[PATCH v2 16/18] spdm: Limit memory consumed by log of received signatures

Lukas Wunner
2024-06-30T20:30:24Z

The SPDM library has just been amended to keep a log of received
signatures and expose it in sysfs.

Limit the log's memory footprint subject to a sysctl parameter. Purge
old signatures when adding a new signature which causes the limit to be
exceeded. Likewise purge old signatures when the sysctl parameter is
reduced.

The latter requires keeping a list of all struct spdm_state and
protecting it with a mutex. It will come in handy when further global
sysctl parameters are added to the SPDM library. Unfortunately an
xarray is not a better option in this case as the xarray-integrated
xa_lock() is a spinlock but purging signatures from sysfs may sleep
(due to kernfs_rwsem).

This functionality is introduced in a separate commit on top of basic
signature exposure to split the code into digestible, reviewable chunks.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
---
Documentation/ABI/testing/sysfs-devices-spdm | 15 ++++
Documentation/admin-guide/sysctl/index.rst | 2 +
Documentation/admin-guide/sysctl/spdm.rst | 33 ++++++++
MAINTAINERS | 1 +
lib/spdm/core.c | 11 +++
lib/spdm/req-sysfs.c | 80 +++++++++++++++++++-
lib/spdm/spdm.h | 10 +++
7 files changed, 150 insertions(+), 2 deletions(-)
create mode 100644 Documentation/admin-guide/sysctl/spdm.rst

diff --git a/Documentation/ABI/testing/sysfs-devices-spdm b/Documentation/ABI/testing/sysfs-devices-spdm
index ae7b3f701ded..8d8ee01672e1 100644
--- a/Documentation/ABI/testing/sysfs-devices-spdm
+++ b/Documentation/ABI/testing/sysfs-devices-spdm
@@ -162,6 +162,21 @@ Description:
dissector needs to be fed the concatenation of "transcript"
and "signature".

+ Because the number prefixed to the filenames is 32 bit, it
+ wraps around to 0 after 4,294,967,295 signatures. The kernel
+ avoids filename collisions on wraparound by purging old files,
+ subject to the limit set by "sysctl spdm.max_signatures_size"
+ (which defaults to 16 MiB). It is advisable to regularly save
+ backups on non-volatile storage to retain access to signatures
+ that have been purged (or across reboots)::
+
+ # tar -u -h -f /path/to/signatures.tar signatures/
+
+ The ctime of each file is the reception time of the signature.
+ However if the signature was received before the device became
+ registered in sysfs, the ctime is the registration time of the
+ device.
+

What: /sys/devices/.../signatures/[0-9]*_type
Date: June 2024
diff --git a/Documentation/admin-guide/sysctl/index.rst b/Documentation/admin-guide/sysctl/index.rst
index 03346f98c7b9..3b48f0039069 100644
--- a/Documentation/admin-guide/sysctl/index.rst
+++ b/Documentation/admin-guide/sysctl/index.rst
@@ -76,6 +76,7 @@ kernel/ global kernel info / tuning
net/ networking stuff, for documentation look in:
<Documentation/networking/>
proc/ <empty>
+spdm/ Security Protocol and Data Model (SPDM)
sunrpc/ SUN Remote Procedure Call (NFS)
vm/ memory management tuning
buffer and cache management
@@ -93,6 +94,7 @@ really like to hear about it :-)
fs
kernel
net
+ spdm
sunrpc
user
vm
diff --git a/Documentation/admin-guide/sysctl/spdm.rst b/Documentation/admin-guide/sysctl/spdm.rst
new file mode 100644
index 000000000000..0f3846c83cd4
--- /dev/null
+++ b/Documentation/admin-guide/sysctl/spdm.rst
@@ -0,0 +1,33 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=================================
+Documentation for /proc/sys/spdm/
+=================================
+
+Copyright (C) 2024 Intel Corporation
+
+This directory allows tuning Security Protocol and Data Model (SPDM)
+parameters. SPDM enables device authentication, measurement, key
+exchange and encrypted sessions.
+
+max_signatures_size
+===================
+
+Maximum amount of memory occupied by the log of signatures (per device,
+in bytes, 16 MiB by default).
+
+The log is meant for re-verification of signatures by remote attestation
+services which do not trust the kernel to have verified the signatures
+correctly or which want to apply policy constraints of their own.
+A signature is computed over the transcript (a concatenation of all
+SPDM messages exchanged with the device during an authentication
+sequence). The transcript can be a few kBytes or up to several MBytes
+in size, hence this parameter prevents the log from consuming too much
+memory.
+
+The kernel always stores the most recent signature in the log even if it
+exceeds ``max_signatures_size``. Additionally as many older signatures
+are kept in the log as this limit allows.
+
+If you reduce the limit, signatures are purged immediately to free up
+memory.
diff --git a/MAINTAINERS b/MAINTAINERS
index 1ed5817e698c..41f35bbb8f1a 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -20154,6 +20154,7 @@ L: linux-pci@vger.kernel.org
S: Maintained
T: git git://git.kernel.org/pub/scm/linux/kernel/git/devsec/spdm.git
F: Documentation/ABI/testing/sysfs-devices-spdm
+F: Documentation/admin-guide/sysctl/spdm.rst
F: drivers/pci/cma*
F: include/linux/spdm.h
F: lib/spdm/
diff --git a/lib/spdm/core.c b/lib/spdm/core.c
index d962a1344760..b6a46bdbb2f9 100644
--- a/lib/spdm/core.c
+++ b/lib/spdm/core.c
@@ -20,6 +20,9 @@
#include <crypto/hash.h>
#include <crypto/public_key.h>

