[RFC v2 6/6] firmware: add firmware signature checking support

From: Luis R. Rodriguez
Date: Wed May 13 2015 - 14:39:21 EST

From: "Luis R. Rodriguez" <mcgrof@xxxxxxxx>

Systems that have module signing currently enabled may
wish to extend vetting of firmware passed to the kernel
as well. We can re-use most of the code for module signing
for firmware signature verification and signing. This will
also later enable re-use of this same code for subsystems
that wish to provide their own cryptographic verification
mechanisms on userspace data needed.

As with module signing, we do a very simple search for a
particular string appended to the firmware. There's both a
config option and a boot parameter which control whether we
accept or fail with unsigned firmware and firmware that are
signed with an unknown key.

If firmware signing is enabled, the kernel will be tainted
if a firmware is loaded that is unsigned or has a signature
for which we don't have the key.

Cc: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
Cc: David Howells <dhowells@xxxxxxxxxx>
Cc: Ming Lei <ming.lei@xxxxxxxxxxxxx>
Cc: Seth Forshee <seth.forshee@xxxxxxxxxxxxx>
Cc: Kyle McMartin <kyle@xxxxxxxxxx>
Signed-off-by: Luis R. Rodriguez <mcgrof@xxxxxxxx>
---
Documentation/firmware_class/signing.txt | 88 +++++++++
drivers/base/Kconfig | 18 ++
drivers/base/firmware_class.c | 214 ++++++++++++++++++++-
.../sysdata-internal.h => include/linux/sysdata.h | 0
kernel/module.c | 2 +-
kernel/sysdata_signing.c | 3 +-
kernel/system_keyring.c | 2 +-
7 files changed, 317 insertions(+), 10 deletions(-)
create mode 100644 Documentation/firmware_class/signing.txt
rename kernel/sysdata-internal.h => include/linux/sysdata.h (100%)

diff --git a/Documentation/firmware_class/signing.txt b/Documentation/firmware_class/signing.txt
new file mode 100644
index 0000000..6e1ce3c
--- /dev/null
+++ b/Documentation/firmware_class/signing.txt
@@ -0,0 +1,88 @@
+ ================================
+ KERNEL FIRMWARE SIGNING FACILITY
+ ================================
+
+CONTENTS
+
+ - Overview.
+ - Configuring firmware signing.
+ - Using signing keys.
+ - Signing firmware files.
+
+
+========
+OVERVIEW
+========
+
+Device drivers which require a firmware to be uploaded onto a device as its own
+device's microcode use any of the following APIs:
+
+ * request_firmware()
+ * request_firmware_direct()
+ * request_firmware_nowait()
+
+The kernel firmware signing facility enables to cryptographically sign
+firmware files on a system using the same keys used for module signing.
+Firmware files's signatures consist of PKCS#7 messages of the respective
+firmware file. A firmware file named foo.bin, would have its respective
+signature on the filesystem as foo.bin.pkcs7. When firmware signature
+checking is enabled (FIRMWARE_SIG) when one of the above APIs is used
+against foo.bin, the file foo.bin.pkcs7 will also be looked for. If
+FIRMWARE_SIG_FORCE is enabled the foo.bin file will only be allowed to
+be returned to callers of the above APIs if and only if the foo.bin.pkcs7
+file is confirmed to be a valid signature of the foo.bin file. If
+FIRMWARE_SIG_FORCE is not enabled and only FIRMWARE_SIG is enabled the
+kernel will be permissive and enabled unsiged firmware files, or firmware
+files with incorrect signatures. If FIRMWARE_SIG is not enabled the
+signature file is ignored completely.
+
+Firmware signing increases security by making it harder to load a malicious
+firmware into the kernel. The firmware signature checking is done by the
+kernel so that it is not necessary to have trusted userspace bits.
+
+============================
+CONFIGURING FIRMWARE SIGNING
+============================
+
+The firmware signing facility is enabled by going to the section:
+
+-> Device Drivers
+ -> Generic Driver Options
+ -> Userspace firmware loading support (FW_LOADER [=y])
+ -> Firmware signature verification (FIRMWARE_SIG [=y])
+
+If you want to not allow unsigned firmware to be loaded you should
+enable:
+
+"Require all firmware to be validly signed", under the same menu.
+
+==================
+USING SIGNING KEYS
+==================
+
+For details on the types of keys used, allowed, and how to generate them
+refer to Documentation/module-signing.txt. We end up with two keys:
+
+ signing_key.priv
+ signing_key.x509
+
+======================
+SIGNING FIRMWARE FILES
+======================
+
+To sign a firmware, use the scripts/sign-file tool available in
+the Linux kernel source tree. The script requires 4 arguments:
+
+ 1. The hash algorithm (e.g., sha256)
+ 2. The private key filename
+ 3. The public key filename
+ 4. The firmware file to be signed
+
+We want to sign the firmware file as a new detached file, for that be
+sure to use the -s flag. The following is an example of how to sign a
+kernel firmware file:
+
+ scripts/sign-file -s sha512 kernel-signkey.priv \
+ kernel-signkey.x509 /lib/firmware/foo.bin
+
+That should have created for you a /lib/firmware/foo.bin.pkcs7 file.
diff --git a/drivers/base/Kconfig b/drivers/base/Kconfig
index 98504ec..a1a6db1 100644
--- a/drivers/base/Kconfig
+++ b/drivers/base/Kconfig
@@ -85,6 +85,24 @@ config FW_LOADER
require userspace firmware loading support, but a module built
out-of-tree does.

