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

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

cmap->start is unsigned, A negative start could result in incorrect
colors. `start' is used as the starting index for the hardware palette,
'start+len-1' is the last index.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
---
>>> vi include/linux/fb.h +478
>>>
>>> Note that struct fb_cmap_user cmap->start in fb_set_user_cmap() is unsigned.
>>> Should there maybe be a test:
>>>
>>> if (cmap->start > MAX || ...)
>>>
>>> (and what should MAX be then?)
>>>

> 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.

Thanks for your explanation, I think this should fix it?

diff --git a/drivers/video/fbcmap.c b/drivers/video/fbcmap.c
index f53b9f1..ea62d87 100644
--- a/drivers/video/fbcmap.c
+++ b/drivers/video/fbcmap.c
@@ -266,7 +266,7 @@ int fb_set_user_cmap(struct fb_cmap_user *cmap, struct fb_info *info)
 		rc = -ENODEV;
 		goto out;
 	}
-	if (cmap->start < 0 || (!info->fbops->fb_setcolreg &&
+	if (cmap->start >= cmap->len || (!info->fbops->fb_setcolreg &&
 				!info->fbops->fb_setcmap)) {
 		rc = -EINVAL;
 		goto out1;


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