+LIST_HEAD(spdm_state_list); /* list of all struct spdm_state */
+DEFINE_MUTEX(spdm_state_mutex); /* protects spdm_state_list */
+
static int spdm_err(struct device *dev, struct spdm_error_rsp *rsp)
{
switch (rsp->error_code) {
@@ -404,6 +407,10 @@ struct spdm_state *spdm_create(struct device *dev, spdm_transport *transport,
mutex_init(&spdm_state->lock);
INIT_LIST_HEAD(&spdm_state->log);

+ mutex_lock(&spdm_state_mutex);
+ list_add_tail(&spdm_state->list, &spdm_state_list);
+ mutex_unlock(&spdm_state_mutex);
+
return spdm_state;
}
EXPORT_SYMBOL_GPL(spdm_create);
@@ -417,6 +424,10 @@ void spdm_destroy(struct spdm_state *spdm_state)
{
u8 slot;

+ mutex_lock(&spdm_state_mutex);
+ list_del(&spdm_state->list);
+ mutex_unlock(&spdm_state_mutex);
+
for_each_set_bit(slot, &spdm_state->provisioned_slots, SPDM_SLOTS)
kvfree(spdm_state->slot[slot]);

diff --git a/lib/spdm/req-sysfs.c b/lib/spdm/req-sysfs.c
index d3c4ca7dbbaa..c782054f8e18 100644
--- a/lib/spdm/req-sysfs.c
+++ b/lib/spdm/req-sysfs.c
@@ -185,6 +185,8 @@ const struct attribute_group spdm_signatures_group = {
.bin_attrs = spdm_signatures_bin_attrs,
};

+static unsigned int spdm_max_log_sz = SZ_16M; /* per device */
+
/**
* struct spdm_log_entry - log entry representing one received SPDM signature
*
@@ -332,13 +334,31 @@ static ssize_t spdm_read_combined_prefix(struct file *file,
return count;
}

-static void spdm_destroy_log_entry(struct spdm_log_entry *log)
+static void spdm_destroy_log_entry(struct spdm_state *spdm_state,
+ struct spdm_log_entry *log)
{
+ spdm_state->log_sz -= log->transcript.size + log->sig.size +
+ sizeof(*log);
+
list_del(&log->list);
kvfree(log->transcript.private);
kfree(log);
}

+static void spdm_shrink_log(struct spdm_state *spdm_state)
+{
+ while (spdm_state->log_sz > spdm_max_log_sz &&
+ !list_is_singular(&spdm_state->log)) {
+ struct spdm_log_entry *log =
+ list_first_entry(&spdm_state->log, typeof(*log), list);
+
+ if (device_is_registered(spdm_state->dev))
+ spdm_unpublish_log_entry(&spdm_state->dev->kobj, log);
+
+ spdm_destroy_log_entry(spdm_state, log);
+ }
+}
+
/**
* spdm_create_log_entry() - Allocate log entry for one received SPDM signature
*
@@ -444,6 +464,11 @@ void spdm_create_log_entry(struct spdm_state *spdm_state,

list_add_tail(&log->list, &spdm_state->log);
spdm_state->log_counter++;
+ spdm_state->log_sz += log->transcript.size + log->sig.size +
+ sizeof(*log);
+
+ /* Purge oldest log entries if max log size is exceeded */
+ spdm_shrink_log(spdm_state);

