2022-08-11 10:37:21

by Andrey Strachuk

Subject: [PATCH] drm/fb-helper: add virtual screen size check to drm_fb_helper_check_var()

Add virtual screen size check to drm_fb_helper_check_var() in
order to validate userspace input.

Found by Linux Verification Center (linuxtesting.org) with syzkaller.

Signed-off-by: Andrey Strachuk <[email protected]>
Fixes: 6c11df58fd1a ("fbmem: Check virtual screen sizes in fb_set_var()")
---
drivers/gpu/drm/drm_fb_helper.c | 10 ++++++++++
1 file changed, 10 insertions(+)

diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
index 2d4cee6a10ff..d21fe89358af 100644
--- a/drivers/gpu/drm/drm_fb_helper.c
+++ b/drivers/gpu/drm/drm_fb_helper.c
@@ -1355,6 +1355,16 @@ int drm_fb_helper_check_var(struct fb_var_screeninfo *var,
(drm_format_info_block_height(fb->format, 0) > 1))
return -EINVAL;

+ /* verify that virtual resolution >= physical resolution */
+ if (var->xres_virtual < var->xres ||
+ var->yres_virtual < var->yres) {
+ drm_dbg_kms(dev, "requested virtual screen size that is "
+ "below the physical size(%ux%u vs. %ux%u)\n",
+ var->xres_virtual, var->yres_virtual,
+ var->xres, var->yres);
+ return -EINVAL;
+ }
+
/*
* Changes struct fb_var_screeninfo are currently not pushed back
* to KMS, hence fail if different settings are requested.
--
2.25.1

2022-08-11 11:57:06

by Geert Uytterhoeven

Subject: Re: [PATCH] drm/fb-helper: add virtual screen size check to drm_fb_helper_check_var()

Hi Andrey,

On Thu, Aug 11, 2022 at 12:23 PM Andrey Strachuk <[email protected]> wrote:
> Add virtual screen size check to drm_fb_helper_check_var() in
> order to validate userspace input.
>
> Found by Linux Verification Center (linuxtesting.org) with syzkaller.
>
> Signed-off-by: Andrey Strachuk <[email protected]>

Thanks for your patch!

> Fixes: 6c11df58fd1a ("fbmem: Check virtual screen sizes in fb_set_var()")

This Fixes tag is misleading: the bug has existed since basically
forever.

> --- a/drivers/gpu/drm/drm_fb_helper.c
> +++ b/drivers/gpu/drm/drm_fb_helper.c
> @@ -1355,6 +1355,16 @@ int drm_fb_helper_check_var(struct fb_var_screeninfo *var,
> (drm_format_info_block_height(fb->format, 0) > 1))
> return -EINVAL;
>
> + /* verify that virtual resolution >= physical resolution */
> + if (var->xres_virtual < var->xres ||
> + var->yres_virtual < var->yres) {
> + drm_dbg_kms(dev, "requested virtual screen size that is "
> + "below the physical size(%ux%u vs. %ux%u)\n",
> + var->xres_virtual, var->yres_virtual,
> + var->xres, var->yres);
> + return -EINVAL;

According to the fbdev rounding rules, invalid values should be rounded
up, if possible. Hence it is better to not return an error, but round up:

if (var->xres_virtual < var->xres)
var->xres_virtual = var->xres;
if (var->yres_virtual < var->yres)
var->yres_virtual = var->yres;

> + }
> +
> /*
> * Changes struct fb_var_screeninfo are currently not pushed back
> * to KMS, hence fail if different settings are requested.

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