fbdev: Make support for userspace interfaces configurable

Add Kconfig option CONFIG_FB_DEVICE and make the virtual fbdev
device optional. If the new option has not been selected, fbdev
does not create files in devfs, sysfs or procfs.

Most modern Linux systems run a DRM-based graphics stack that uses
the kernel's framebuffer console, but has otherwise deprecated fbdev
support. Yet fbdev userspace interfaces are still present.

The option makes it possible to use the fbdev subsystem as console
implementation without support for userspace. This closes potential
entry points to manipulate kernel or I/O memory via framebuffers. It
also prevents the execution of driver code via ioctl or sysfs, both
of which might allow malicious software to exploit bugs in the fbdev
code.

A small number of fbdev drivers require struct fbinfo.dev to be
initialized, usually for the support of sysfs interface. Make these
drivers depend on FB_DEVICE. They can later be fixed if necessary.

v3:
	* effect -> affect in Kconfig help (Daniel)
v2:
	* set FB_DEVICE default to y (Geert)
	* comment on {get,put}_device() (Sam)
	* Kconfig fixes (Sam)
	* add TODO item about FB_DEVICE dependencies (Sam)
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230613110953.24176-39-tzimmermann@suse.de

Documentation/gpu/todo.rst

@@ -443,6 +443,19 @@ Contact: Thomas Zimmermann <tzimmermann@suse.de>
 
 Level: Starter
 
+Remove driver dependencies on FB_DEVICE
+---------------------------------------
+
+A number of fbdev drivers provide attributes via sysfs and therefore depend
+on CONFIG_FB_DEVICE to be selected. Review each driver and attempt to make
+any dependencies on CONFIG_FB_DEVICE optional. At the minimum, the respective
+code in the driver could be conditionalized via ifdef CONFIG_FB_DEVICE. Not
+all drivers might be able to drop CONFIG_FB_DEVICE.
+
+Contact: Thomas Zimmermann <tzimmermann@suse.de>
+
+Level: Starter
+
 
 Core refactorings
 =================

drivers/staging/fbtft/Kconfig

@@ -2,6 +2,7 @@
 menuconfig FB_TFT
 	tristate "Support for small TFT LCD display modules"
 	depends on FB && SPI
+	depends on FB_DEVICE
 	depends on GPIOLIB || COMPILE_TEST
 	select FB_SYS_FILLRECT
 	select FB_SYS_COPYAREA

drivers/video/fbdev/Kconfig

@@ -57,6 +57,16 @@ config FIRMWARE_EDID
 	  combination with certain motherboards and monitors are known to
 	  suffer from this problem.
 
+config FB_DEVICE
+	bool "Provide legacy /dev/fb* device"
+	depends on FB
+	default y
+	help
+	  Say Y here if you want the legacy /dev/fb* device file and
+	  interfaces within sysfs anc procfs. It is only required if you
+	  have userspace programs that depend on fbdev for graphics output.
+	  This does not affect the framebuffer console. If unsure, say N.
+
 config FB_DDC
 	tristate
 	depends on FB
@@ -1545,6 +1555,7 @@ config FB_3DFX_I2C
 config FB_VOODOO1
 	tristate "3Dfx Voodoo Graphics (sst1) support"
 	depends on FB && PCI
+	depends on FB_DEVICE
 	select FB_CFB_FILLRECT
 	select FB_CFB_COPYAREA
 	select FB_CFB_IMAGEBLIT
@@ -1862,6 +1873,7 @@ config FB_SH_MOBILE_LCDC
 	tristate "SuperH Mobile LCDC framebuffer support"
 	depends on FB && HAVE_CLK && HAS_IOMEM
 	depends on SUPERH || ARCH_RENESAS || COMPILE_TEST
+	depends on FB_DEVICE
 	select FB_SYS_FILLRECT
 	select FB_SYS_COPYAREA
 	select FB_SYS_IMAGEBLIT
@@ -1930,6 +1942,7 @@ config FB_SMSCUFX
 config FB_UDL
 	tristate "Displaylink USB Framebuffer support"
 	depends on FB && USB
+	depends on FB_DEVICE
 	select FB_MODE_HELPERS
 	select FB_SYS_FILLRECT
 	select FB_SYS_COPYAREA