/* Steal transcript pointer ahead of spdm_free_transcript() */
spdm_state->transcript = NULL;
@@ -504,5 +529,56 @@ void spdm_destroy_log(struct spdm_state *spdm_state)
struct spdm_log_entry *log, *tmp;

list_for_each_entry_safe(log, tmp, &spdm_state->log, list)
- spdm_destroy_log_entry(log);
+ spdm_destroy_log_entry(spdm_state, log);
+}
+
+#ifdef CONFIG_SYSCTL
+static int proc_max_log_sz(struct ctl_table *table, int write,
+ void *buffer, size_t *lenp, loff_t *ppos)
+{
+ unsigned int old_max_log_sz = spdm_max_log_sz;
+ struct spdm_state *spdm_state;
+ int rc;
+
+ rc = proc_douintvec_minmax(table, write, buffer, lenp, ppos);
+ if (rc)
+ return rc;
+
+ /* Purge oldest log entries if max log size has been reduced */
+ if (write && spdm_max_log_sz < old_max_log_sz) {
+ mutex_lock(&spdm_state_mutex);
+ list_for_each_entry(spdm_state, &spdm_state_list, list) {
+ mutex_lock(&spdm_state->lock);
+ spdm_shrink_log(spdm_state);
+ mutex_unlock(&spdm_state->lock);
+ }
+ mutex_unlock(&spdm_state_mutex);
+ }
+
+ return 0;
+}
+
+static struct ctl_table spdm_ctl_table[] = {
+ {
+ .procname = "max_signatures_size",
+ .data = &spdm_max_log_sz,
+ .maxlen = sizeof(spdm_max_log_sz),
+ .mode = 0644,
+ .proc_handler = proc_max_log_sz,
+ .extra1 = SYSCTL_ZERO,
+ /*
+ * 2 GiB limit avoids filename collision on
+ * wraparound of unsigned 32-bit log_counter
+ */
+ .extra2 = SYSCTL_INT_MAX,
+ },
+ { }
+};
+
+static int __init spdm_init(void)
+{
+ register_sysctl_init("spdm", spdm_ctl_table);
+ return 0;
}
+fs_initcall(spdm_init);
+#endif /* CONFIG_SYSCTL */
diff --git a/lib/spdm/spdm.h b/lib/spdm/spdm.h
index a63c2922af5d..448107c92db7 100644
--- a/lib/spdm/spdm.h
+++ b/lib/spdm/spdm.h
@@ -420,6 +420,8 @@ struct spdm_error_rsp {
* @dev: Responder device. Used for error reporting and passed to @transport.
* Attributes in sysfs appear below this device's directory.
* @lock: Serializes multiple concurrent spdm_authenticate() calls.
+ * @list: List node. Added to spdm_state_list. Used to iterate over all
+ * SPDM-capable devices when a global sysctl parameter is changed.
* @authenticated: Whether device was authenticated successfully.
* @dev: Responder device. Used for error reporting and passed to @transport.
* @transport: Transport function to perform one message exchange.
@@ -468,12 +470,16 @@ struct spdm_error_rsp {
* @transcript_max: Allocation size of @transcript. Multiple of PAGE_SIZE.
* @log: Linked list of past authentication events. Each list entry is of type
* struct spdm_log_entry and is exposed as several files in sysfs.
+ * @log_sz: Memory occupied by @log (in bytes) to enforce the limit set by
+ * spdm_max_log_sz. Includes, for every entry, the struct spdm_log_entry
+ * itself and the transcript with trailing signature.
* @log_counter: Number of generated log entries so far. Will be prefixed to
* the sysfs files of the next generated log entry.
*/
struct spdm_state {
struct device *dev;
struct mutex lock;
+ struct list_head list;
unsigned int authenticated:1;

/* Transport */
@@ -513,9 +519,13 @@ struct spdm_state {

/* Signatures Log */
struct list_head log;
+ size_t log_sz;
u32 log_counter;
};

+extern struct list_head spdm_state_list;
+extern struct mutex spdm_state_mutex;
+
ssize_t spdm_exchange(struct spdm_state *spdm_state,
void *req, size_t req_sz, void *rsp, size_t rsp_sz);

--
2.43.0