stm class: Prevent division by zero

commit bf7cbaae upstream.

Using STP_POLICY_ID_SET ioctl command with dummy_stm device, or any STM
device that supplies zero mmio channel size, will trigger a division by
zero bug in the kernel.

Prevent this by disallowing channel widths other than 1 for such devices.
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Fixes: 7bd1d409 ("stm class: Introduce an abstraction for System Trace Module devices")
CC: stable@vger.kernel.org # v4.4+
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/hwtracing/stm/core.c

@@ -553,7 +553,7 @@ static int stm_char_policy_set_ioctl(struct stm_file *stmf, void __user *arg)
 {
 	struct stm_device *stm = stmf->stm;
 	struct stp_policy_id *id;
-	int ret = -EINVAL;
+	int ret = -EINVAL, wlimit = 1;
 	u32 size;
 
 	if (stmf->output.nr_chans)
@@ -581,8 +581,10 @@ static int stm_char_policy_set_ioctl(struct stm_file *stmf, void __user *arg)
 	if (id->__reserved_0 || id->__reserved_1)
 		goto err_free;
 
-	if (id->width < 1 ||
-	    id->width > PAGE_SIZE / stm->data->sw_mmiosz)
+	if (stm->data->sw_mmiosz)
+		wlimit = PAGE_SIZE / stm->data->sw_mmiosz;
+
+	if (id->width < 1 || id->width > wlimit)
 		goto err_free;
 
 	ret = stm_file_assign(stmf, id->id, id->width);