drm/i915: prevent division by zero when asking for chipset power

commit 4ed0b57 upstream.

This prevents an in-kernel division by zero which happens when we are
asking for i915_chipset_val too quickly, or within a race condition
between the power monitoring thread and userspace accesses via debugfs.

The issue can be reproduced easily via the following command:
while ``; do cat /sys/kernel/debug/dri/0/i915_emon_status; done

This is particularly dangerous because it can be triggered by
a non-privileged user by just reading the debugfs entry.

This issue was also found independently by Konstantin Belousov
<[email protected]>, who proposed a similar patch.

Reported-by: Konstantin Belousov <[email protected]>
Acked-by: Jesse Barnes <[email protected]>
Acked-by: Keith Packard <[email protected]>
Reviewed-by: Chris Wilson <[email protected]>
Signed-off-by: Eugeni Dodonov <[email protected]>
Signed-off-by: Keith Packard <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

drivers/gpu/drm/i915/i915_dma.c

@@ -1451,6 +1451,14 @@ unsigned long i915_chipset_val(struct drm_i915_private *dev_priv)
 
 	diff1 = now - dev_priv->last_time1;
 
+	/* Prevent division-by-zero if we are asking too fast.
+	 * Also, we don't get interesting results if we are polling
+	 * faster than once in 10ms, so just return the saved value
+	 * in such cases.
+	 */
+	if (diff1 <= 10)
+		return dev_priv->chipset_power;
+
 	count1 = I915_READ(DMIEC);
 	count2 = I915_READ(DDREC);
 	count3 = I915_READ(CSIEC);
@@ -1481,6 +1489,8 @@ unsigned long i915_chipset_val(struct drm_i915_private *dev_priv)
 	dev_priv->last_count1 = total_count;
 	dev_priv->last_time1 = now;
 
+	dev_priv->chipset_power = ret;
+
 	return ret;
 }
 

drivers/gpu/drm/i915/i915_drv.h

@@ -702,6 +702,7 @@ typedef struct drm_i915_private {
 
 	u64 last_count1;
 	unsigned long last_time1;
+	unsigned long chipset_power;
 	u64 last_count2;
 	struct timespec last_time2;
 	unsigned long gfx_power;