fbdev: color map copying bounds checking

Copying color maps to userspace doesn't check the value of to->start, which will cause kernel heap buffer OOB read due to signedness wraps.

CVE-2016-8405

Reported-by Peter Pi (@heisecode) of Trend Micro

Fixes 1da177e4c3f4 ("Linux-2.6.12-rc2")

Cc [email protected]

Signed-off-by Kees Cook <[email protected]>

---
 drivers/video/fbdev/core/fbcmap.c | 26 ++++++++++++++------------
 1 file changed, 14 insertions(+), 12 deletions(-)

drivers/video/fbdev/core/fbcmap.c +14 -13

index f89245b8ba8e..68a113594808 100644
@@ -163,17 +163,18 @@ void fb_dealloc_cmap(struct fb_cmap *cmap)
 
 int fb_copy_cmap(const struct fb_cmap *from, struct fb_cmap *to)
 {
-       int tooff = 0, fromoff = 0;
-       int size;
+       unsigned int tooff = 0, fromoff = 0;
+       size_t size;
 
        if (to->start > from->start)
                fromoff = to->start - from->start;
        else
                tooff = from->start - to->start;
-       size = to->len - tooff;
-       if (size > (int) (from->len - fromoff))
-               size = from->len - fromoff;
-       if (size <= 0)
+       if (fromoff >= from->len || tooff >= to->len)
+               return -EINVAL;
+
+       size = min_t(size_t, to->len - tooff, from->len - fromoff);
+       if (size = 0)
                return -EINVAL;
        size *= sizeof(u16);
 

@@ -187,17 +188,18 @@ int fb_copy_cmap(const struct fb_cmap *from, struct fb_cmap *to)
 
 int fb_cmap_to_user(const struct fb_cmap *from, struct fb_cmap_user *to)
 {
-       int tooff = 0, fromoff = 0;
-       int size;
+       unsigned int tooff = 0, fromoff = 0;
+       size_t size;
 
        if (to->start > from->start)
                fromoff = to->start - from->start;
        else
                tooff = from->start - to->start;
-       size = to->len - tooff;
-       if (size > (int) (from->len - fromoff))
-               size = from->len - fromoff;
-       if (size <= 0)
+       if (fromoff >= from->len || tooff >= to->len)
+               return -EINVAL;
+
+       size = min_t(size_t, to->len - tooff, from->len - fromoff);
+       if (size = 0)
                return -EINVAL;
        size *= sizeof(u16);
 
--

2.7.4

--
Kees Cook
Nexus Security