drivers/video/fbdev/core/Makefile

@@ -2,12 +2,13 @@
 obj-$(CONFIG_FB_NOTIFY)           += fb_notify.o
 obj-$(CONFIG_FB)                  += fb.o
 fb-y                              := fb_backlight.o \
-                                     fb_chrdev.o \
                                      fb_info.o \
-                                     fb_procfs.o \
-                                     fbmem.o fbmon.o fbcmap.o fbsysfs.o \
+                                     fbmem.o fbmon.o fbcmap.o \
                                      modedb.o fbcvt.o fb_cmdline.o fb_io_fops.o
 fb-$(CONFIG_FB_DEFERRED_IO)       += fb_defio.o
+fb-$(CONFIG_FB_DEVICE)            += fb_chrdev.o \
+                                     fb_procfs.o \
+                                     fbsysfs.o
 
 ifeq ($(CONFIG_FRAMEBUFFER_CONSOLE),y)
 fb-y				  += fbcon.o bitblit.o softcursor.o

drivers/video/fbdev/core/fb_internal.h

@@ -3,12 +3,22 @@
 #ifndef _FB_INTERNAL_H
 #define _FB_INTERNAL_H
 
+#include <linux/device.h>
 #include <linux/fb.h>
 #include <linux/mutex.h>
 
 /* fb_devfs.c */
+#if defined(CONFIG_FB_DEVICE)
 int fb_register_chrdev(void);
 void fb_unregister_chrdev(void);
+#else
+static inline int fb_register_chrdev(void)
+{
+	return 0;
+}
+static inline void fb_unregister_chrdev(void)
+{ }
+#endif
 
 /* fbmem.c */
 extern struct class *fb_class;
@@ -19,11 +29,39 @@ struct fb_info *get_fb_info(unsigned int idx);
 void put_fb_info(struct fb_info *fb_info);
 
 /* fb_procfs.c */
+#if defined(CONFIG_FB_DEVICE)
 int fb_init_procfs(void);
 void fb_cleanup_procfs(void);
+#else
+static inline int fb_init_procfs(void)
+{
+	return 0;
+}
+static inline void fb_cleanup_procfs(void)
+{ }
+#endif
 
 /* fbsysfs.c */
+#if defined(CONFIG_FB_DEVICE)
 int fb_device_create(struct fb_info *fb_info);
 void fb_device_destroy(struct fb_info *fb_info);
+#else
+static inline int fb_device_create(struct fb_info *fb_info)
+{
+	/*
+	 * Acquire a reference on the parent device to avoid
+	 * unplug operations behind our back. With the fbdev
+	 * device enabled, this is performed within register_device().
+	 */
+	get_device(fb_info->device);
+
+	return 0;
+}
+static inline void fb_device_destroy(struct fb_info *fb_info)
+{
+	/* Undo the get_device() from fb_device_create() */
+	put_device(fb_info->device);
+}
+#endif
 
 #endif

drivers/video/fbdev/omap2/omapfb/Kconfig

@@ -5,9 +5,9 @@ config OMAP2_VRFB
 menuconfig FB_OMAP2
 	tristate "OMAP2+ frame buffer support"
 	depends on FB
+	depends on FB_DEVICE
 	depends on DRM_OMAP = n
 	depends on GPIOLIB
-
 	select FB_OMAP2_DSS
 	select OMAP2_VRFB if ARCH_OMAP2 || ARCH_OMAP3
 	select FB_CFB_FILLRECT

include/linux/fb.h

@@ -481,7 +481,9 @@ struct fb_info {
 
 	const struct fb_ops *fbops;
 	struct device *device;		/* This is the parent */
+#if defined(CONFIG_FB_DEVICE)
 	struct device *dev;		/* This is this fb device */
+#endif
 	int class_flag;                    /* private sysfs flags */
 #ifdef CONFIG_FB_TILEBLITTING
 	struct fb_tile_ops *tileops;    /* Tile Blitting */