udev: Add id program and rule for FIDO security tokens

Add a fido_id program meant to be run for devices in the hidraw
subsystem via an IMPORT directive. The program parses the HID report
descriptor and assigns the ID_SECURITY_TOKEN environment variable if a
declared usage matches the FIDO_CTAPHID_USAGE declared in the FIDO CTAP
specification. This replaces the previous approach of whitelisting all
known security token models manually.

This commit is accompanied by a test suite and a fuzzer target for the
descriptor parsing routine.

Fixes: #11996.

4 files changed, 199 insertions, 0 deletions

diff --git a/src/udev/fido_id/fido_id.c b/src/udev/fido_id/fido_id.c
new file mode 100644
index 0000000000..5afd5ddb80
--- /dev/null
+++ b/src/udev/fido_id/fido_id.c
@@ -0,0 +1,96 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
+/*
+ * Identifies FIDO CTAP1 ("U2F")/CTAP2 security tokens based on the usage declared in their report
+ * descriptor and outputs suitable environment variables.
+ *
+ * Inspired by Andrew Lutomirski's 'u2f-hidraw-policy.c'
+ */
+
+#include <errno.h>
+#include <fcntl.h>
+#include <linux/hid.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+#include "device-internal.h"
+#include "device-private.h"
+#include "device-util.h"
+#include "fd-util.h"
+#include "fido_id_desc.h"
+#include "log.h"
+#include "macro.h"
+#include "main-func.h"
+#include "path-util.h"
+#include "string-util.h"
+#include "udev-util.h"
+
+static int run(int argc, char **argv) {
+        _cleanup_(sd_device_unrefp) struct sd_device *device = NULL;
+        _cleanup_free_ char *desc_path = NULL;
+        _cleanup_close_ int fd = -1;
+
+        struct sd_device *hid_device;
+        const char *sys_path;
+        uint8_t desc[HID_MAX_DESCRIPTOR_SIZE];
+        ssize_t desc_len;
+
+        int r;
+
+        log_set_target(LOG_TARGET_AUTO);
+        udev_parse_config();
+        log_parse_environment();
+        log_open();
+
+        if (argc > 2)
+                return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "Usage: hidraw_id [SYSFS_PATH]");
+
+        if (argc == 1) {
+                r = device_new_from_strv(&device, environ);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to get current device from environment: %m");
+        } else {
+                r = sd_device_new_from_syspath(&device, argv[1]);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to get device from syspath: %m");
+        }
+
+        r = sd_device_get_parent(device, &hid_device);
+        if (r < 0)
+                return log_device_error_errno(device, r, "Failed to get parent HID device: %m");
+
+        r = sd_device_get_syspath(hid_device, &sys_path);
+        if (r < 0)
+                return log_device_error_errno(hid_device, r, "Failed to get syspath for HID device: %m");
+
+        desc_path = path_join(sys_path, "report_descriptor");
+        if (!desc_path)
+                return log_oom();
+
+        fd = open(desc_path, O_RDONLY | O_NOFOLLOW | O_CLOEXEC);
+        if (fd < 0)
+                return log_device_error_errno(hid_device, errno,
+                                              "Failed to open report descriptor at '%s': %m", desc_path);
+
+        desc_len = read(fd, desc, sizeof(desc));
+        if (desc_len < 0)
+                return log_device_error_errno(hid_device, errno,
+                                              "Failed to read report descriptor at '%s': %m", desc_path);
+        if (desc_len == 0)
+                return log_device_debug_errno(hid_device, SYNTHETIC_ERRNO(EINVAL),
+                                              "Empty report descriptor at '%s'.", desc_path);
+
+        r = is_fido_security_token_desc(desc, desc_len);
+        if (r < 0)
+                return log_device_debug_errno(hid_device, r,
+                                              "Failed to parse report descriptor at '%s'.", desc_path);
+        if (r > 0) {
+                printf("ID_FIDO_TOKEN=1\n");
+                printf("ID_SECURITY_TOKEN=1\n");
+        }
+
+        return 0;
+}
+
+DEFINE_MAIN_FUNCTION(run);
diff --git a/src/udev/fido_id/fido_id_desc.c b/src/udev/fido_id/fido_id_desc.c
new file mode 100644
index 0000000000..bbfcf93709
--- /dev/null
+++ b/src/udev/fido_id/fido_id_desc.c
@@ -0,0 +1,92 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
+/* Inspired by Andrew Lutomirski's 'u2f-hidraw-policy.c' */
+
+#include <errno.h>
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdint.h>
+
+#include "fido_id_desc.h"
+
+#define HID_RPTDESC_FIRST_BYTE_LONG_ITEM 0xfeu
+#define HID_RPTDESC_TYPE_GLOBAL 0x1u
+#define HID_RPTDESC_TYPE_LOCAL 0x2u
+#define HID_RPTDESC_TAG_USAGE_PAGE 0x0u
