intel_th: msu: Fix an off-by-one in attribute store

The 'nr_pages' attribute of the 'msc' subdevices parses a comma-separated
list of window sizes, passed from userspace. However, there is a bug in
the string parsing logic wherein it doesn't exclude the comma character
from the range of characters as it consumes them. This leads to an
out-of-bounds access given a sufficiently long list. For example:

> # echo 8,8,8,8 > /sys/bus/intel_th/devices/0-msc0/nr_pages
> ==================================================================
> BUG: KASAN: slab-out-of-bounds in memchr+0x1e/0x40
> Read of size 1 at addr ffff8803ffcebcd1 by task sh/825
>
> CPU: 3 PID: 825 Comm: npktest.sh Tainted: G        W         4.20.0-rc1+
> Call Trace:
>  dump_stack+0x7c/0xc0
>  print_address_description+0x6c/0x23c
>  ? memchr+0x1e/0x40
>  kasan_report.cold.5+0x241/0x308
>  memchr+0x1e/0x40
>  nr_pages_store+0x203/0xd00 [intel_th_msu]

Fix this by accounting for the comma character.
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Fixes: ba82664c ("intel_th: Add Memory Storage Unit driver")
Cc: stable@vger.kernel.org # v4.4+
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/hwtracing/intel_th/msu.c

@@ -1423,7 +1423,8 @@ nr_pages_store(struct device *dev, struct device_attribute *attr,
 		if (!end)
 			break;
 
-		len -= end - p;
+		/* consume the number and the following comma, hence +1 */
+		len -= end - p + 1;
 		p = end + 1;
 	} while (len);