fbdev/omapfb: fix omapfb_memory_read infoleak

OMAPFB_MEMORY_READ ioctl reads pixels from the LCD's memory and copies
them to a userspace buffer. The code has two issues:

- The user provided width and height could be large enough to overflow
  the calculations
- The copy_to_user() can copy uninitialized memory to the userspace,
  which might contain sensitive kernel information.

Fix these by limiting the width & height parameters, and only copying
the amount of data that we actually received from the LCD.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reported-by: Jann Horn <jannh@google.com>
Cc: stable@vger.kernel.org
Cc: security@kernel.org
Cc: Will Deacon <will.deacon@arm.com>
Cc: Jann Horn <jannh@google.com>
Cc: Tony Lindgren <tony@atomide.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>

drivers/video/fbdev/omap2/omapfb/omapfb-ioctl.c

@@ -496,6 +496,9 @@ static int omapfb_memory_read(struct fb_info *fbi,
 	if (!access_ok(VERIFY_WRITE, mr->buffer, mr->buffer_size))
 		return -EFAULT;
 
+	if (mr->w > 4096 || mr->h > 4096)
+		return -EINVAL;
+
 	if (mr->w * mr->h * 3 > mr->buffer_size)
 		return -EINVAL;
 
@@ -509,7 +512,7 @@ static int omapfb_memory_read(struct fb_info *fbi,
 			mr->x, mr->y, mr->w, mr->h);
 
 	if (r > 0) {
-		if (copy_to_user(mr->buffer, buf, mr->buffer_size))
+		if (copy_to_user(mr->buffer, buf, r))
 			r = -EFAULT;
 	}