hw/usb: Add U2F device autoscan to passthru mode

This patch adds an autoscan to let u2f-passthru choose the first U2F
device it finds.

The autoscan is performed using libudev with an enumeration of all the
hidraw devices present on the host.

The first device which happens to be a U2F device is taken to do the
passtru.

Signed-off-by: César Belley <cesar.belley@lse.epita.fr>
Message-id: 20200826114209.28821-13-cesar.belley@lse.epita.fr
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

docs/u2f.txt

@@ -42,6 +42,10 @@ on libu2f-emu: configuring and building:
 
     ./configure --enable-u2f && make
 
+The pass-through mode is built by default on Linux. To take advantage
+of the autoscan option it provides, make sure you have a working libudev
+installed on the host.
+
 
 3. Using u2f-emulated
 
@@ -90,6 +94,11 @@ On the host specify the u2f-passthru device with a suitable hidraw:
 
     qemu -usb -device u2f-passthru,hidraw=/dev/hidraw0
 
+Alternately, the u2f-passthru device can autoscan to take the first
+U2F device it finds on the host (this requires a working libudev):
+
+    qemu -usb -device u2f-passthru
+
 
 5. Libu2f-emu
 

hw/usb/meson.build

@@ -52,7 +52,7 @@ endif
 
 # U2F
 softmmu_ss.add(when: 'CONFIG_USB_U2F', if_true: files('u2f.c'))
-softmmu_ss.add(when: ['CONFIG_LINUX', 'CONFIG_USB_U2F'], if_true: files('u2f-passthru.c'))
+softmmu_ss.add(when: ['CONFIG_LINUX', 'CONFIG_USB_U2F'], if_true: [libudev, files('u2f-passthru.c')])
 if u2f.found()
   softmmu_ss.add(when: 'CONFIG_USB_U2F', if_true: [u2f, files('u2f-emulated.c')])
 endif

hw/usb/u2f-passthru.c

@@ -378,6 +378,84 @@ static bool u2f_passthru_is_u2f_device(int fd)
                   sizeof(u2f_hid_report_desc_header)) == 0;
 }
 
+#ifdef CONFIG_LIBUDEV
+static int u2f_passthru_open_from_device(struct udev_device *device)
+{
+    const char *devnode = udev_device_get_devnode(device);
+
+    int fd = qemu_open(devnode, O_RDWR);
+    if (fd < 0) {
+        return -1;
+    } else if (!u2f_passthru_is_u2f_device(fd)) {
+        qemu_close(fd);
+        return -1;
+    }
+    return fd;
+}
+
+static int u2f_passthru_open_from_enumerate(struct udev *udev,
+                                            struct udev_enumerate *enumerate)
+{
+    struct udev_list_entry *devices, *entry;
+    int ret, fd;
+
+    ret = udev_enumerate_scan_devices(enumerate);
+    if (ret < 0) {
+        return -1;
+    }
+
+    devices = udev_enumerate_get_list_entry(enumerate);
+    udev_list_entry_foreach(entry, devices) {
+        struct udev_device *device;
+        const char *syspath = udev_list_entry_get_name(entry);
+
+        if (syspath == NULL) {
+            continue;
+        }
+
+        device = udev_device_new_from_syspath(udev, syspath);
+        if (device == NULL) {
+            continue;
+        }
+
+        fd = u2f_passthru_open_from_device(device);
+        udev_device_unref(device);
+        if (fd >= 0) {
+            return fd;
+        }
+    }
+    return -1;
+}
+
+static int u2f_passthru_open_from_scan(void)
+{
+    struct udev *udev;
+    struct udev_enumerate *enumerate;
+    int ret, fd = -1;
+
+    udev = udev_new();
+    if (udev == NULL) {
+        return -1;
+    }
+
+    enumerate = udev_enumerate_new(udev);
+    if (enumerate == NULL) {
+        udev_unref(udev);
+        return -1;
+    }
+
+    ret = udev_enumerate_add_match_subsystem(enumerate, "hidraw");
+    if (ret >= 0) {
+        fd = u2f_passthru_open_from_enumerate(udev, enumerate);
+    }
+
+    udev_enumerate_unref(enumerate);
+    udev_unref(udev);
+
+    return fd;
+}
+#endif
+
 static void u2f_passthru_unrealize(U2FKeyState *base)
 {
     U2FPassthruState *key = PASSTHRU_U2F_KEY(base);
@@ -392,22 +470,31 @@ static void u2f_passthru_realize(U2FKeyState *base, Error **errp)
     int fd;
 
     if (key->hidraw == NULL) {
+#ifdef CONFIG_LIBUDEV
+        fd = u2f_passthru_open_from_scan();
+        if (fd < 0) {
+            error_setg(errp, "%s: Failed to find a U2F USB device",
+                       TYPE_U2F_PASSTHRU);
+            return;
+        }
+#else
         error_setg(errp, "%s: Missing hidraw", TYPE_U2F_PASSTHRU);
         return;
-    }
+#endif
+    } else {
+        fd = qemu_open(key->hidraw, O_RDWR);
+        if (fd < 0) {
+            error_setg(errp, "%s: Failed to open %s", TYPE_U2F_PASSTHRU,
+                       key->hidraw);
+            return;
+        }
 
-    fd = qemu_open(key->hidraw, O_RDWR);
-    if (fd < 0) {
-        error_setg(errp, "%s: Failed to open %s", TYPE_U2F_PASSTHRU,
-                   key->hidraw);
-        return;
-    }
-
-    if (!u2f_passthru_is_u2f_device(fd)) {
-        qemu_close(fd);
-        error_setg(errp, "%s: Passed hidraw does not represent "
-                   "a U2F HID device", TYPE_U2F_PASSTHRU);
-        return;
+        if (!u2f_passthru_is_u2f_device(fd)) {
+            qemu_close(fd);
+            error_setg(errp, "%s: Passed hidraw does not represent "
+                       "a U2F HID device", TYPE_U2F_PASSTHRU);
+            return;
+        }
     }
     key->hidraw_fd = fd;
     u2f_passthru_reset(key);