PLPKS in bytes, represented in
> + ASCII decimal format.

Similarly here I think the description should be written in a way that
is agnostic about the backend. So eg. "Contains the total size of the
key store in bytes".

> +What: /sys/firmware/secvar/config/used_space
> +Date: December 2022
> +Contact: Nayna Jain <nayna@linux.ibm.com>
> +Description: RO file, only present if the secvar implementation is PLPKS.
> +
> + Contains the current space consumed of the PLPKS in bytes,
> + represented in ASCII decimal format.
> +
> +What: /sys/firmware/secvar/config/supported_policies
> +Date: December 2022
> +Contact: Nayna Jain <nayna@linux.ibm.com>
> +Description: RO file, only present if the secvar implementation is PLPKS.
> +
> + Contains a 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.
> +
> +What: /sys/firmware/secvar/config/signed_update_algorithms
> +Date: December 2022
> +Contact: Nayna Jain <nayna@linux.ibm.com>
> +Description: RO file, only present if the secvar implementation is PLPKS.
> +
> + Contains a bitmask of flags indicating which algorithms the
> + hypervisor supports objects to be signed with when modifying
> + secvars, represented as a 16 byte hexadecimal ASCII string.
> + Consult the hypervisor documentation for what these flags mean.

Can this at least say "as defined in PAPR version X section Y"?

> diff --git a/arch/powerpc/platforms/pseries/Kconfig b/arch/powerpc/platforms/pseries/Kconfig
> index a3b4d99567cb..94e08c405d50 100644
> --- a/arch/powerpc/platforms/pseries/Kconfig
> +++ b/arch/powerpc/platforms/pseries/Kconfig
> @@ -162,6 +162,19 @@ config PSERIES_PLPKS
>
> If unsure, select N.
>
> +config PSERIES_PLPKS_SECVAR
> + depends on PSERIES_PLPKS
> + depends on PPC_SECURE_BOOT
> + bool "Support for the PLPKS secvar interface"
> + help
> + PowerVM can support dynamic secure boot with user-defined keys
> + through the PLPKS. Keystore objects used in dynamic secure boot
> + can be exposed to the kernel and userspace through the powerpc
> + secvar infrastructure. Select this to enable the PLPKS backend
> + for secvars for use in pseries dynamic secure boot.
> +
> + If unsure, select N.

I don't think we need that config option at all, or if we do it should
not be user selectable and just enabled automatically by PSERIES_PLPKS.

> diff --git a/arch/powerpc/platforms/pseries/Makefile b/arch/powerpc/platforms/pseries/Makefile
> index 92310202bdd7..807756991f9d 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_PSERIES_PLPKS_SECVAR) += plpks-secvar.o

I'm not convinced the secvar parts need to be in a separate C file.

If it was all in plpks.c we could avoid all/most of plpks.h and a bunch
of accessors and so on.

But I don't feel that strongly about it if you think it's better separate.

> diff --git a/arch/powerpc/platforms/pseries/plpks-secvar.c b/arch/powerpc/platforms/pseries/plpks-secvar.c
> new file mode 100644
> index 000000000000..8298f039bef4
> --- /dev/null
> +++ b/arch/powerpc/platforms/pseries/plpks-secvar.c
> @@ -0,0 +1,245 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * Secure variable implementation using the PowerVM LPAR Platform KeyStore (PLPKS)
> + *
> + * Copyright 2022, 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 <asm/secvar.h>
> +#include "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);

For those last two I wonder if we should be decoding the integer values
into a comma separated list of named flags?

Just blatting out the integer values is a bit gross. It's not helpful
for shell scripts, and a consumer written in C has to strtoull() the
value back into an integer before it can decode it.

> +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 u16 get_ucs2name(const char *name, uint8_t **ucs2_name)
> +{
> + int namelen = strlen(name) * 2;
> + *ucs2_name = kzalloc(namelen, GFP_KERNEL);
> +
> + if (!*ucs2_name)
> + return 0;
> +
> + for (int i = 0; name[i]; i++) {
> + (*ucs2_name)[i * 2] = name[i];
> + (*ucs2_name)[i * 2 + 1] = '\0';
> + }
> +
> + return namelen;
> +}

There are some ucs2 routines in lib/ucs2_string.c, can we use any of
them?

> +static u32 get_policy(const char *name)
> +{
> + if ((strcmp(name, "db") == 0) ||
> + (strcmp(name, "dbx") == 0) ||
> + (strcmp(name, "grubdb") == 0) ||
> + (strcmp(name, "sbat") == 0))
> + return (WORLDREADABLE | SIGNEDUPDATE);
> + else
> + return SIGNEDUPDATE;
> +}
> +
> +#define PLPKS_SECVAR_COUNT 8

I don't think we need that. Just declare the array as unsized and then
use ARRAY_SIZE(var_names) in plpks_get_next_variable().

