[RFC v3 2/2] firmware: add firmware signature checking support
From: Luis R. Rodriguez
Date: Mon May 18 2015 - 20:52:09 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
requirements on userspace data needed.
Contrary to module signing the signature files are expected
to be completely detached for practical and licensing puposes.
If you have foo.bin, you'll need foo.bin.p7s file present
for firmware signing.
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 permissively we'll only log into the kernel ring buffer
when use of an unsigned firmware file is used. If firmware
signing is in enforced mode the kernel will not allow invalid
or unsigned firmware files to be loaded into the kernel.
Contrary to module signing we do not taint the kernel in
the permissive fw signing mode due to restrictions on the
firmware_class API, extensions to enable this are expected
however in the future.
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>
Cc: David Woodhouse <David.Woodhouse@xxxxxxxxx>
Signed-off-by: Luis R. Rodriguez <mcgrof@xxxxxxxx>
---
Documentation/firmware_class/signing.txt | 90 +++++++++++++
drivers/base/Kconfig | 18 +++
drivers/base/firmware_class.c | 213 ++++++++++++++++++++++++++++++-
3 files changed, 314 insertions(+), 7 deletions(-)
create mode 100644 Documentation/firmware_class/signing.txt
diff --git a/Documentation/firmware_class/signing.txt b/Documentation/firmware_class/signing.txt
new file mode 100644
index 0000000..b03f654
--- /dev/null
+++ b/Documentation/firmware_class/signing.txt
@@ -0,0 +1,90 @@
+ ================================
+ 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.p7s. When firmware signature
+checking is enabled (FIRMWARE_SIG) and when one of the above APIs is used
+against foo.bin, the file foo.bin.p7s 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.p7s
+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 unsigned 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" (FIRMWARE_SIG_FORCE [=y]),
+under the same menu.
+
+==================
+USING SIGNING KEYS
+==================
+
+The same key types used for module signing can be used for firmware
+signing. For details on that refer to Documentation/module-signing.txt.
+
+You will need:
+
+ A) A DER-encoded X.509 certificate containing the public key.
+ B) A DER-encoded PKCS#7 message containing the signatures, these are
+ the .p7s files.
+ C) A binary blob that is the detached data for the PKCS#7 message, this
+ is the firmware files
+
+A) is must be made available to the kernel. One way to do this is to provide a
+DER-encoded in the source directory as <name>.x509 when you build the kernel.
+
+======================
+SIGNING FIRMWARE FILES
+======================
+
+To generate a DER-encoded PKCS#7 signature message for each firmware file
+you can use openssl as follows:
+
+ openssl smime -sign -in $FIRMWARE_BLOB_NAME \
+ -outform DER \
+ -inkey $PRIVATE_KEY_FILE_IN_PEM_FORM \
+ -signer $X509_CERT_FILE_IN_PEM_FORM \
+ -nocerts -md $DIGEST_ALGORITHM -binary > \
+ $(FIRMWARE_BLOB_NAME).p7s
diff --git a/drivers/base/Kconfig b/drivers/base/Kconfig
index 98504ec..fa076ea 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.p7s 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..97cab65 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 <keys/system_keyring.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 = ".p7s";
+ 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 +553,42 @@ 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 = system_verify_data(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 +1294,22 @@ 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 +1367,15 @@ _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;
--
2.3.2.209.gd67f9d5.dirty
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