+config FIRMWARE_SIG
+ bool "Firmware signature verification"
+ depends on FW_LOADER
+ select SYSDATA_SIG
+ help
+ Check firmware files for valid signatures upon load: if the firmware
+ was called foo.bin, a respective foo.bin.pkcs7 is expected to be
+ present as the signature. For more information see
+ Documentation/firmware_class/signing.txt
+
+config FIRMWARE_SIG_FORCE
+ bool "Require all firmware to be validly signed"
+ depends on FIRMWARE_SIG
+ help
+ Reject unsigned files or signed files for which we don't have a
+ key. Without this, you'll only get a record on the kernel ring
+ buffer of firmware files loaded without a signature.
+
config FIRMWARE_IN_KERNEL
bool "Include in-kernel firmware blobs in kernel binary"
depends on FW_LOADER
diff --git a/drivers/base/firmware_class.c b/drivers/base/firmware_class.c
index 134dd77..f1d0a41 100644
--- a/drivers/base/firmware_class.c
+++ b/drivers/base/firmware_class.c
@@ -29,6 +29,7 @@
#include <linux/syscore_ops.h>
#include <linux/reboot.h>
#include <linux/security.h>
+#include <linux/sysdata.h>

#include <generated/utsrelease.h>

@@ -38,6 +39,11 @@ MODULE_AUTHOR("Manuel Estrada Sainz");
MODULE_DESCRIPTION("Multi purpose firmware loading support");
MODULE_LICENSE("GPL");

+static bool fw_sig_enforce = IS_ENABLED(CONFIG_FIRMWARE_SIG_FORCE);
+#ifndef CONFIG_FIRMWARE_SIG_FORCE
+module_param(fw_sig_enforce, bool_enable_only, 0644);
+#endif /* !CONFIG_FIRMWARE_SIG_FORCE */
+
/* Builtin firmware support */

#ifdef CONFIG_FW_LOADER
@@ -142,6 +148,9 @@ struct firmware_buf {
unsigned long status;
void *data;
size_t size;
+ void *data_sig;
+ size_t size_sig;
+ bool sig_ok;
#ifdef CONFIG_FW_LOADER_USER_HELPER
bool is_paged_buf;
bool need_uevent;
@@ -151,6 +160,7 @@ struct firmware_buf {
struct list_head pending_list;
#endif
const char *fw_id;
+ const char *fw_sig;
};

struct fw_cache_entry {
@@ -180,17 +190,33 @@ static struct firmware_buf *__allocate_fw_buf(const char *fw_name,
struct firmware_cache *fwc)
{
struct firmware_buf *buf;
+ const char *sign_ext = ".pkcs7";
+ char *signed_name;
+
+ signed_name = kzalloc(PATH_MAX, GFP_ATOMIC);
+ if (!signed_name)
+ return NULL;

buf = kzalloc(sizeof(*buf), GFP_ATOMIC);
- if (!buf)
+ if (!buf) {
+ kfree(signed_name);
return NULL;
+ }

buf->fw_id = kstrdup_const(fw_name, GFP_ATOMIC);
if (!buf->fw_id) {
+ kfree(signed_name);
kfree(buf);
return NULL;
}

+ strcpy(signed_name, buf->fw_id);
+ strncat(signed_name, sign_ext, strlen(sign_ext));
+ buf->fw_sig = kstrdup_const(signed_name, GFP_ATOMIC);
+ if (!buf->fw_sig)
+ goto out;
+
+
kref_init(&buf->ref);
buf->fwc = fwc;
init_completion(&buf->completion);
@@ -201,6 +227,11 @@ static struct firmware_buf *__allocate_fw_buf(const char *fw_name,
pr_debug("%s: fw-%s buf=%p\n", __func__, fw_name, buf);

return buf;
+out:
+ kfree(signed_name);
+ kfree_const(buf->fw_id);
+ kfree(buf);
+ return NULL;
}

static struct firmware_buf *__fw_lookup_buf(const char *fw_name)
@@ -262,6 +293,7 @@ static void __fw_free_buf(struct kref *ref)
#endif
vfree(buf->data);
kfree_const(buf->fw_id);
+ kfree_const(buf->fw_sig);
kfree(buf);
}

