Re: fbcmap: non working tests on unsigned cmap->start

[Roel Kluin <roel.kluin@gmail.com>]

Ville Syrjälä wrote:
> On Sun, Apr 26, 2009 at 02:20:47PM +0200, Roel Kluin wrote:
>> cmap->start is unsigned,

>>> On Thu, Apr 23, 2009 at 01:41:10PM -0700, Andrew Morton wrote:
>>>> argh.
>>>>
>>>> - Perhaps userspace can kill the kernel by sending a "negative"
>>>>   `start'.  Removing the test will make it even less likely that we'll
>>>>   fix this bug.
>>> Shouldn't happen. 'start' is used as the starting index for the hardware
>>> palette, 'start+len-1' is the last index. All drivers should already check
>>> the passed values since the maximum index depends on the display mode.
>>> And I suppose the worst thing that could happen if the driver fails to
>>> check the values would be incorrect colors.

> You would rather need something like
> 'if (start+len > 1 << max(red.len, green.len, blue.len, transp.len))'

what do you mean with `red.len'? is that `info->var.red.length'?

> and a check to make sure that start+len doesn't overflow.
> 
> Oh and I guess it should also check that the visual is pseudocolor or
> directcolor.

I am fairly new so please review carefully.

Not yet signed off-by: Roel Kluin <roel.kluin@gmail.com>
---
diff --git a/drivers/video/fbcmap.c b/drivers/video/fbcmap.c
index f53b9f1..b34e74e 100644
--- a/drivers/video/fbcmap.c
+++ b/drivers/video/fbcmap.c
@@ -249,6 +249,7 @@ int fb_set_user_cmap(struct fb_cmap_user *cmap, struct fb_info *info)
 {
 	int rc, size = cmap->len * sizeof(u16);
 	struct fb_cmap umap;
+	__u32 rgba_max = 0;
 
 	memset(&umap, 0, sizeof(struct fb_cmap));
 	rc = fb_alloc_cmap(&umap, cmap->len, cmap->transp != NULL);
@@ -261,13 +262,27 @@ int fb_set_user_cmap(struct fb_cmap_user *cmap, struct fb_info *info)
 		rc = -EFAULT;
 		goto out;
 	}
+
+	if (cmap->start + cmap->len < cmap->start) {
+		rc = -EINVAL;
+		goto out;
+	}
+
 	umap.start = cmap->start;
 	if (!lock_fb_info(info)) {
 		rc = -ENODEV;
 		goto out;
 	}
-	if (cmap->start < 0 || (!info->fbops->fb_setcolreg &&
-				!info->fbops->fb_setcmap)) {
+
+	rgba_max = max(info->var.red.length, info->var.green.length);
+	rgba_max = max(rgba_max, info->var.blue.length);
+	rgba_max = max(rgba_max, info->var.transp.length);
+
+	if (cmap->start + cmap->len > 1 << rgba_max ||
+			!(info->fbops->fb_setcolreg ||
+				info->fbops->fb_setcmap) ||
+			!(info->fix.visual == FB_VISUAL_PSEUDOCOLOR ||
+				info->fix.visual == FB_VISUAL_TRUECOLOR)) {
 		rc = -EINVAL;
 		goto out1;
 	}

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