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

[Andrey Strachuk <strochuk@ispras.ru>]

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 <strochuk@ispras.ru>
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