[PATCH v3] fbdev: vfb: Validate framebuffer geometry and memory size

FBIOPUT_VSCREENINFO can supply a virtual width large enough to
overflow get_line_length()'s int multiplication and bypass the
video-memory limit, allowing fbcon to write past the framebuffer. The
offset additions can also wrap, while an excessive videomemorysize
module parameter can be truncated when PAGE_ALIGN() is assigned to
unsigned int.

Cap framebuffer geometry at 8192 x 8192 and video memory at 512 MiB,
keeping the stride, framebuffer-size, and generic fbdev drawing
calculations within their destination types.

KASAN reports:

  BUG: KASAN: vmalloc-out-of-bounds in sys_fillrect
    drivers/video/fbdev/core/sysfillrect.c:24
  Write of size 8 at addr ffa0000001671000 by task poc/198
  sys_fillrect
    drivers/video/fbdev/core/sysfillrect.c:24
  bit_clear_margins
    drivers/video/fbdev/core/bitblit.c:247
  fbcon_clear_margins.constprop.0
    drivers/video/fbdev/core/fbcon.c:1410
  fbcon_switch
    drivers/video/fbdev/core/fbcon.c:2261
  redraw_screen
    drivers/tty/vt/vt.c:994
  fbcon_modechanged
    drivers/video/fbdev/core/fbcon.c:2693
  fbcon_update_vcs
    drivers/video/fbdev/core/fbcon.c:2732

Cc: <redacted>
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: <redacted>
Suggested-by: Helge Deller <deller@gmx.de>
Assisted-by: LLM
Signed-off-by: Weiming Shi <redacted>
---
Changes in v3:
- Clamp visible and virtual resolutions and offsets instead of rejecting
  oversized requests.
- Clamp videomemorysize before PAGE_ALIGN() instead of rejecting it.
- Add unsigned suffixes to the limit constants.

Changes in v2:
- Limit virtual geometry and offsets to 8192 x 8192. This retains 8K mode
  support while bounding all generic fbdev drawing calculations.
- Cap videomemorysize at 512 MiB.
- Keep the existing arithmetic types now that all inputs are bounded.

 drivers/video/fbdev/vfb.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/drivers/video/fbdev/vfb.c b/drivers/video/fbdev/vfb.c
index de137b2bdaed..deaa3e9c758d 100644
--- a/drivers/video/fbdev/vfb.c
+++ b/drivers/video/fbdev/vfb.c

@@ -31,6 +31,9 @@
      */
 
 #define VIDEOMEMSIZE	(1*1024*1024)	/* 1 MB */
+#define MAX_VIDEOMEMSIZE	(512UL * 1024 * 1024)
+#define MAX_XRES		8192U
+#define MAX_YRES		8192U
 
 static void *videomemory;
 static u_long videomemorysize = VIDEOMEMSIZE;

@@ -142,6 +145,10 @@ static int vfb_check_var(struct fb_var_screeninfo *var,
 		var->xres = 1;
 	if (!var->yres)
 		var->yres = 1;
+	var->xres = min(var->xres, MAX_XRES);
+	var->yres = min(var->yres, MAX_YRES);
+	var->xres_virtual = min(var->xres_virtual, MAX_XRES);
+	var->yres_virtual = min(var->yres_virtual, MAX_YRES);
 	if (var->xres > var->xres_virtual)
 		var->xres_virtual = var->xres;
 	if (var->yres > var->yres_virtual)

@@ -159,6 +166,9 @@ static int vfb_check_var(struct fb_var_screeninfo *var,
 	else
 		return -EINVAL;
 
+	var->xoffset = min(var->xoffset, MAX_XRES - var->xres);
+	var->yoffset = min(var->yoffset, MAX_YRES - var->yres);
+
 	if (var->xres_virtual < var->xoffset + var->xres)
 		var->xres_virtual = var->xoffset + var->xres;
 	if (var->yres_virtual < var->yoffset + var->yres)

@@ -434,9 +444,12 @@ static int __init vfb_setup(char *options)
 static int vfb_probe(struct platform_device *dev)
 {
 	struct fb_info *info;
-	unsigned int size = PAGE_ALIGN(videomemorysize);
+	unsigned int size;
 	int retval = -ENOMEM;
 
+	videomemorysize = min(videomemorysize, MAX_VIDEOMEMSIZE);
+	size = PAGE_ALIGN(videomemorysize);
+
 	/*
 	 * For real video cards we use ioremap.
 	 */

-- 
2.56.0