@@ -325,7 +357,84 @@ fail:
return rc;
}

+#ifdef CONFIG_FIRMWARE_SIG_FORCE
+static struct file *get_filesystem_file_sig(const char *sig_name)
+{
+ return filp_open(sig_name, O_RDONLY, 0);
+}
+
+static bool get_filesystem_file_sig_ok(struct file *file_sig)
+{
+ if (IS_ERR(file_sig))
+ return -EINVAL;
+ return 0;
+}
+
+static int read_file_signature_contents(struct file *file_sig,
+ struct firmware_buf *fw_buf)
+{
+ int rc;
+
+ rc = __read_file_contents(file_sig,
+ &fw_buf->data_sig,
+ &fw_buf->size_sig);
+ if (rc)
+ return rc;
+
+ return 0;
+}
+
+#elif CONFIG_FIRMWARE_SIG
+static struct file *get_filesystem_file_sig(const char *sig_name)
+{
+ struct file *file;
+
+ file = filp_open(sig_name, O_RDONLY, 0);
+ if (IS_ERR(file))
+ pr_info("singature %s not present, but this is OK\n", sig_name);
+
+ return file;
+}
+
+static bool get_filesystem_file_sig_ok(struct file *file_sig)
+{
+ return 0;
+}
+
+static int read_file_signature_contents(struct file *file_sig,
+ struct firmware_buf *fw_buf)
+{
+ int rc;
+
+ rc = __read_file_contents(file,
+ &fw_buf->data_sig,
+ &fw_buf->size_sig);
+ if (rc)
+ pr_info("could not read signature %s, but this is OK\n",
+ fw_buf->fw_sig);
+
+ return 0;
+}
+#else
+static struct file *get_filesystem_file_sig(const char *sig_name)
+{
+ return NULL;
+}
+
+static bool get_filesystem_file_sig_ok(struct file *file_sig)
+{
+ return 0;
+}
+
+static int read_file_signature_contents(struct file *file_sig,
+ struct firmware_buf *fw_buf)
+{
+ return 0;
+}
+#endif
+
static int fw_read_file_contents(struct file *file,
+ struct file *file_sig,
struct firmware_buf *fw_buf)
{
int rc;
@@ -336,6 +445,10 @@ static int fw_read_file_contents(struct file *file,
if (rc)
return rc;

+ rc = read_file_signature_contents(file_sig, fw_buf);
+ if (rc)
+ return rc;
+
return 0;
}

@@ -343,15 +456,20 @@ static int fw_get_filesystem_firmware(struct device *device,
struct firmware_buf *buf)
{
int i, len;
- int rc = -ENOENT;
- char *path;
+ int rc = -ENOMEM;
+ char *path, *path_sig = NULL;

path = __getname();
if (!path)
return -ENOMEM;

+ path_sig = __getname();
+ if (!path_sig)
+ goto out;
+
for (i = 0; i < ARRAY_SIZE(fw_path); i++) {
struct file *file;
+ struct file *file_sig;

/* skip the unset customized path */
if (!fw_path[i][0])
@@ -364,18 +482,43 @@ static int fw_get_filesystem_firmware(struct device *device,
break;
}

+ len = snprintf(path_sig, PATH_MAX, "%s/%s",
+ fw_path[i], buf->fw_sig);
+ if (len >= PATH_MAX) {
+ rc = -ENAMETOOLONG;
+ break;
+ }
+
file = filp_open(path, O_RDONLY, 0);
- if (IS_ERR(file))
+ if (IS_ERR(file)) {
+ rc = -ENOENT;
continue;
- rc = fw_read_file_contents(file, buf);
+ }
+
+ file_sig = get_filesystem_file_sig(path_sig);
+ rc = get_filesystem_file_sig_ok(file_sig);
+ if (rc) {
+ fput(file);
+ if (!IS_ERR(file_sig))
+ fput(file_sig);
+ continue;
+ }
+
+ rc = fw_read_file_contents(file, file_sig, buf);
+
fput(file);
+ if (!IS_ERR(file_sig))
+ fput(file_sig);
+
if (rc)
dev_warn(device, "firmware, attempted to load %s, but failed with error %d\n",
path, rc);
else
break;
}
+out:
__putname(path);
+ __putname(path_sig);

if (!rc) {
dev_dbg(device, "firmware: direct-loading firmware %s\n",
@@ -410,11 +553,43 @@ static void fw_set_page_data(struct firmware_buf *buf, struct firmware *fw)
fw->size = buf->size;
fw->data = buf->data;