> +static char *var_names[PLPKS_SECVAR_COUNT] = {
> + "PK",
> + "KEK",
> + "db",
> + "dbx",
> + "grubdb",
> + "sbat",
> + "moduledb",
> + "trustedcadb",
> +};
> +
> +static int plpks_get_variable(const char *key, uint64_t key_len,
> + u8 *data, uint64_t *data_size)
> +{
> + struct plpks_var var = {0};
> + u16 ucs2_namelen;
> + u8 *ucs2_name;
> + int rc = 0;
> +
> + ucs2_namelen = get_ucs2name(key, &ucs2_name);
> + if (!ucs2_namelen)
> + return -ENOMEM;
> +
> + var.name = ucs2_name;
> + var.namelen = ucs2_namelen;
> + var.os = PLPKS_VAR_LINUX;
> + rc = plpks_read_os_var(&var);
> +
> + if (rc)
> + goto err;
> +
> + *data_size = var.datalen + sizeof(var.policy);
> +
> + // We can be called with data = NULL to just get the object size.
> + if (data) {
> + memcpy(data, &var.policy, sizeof(var.policy));
> + memcpy(data + sizeof(var.policy), var.data, var.datalen);
> + }

There's a lot of allocation and copying going on. The secvar-sysfs.c
data_read() has kzalloc'ed data, but only after already doing the hcall
to get the size. Then plpks_read_os_var() does an allocation for the
hcall and then another allocation of the exact data size. Then data_read()
does another copy into the sysfs supplied buffer.

So to read a single variable we do the hcall twice, and allocate/copy
the content of the variable 4 times?

- Hypervisor into "output" in plpks_read_var().
- "output" into "var->data" in plpks_read_var().
- "var.data" into "data" in plpks_get_variable().
- "data" into "buf" in data_read().

As long as maxobjsize is < PAGE_SIZE I think we could do the hcall
directly into "buf". Maybe we want to avoid writing into "buf" directly
in case the hcall fails or something, but the other 3 copies seem
unnecessary.

> + kfree(var.data);
> +err:
> + kfree(ucs2_name);
> + return rc;
> +}
> +
> +static int plpks_set_variable(const char *key, uint64_t key_len,
> + u8 *data, uint64_t data_size)
> +{
> + struct plpks_var var = {0};
> + u16 ucs2_namelen;
> + u8 *ucs2_name;
> + 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;
> +
> + ucs2_namelen = get_ucs2name(key, &ucs2_name);
> + if (!ucs2_namelen)
> + return -ENOMEM;
> +
> + memcpy(&flags, data, sizeof(flags));
> +
> + var.datalen = data_size - sizeof(flags);
> + var.data = kzalloc(var.datalen, GFP_KERNEL);
> + if (!var.data) {
> + rc = -ENOMEM;
> + goto err;
> + }
> +
> + memcpy(var.data, data + sizeof(flags), var.datalen);
> +
> + var.name = ucs2_name;
> + var.namelen = ucs2_namelen;
> + var.os = PLPKS_VAR_LINUX;
> + var.policy = get_policy(key);
> +
> + rc = plpks_signed_update_var(var, flags);
> +
> + kfree(var.data);
> +err:
> + kfree(ucs2_name);
> + return rc;
> +}
> +
> +/*
> + * get_next() in the secvar API is designed for the OPAL API.
> + * If *key is 0, it returns the first variable in the keystore.
> + * Otherwise, you pass the name of a key and it returns next in line.
> + *
> + * We're going to cheat here - since we have fixed keys and don't care about
> + * key_len, we can just use it as an index.
> + */

That's kinda gross. Just change the ops API to do what we need? Either
add a separate get-by-index routine or change the existing one and
update the only other implementation.

> +static int plpks_get_next_variable(const char *key, uint64_t *key_len, uint64_t keybufsize)
> +{
> + if (!key || !key_len)
> + return -EINVAL;
> +
> + if (*key_len >= PLPKS_SECVAR_COUNT)
> + return -ENOENT;
> +
> + if (strscpy((char *)key, var_names[(*key_len)++], keybufsize) < 0)
> + return -E2BIG;
> +
> + return 0;
> +}
> +
> +// 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.
> +static ssize_t plpks_secvar_format(char *buf)
> +{
> + struct plpks_var var = {0};
> + ssize_t ret;
> +
> + var.component = NULL;
> + // Only the signed variables have ucs2-encoded names, this one doesn't
> + var.name = "SB_VERSION";

Is that specified somewhere?

> + var.namelen = 10;
> + var.datalen = 0;
> + var.data = NULL;
> +
> + // 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)
> + return sysfs_emit(buf, "ibm,plpks-sb-unknown\n");
> +
> + pr_err("Error %ld reading SB_VERSION from firmware\n", ret);
> + return ret;

I'm not sure you should pass that raw error back to sysfs. Some of the
values could be confusing, eg. if you return -EINVAL it looks like a
parameter to the read() syscall was invalid. Might be better to just
return -EIO.

> + }
> +
> + // Hypervisor defines SB_VERSION as a "1 byte unsigned integer value"
> + ret = sysfs_emit(buf, "ibm,plpks-sb-%hhu\n", var.data[0]);

The rest of the name string is just made up by us?

> + kfree(var.data);
> + return ret;
> +}
> +
> +static int plpks_max_size(uint64_t *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 = (uint64_t)plpks_get_maxobjectsize() + 8;
> +
> + return 0;
> +}
> +
> +
> +static const struct secvar_operations plpks_secvar_ops = {
> + .get = plpks_get_variable,
> + .get_next = plpks_get_next_variable,
> + .set = plpks_set_variable,
> + .format = plpks_secvar_format,
> + .max_size = plpks_max_size,
> +};
> +
> +static int plpks_secvar_init(void)
> +{
> + if (!plpks_is_available())
> + return -ENODEV;
> +
> + set_secvar_ops(&plpks_secvar_ops);
> + set_secvar_config_attrs(config_attrs);
> + return 0;
> +}
> +device_initcall(plpks_secvar_init);

That must be a machine_device_initcall(pseries, ...), otherwise we will
blow up doing a hcall on powernv in plpks_is_available().

cheers