+#define HID_RPTDESC_TAG_USAGE 0x0u
+
+/*
+ * HID usage for FIDO CTAP1 ("U2F") and CTAP2 security tokens.
+ * https://fidoalliance.org/specs/fido-u2f-v1.0-ps-20141009/fido-u2f-u2f_hid.h-v1.0-ps-20141009.txt
+ * https://fidoalliance.org/specs/fido-v2.0-ps-20190130/fido-client-to-authenticator-protocol-v2.0-ps-20190130.html#usb-discovery
+ * https://www.usb.org/sites/default/files/hutrr48.pdf
+ */
+#define FIDO_FULL_USAGE_CTAPHID 0xf1d00001u
+
+/*
+ * Parses a HID report descriptor and identifies FIDO CTAP1 ("U2F")/CTAP2 security tokens based on their
+ * declared usage.
+ * A positive return value indicates that the report descriptor belongs to a FIDO security token.
+ * https://www.usb.org/sites/default/files/documents/hid1_11.pdf (Section 6.2.2)
+ */
+int is_fido_security_token_desc(const uint8_t *desc, size_t desc_len) {
+        uint32_t usage = 0;
+
+        for (size_t pos = 0; pos < desc_len; ) {
+                uint8_t tag, type, size_code;
+                size_t size;
+                uint32_t value;
+
+                /* Report descriptors consists of short items (1-5 bytes) and long items (3-258 bytes). */
+                if (desc[pos] == HID_RPTDESC_FIRST_BYTE_LONG_ITEM) {
+                        /* No long items are defined in the spec; skip them.
+                         * The length of the data in a long item is contained in the byte after the long
+                         * item tag. The header consists of three bytes: special long item tag, length,
+                         * actual tag. */
+                        if (pos + 1 >= desc_len)
+                                return -EINVAL;
+                        pos += desc[pos + 1] + 3;
+                        continue;
+                }
+
+                /* The first byte of a short item encodes tag, type and size. */
+                tag = desc[pos] >> 4;          /* Bits 7 to 4 */
+                type = (desc[pos] >> 2) & 0x3; /* Bits 3 and 2 */
+                size_code = desc[pos] & 0x3;   /* Bits 1 and 0 */
+                /* Size is coded as follows:
+                 * 0 -> 0 bytes, 1 -> 1 byte, 2 -> 2 bytes, 3 -> 4 bytes
+                 */
+                size = size_code < 3 ? size_code : 4;
+                /* Consume header byte. */
+                pos++;
+
+                /* Extract the item value coded on size bytes. */
+                if (pos + size > desc_len)
+                        return -EINVAL;
+                value = 0;
+                for (size_t i = 0; i < size; i++)
+                        value |= (uint32_t) desc[pos + i] << (8 * i);
+                /* Consume value bytes. */
+                pos += size;
+
+                if (type == HID_RPTDESC_TYPE_GLOBAL && tag == HID_RPTDESC_TAG_USAGE_PAGE) {
+                        /* A usage page is a 16 bit value coded on at most 16 bits. */
+                        if (size > 2)
+                                return -EINVAL;
+                        /* A usage page sets the upper 16 bits of a following usage. */
+                        usage = (value & 0x0000ffffu) << 16;
+                }
+
+                if (type == HID_RPTDESC_TYPE_LOCAL && tag == HID_RPTDESC_TAG_USAGE) {
+                        /* A usage is a 32 bit value, but is prepended with the current usage page if
+                         * coded on less than 4 bytes (that is, at most 2 bytes). */
+                        if (size == 4)
+                                usage = value;
+                        else
+                                usage = (usage & 0xffff0000u) | (value & 0x0000ffffu);
+                        if (usage == FIDO_FULL_USAGE_CTAPHID)
+                                return 1;
+                }
+        }
+
+        return 0;
+}
diff --git a/src/udev/fido_id/fido_id_desc.h b/src/udev/fido_id/fido_id_desc.h
new file mode 100644
index 0000000000..c813a3a454
--- /dev/null
+++ b/src/udev/fido_id/fido_id_desc.h
@@ -0,0 +1,8 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
+
+#pragma once
+
+#include <stddef.h>
+#include <stdint.h>
+
+int is_fido_security_token_desc(const uint8_t *desc, size_t desc_len);
diff --git a/src/udev/meson.build b/src/udev/meson.build
index 511fe428b9..48573cdb60 100644
--- a/src/udev/meson.build
+++ b/src/udev/meson.build
@@ -165,6 +165,9 @@ libudev_core = static_library(
 
 foreach prog : [['ata_id/ata_id.c'],
                 ['cdrom_id/cdrom_id.c'],
+                ['fido_id/fido_id.c',
+                 'fido_id/fido_id_desc.c',
+                 'fido_id/fido_id_desc.h'],
                 ['scsi_id/scsi_id.c',
                  'scsi_id/scsi_id.h',
                  'scsi_id/scsi_serial.c',