- pr_debug("%s: fw-%s buf=%p data=%p size=%u\n",
+ pr_debug("%s: fw-%s buf=%p data=%p size=%u sig_ok=%d\n",
__func__, buf->fw_id, buf, buf->data,
- (unsigned int)buf->size);
+ (unsigned int)buf->size, buf->sig_ok);
}

+#ifdef CONFIG_FIRMWARE_SIG
+static int firmware_sig_check(struct firmware *fw, const char *name)
+{
+ int err = -ENOKEY;
+ struct firmware_buf *buf = fw->priv;
+ const void *data = buf->data;
+ const void *data_sig = buf->data_sig;
+
+ err = data_verify_pkcs7(data, buf->size,
+ data_sig, buf->size_sig);
+ if (!err) {
+ buf->sig_ok = true;
+ fw_set_page_data(buf, fw);
+ return 0;
+ }
+
+ /* Not having a signature is only an error if we're strict. */
+ if (err == -ENOKEY && !fw_sig_enforce)
+ err = 0;
+
+ fw_set_page_data(buf, fw);
+
+ return err;
+}
+#else /* !CONFIG_FIRMWARE_SIG */
+static int firmware_sig_check(struct firmware *fw, const char *name)
+{
+ return 0;
+}
+#endif /* !CONFIG_MODULE_SIG */
+
+
#ifdef CONFIG_PM_SLEEP
static void fw_name_devm_release(struct device *dev, void *res)
{
@@ -1120,6 +1295,22 @@ static int assign_firmware_buf(struct firmware *fw, struct device *device,
return 0;
}

+#ifdef CONFIG_FIRMWARE_SIG
+static void fw_check_sig_ok(const struct firmware *fw, const char *name)
+{
+ struct firmware_buf *buf = fw->priv;
+
+ if (!buf->sig_ok)
+ pr_notice_once("%s: firmware verification failed: signature "
+ "and/or required key missing\n", name);
+}
+#else
+static void fw_check_sig_ok(const struct firmware *fw, const char *name)
+{
+ return;
+}
+#endif
+
/* called from request_firmware() and request_firmware_work_func() */
static int
_request_firmware(const struct firmware **firmware_p, const char *name,
@@ -1177,6 +1368,15 @@ _request_firmware(const struct firmware **firmware_p, const char *name,
usermodehelper_read_unlock();

out:
+ if (ret >= 0) {
+ ret = firmware_sig_check(fw, name);
+ if (ret)
+ goto out_bad_sig;
+ fw_check_sig_ok(fw, name);
+ }
+
+ out_bad_sig:
+
if (ret < 0) {
release_firmware(fw);
fw = NULL;
diff --git a/kernel/sysdata-internal.h b/include/linux/sysdata.h
similarity index 100%
rename from kernel/sysdata-internal.h
rename to include/linux/sysdata.h
diff --git a/kernel/module.c b/kernel/module.c
index 6a3f629..0a97256 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -59,8 +59,8 @@
#include <linux/jump_label.h>
#include <linux/pfn.h>
#include <linux/bsearch.h>
+#include <linux/sysdata.h>
#include <uapi/linux/module.h>
-#include "sysdata-internal.h"

#define CREATE_TRACE_POINTS
#include <trace/events/module.h>
diff --git a/kernel/sysdata_signing.c b/kernel/sysdata_signing.c
index 6759c54..32d5f92 100644
--- a/kernel/sysdata_signing.c
+++ b/kernel/sysdata_signing.c
@@ -11,10 +11,10 @@

#include <linux/kernel.h>
#include <linux/err.h>
+#include <linux/sysdata.h>
#include <keys/system_keyring.h>
#include <crypto/public_key.h>
#include <crypto/pkcs7.h>
-#include "sysdata-internal.h"

/*
* System Data signature information block.
@@ -117,3 +117,4 @@ int sysdata_verify_sig(const void *data, unsigned long *_len)

return data_verify_pkcs7(data, len, data + len, sig_len);
}
+EXPORT_SYMBOL_GPL(sysdata_verify_sig);
diff --git a/kernel/system_keyring.c b/kernel/system_keyring.c
index 1eb0c86..a0b8653 100644
--- a/kernel/system_keyring.c
+++ b/kernel/system_keyring.c
@@ -14,9 +14,9 @@
#include <linux/sched.h>
#include <linux/cred.h>
#include <linux/err.h>
+#include <linux/sysdata.h>
#include <keys/asymmetric-type.h>
#include <keys/system_keyring.h>
-#include "sysdata-internal.h"

struct key *system_trusted_keyring;
EXPORT_SYMBOL_GPL(system_trusted_keyring);
--
2.3.2.209.gd67f9d5.dirty

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