[07/75] drm/i915: prevent division by zero when asking for chipset power

[Greg KH]

3.1-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Eugeni Dodonov <eugeni.dodonov@xxxxxxxxx>

commit 4ed0b577457eb6aeb7cdc7e7316576e63d15abb2 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
<kostikbel@xxxxxxxxx>, who proposed a similar patch.

Reported-by: Konstantin Belousov <kostikbel@xxxxxxxxx>
Acked-by: Jesse Barnes <jbarnes@xxxxxxxxxxxxxxxx>
Acked-by: Keith Packard <keithp@xxxxxxxxxx>
Reviewed-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Eugeni Dodonov <eugeni.dodonov@xxxxxxxxx>
Signed-off-by: Keith Packard <keithp@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>

---
 drivers/gpu/drm/i915/i915_dma.c |   10 ++++++++++
 drivers/gpu/drm/i915/i915_drv.h |    1 +
 2 files changed, 11 insertions(+)

--- a/drivers/gpu/drm/i915/i915_dma.c
+++ b/drivers/gpu/drm/i915/i915_dma.c
@@ -1453,6 +1453,14 @@ unsigned long i915_chipset_val(struct dr
 
 	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);
@@ -1483,6 +1491,8 @@ unsigned long i915_chipset_val(struct dr
 	dev_priv->last_count1 = total_count;
 	dev_priv->last_time1 = now;
 
+	dev_priv->chipset_power = ret;
+
 	return ret;
 }
 
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -707,6 +707,7 @@ 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;


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