Hi Weiming,
On Tue, 27 Oct 2026 at 17:16, Weiming Shi <[email protected]> wrote:
> 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: [email protected]
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Reported-by: <[email protected]>
> Suggested-by: Helge Deller <[email protected]>
> Assisted-by: LLM
> Signed-off-by: Weiming Shi <[email protected]>
Thanks for your patch, which is now commit f93dd31db3e23e61 ("fbdev:
vfb: Validate framebuffer geometry and memory size") in fbdev/for-next.
> --- 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)
Is there any point in having a larger limit than 256 MiB
(i.e. MAX_XRES * MAX_YRES * 32 bpp)?
> +#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);
This violates fbdev API rounding rules:
1. If a value doesn't fit, it must be rounded up by the driver,
2. If a value is too large, -EINVAL must be returned.
> 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);
Likewise.
> +
> 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);
Perhaps return -EINVAL, too?
> +
> /*
> * For real video cards we use ioremap.
> */
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- [email protected]
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
