[PATCH 30/30] fbdev: Make support for userspace interfaces configurable

Thomas Zimmermann
2023-06-05T14:48:25Z

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 a files in devfs or sysfs.

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.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
---
drivers/staging/fbtft/Kconfig | 1 +
drivers/video/fbdev/Kconfig | 12 +++++++++
drivers/video/fbdev/core/Makefile | 7 +++---
drivers/video/fbdev/core/fb_internal.h | 32 ++++++++++++++++++++++++
drivers/video/fbdev/omap2/omapfb/Kconfig | 2 +-
include/linux/fb.h | 2 ++
6 files changed, 52 insertions(+), 4 deletions(-)

diff --git a/drivers/staging/fbtft/Kconfig b/drivers/staging/fbtft/Kconfig
index 4d29e8c1014e..5dda3c65a38e 100644
--- a/drivers/staging/fbtft/Kconfig
+++ b/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
diff --git a/drivers/video/fbdev/Kconfig b/drivers/video/fbdev/Kconfig
index 6df9bd09454a..48d9a14f889c 100644
--- a/drivers/video/fbdev/Kconfig
+++ b/drivers/video/fbdev/Kconfig
@@ -57,6 +57,15 @@ 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
+ help
+ Say Y here if you want the legacy /dev/fb* device file. It's
+ only required if you have userspace programs that depend on
+ fbdev for graphics output. This does not effect the framebuffer
+ console.
+
config FB_DDC
tristate
depends on FB
@@ -1545,6 +1554,7 @@ 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 +1872,7 @@ 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 +1941,7 @@ 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
diff --git a/drivers/video/fbdev/core/Makefile b/drivers/video/fbdev/core/Makefile
index 125d24f50c36..d5e8772620f8 100644
--- a/drivers/video/fbdev/core/Makefile
+++ b/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_devfs.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_devfs.o \
+ fb_procfs.o \
+ fbsysfs.o

ifeq ($(CONFIG_FRAMEBUFFER_CONSOLE),y)
fb-y += fbcon.o bitblit.o softcursor.o
diff --git a/drivers/video/fbdev/core/fb_internal.h b/drivers/video/fbdev/core/fb_internal.h
index 0b43c0cd5096..b8a28466db79 100644
--- a/drivers/video/fbdev/core/fb_internal.h
+++ b/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 +29,33 @@ 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)
+{
+ get_device(fb_info->device); // as in device_add()
+
+ return 0;
+}
+static inline void fb_device_destroy(struct fb_info *fb_info)
+{
+ put_device(fb_info->device); // as in device_del()
+}
+#endif

#endif
diff --git a/drivers/video/fbdev/omap2/omapfb/Kconfig b/drivers/video/fbdev/omap2/omapfb/Kconfig
index 69f9cb03507e..21069fdb7cc2 100644
--- a/drivers/video/fbdev/omap2/omapfb/Kconfig
+++ b/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
diff --git a/include/linux/fb.h b/include/linux/fb.h
index 541a0e3ce21f..40ed1028160c 100644
--- a/include/linux/fb.h
+++ b/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 */
--
2.40.1