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

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@gmail.com>, who proposed a similar patch.

Reported-by: Konstantin Belousov <kostikbel@gmail.com>
Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Acked-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: <stable@vger.kernel.org>
Signed-off-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Keith Packard <keithp@keithp.com>

2 files changed, 11 insertions, 0 deletions

diff --git a/drivers/gpu/drm/i915/i915_dma.c b/drivers/gpu/drm/i915/i915_dma.c
index a9533c54c93..a9ae374861e 100644
--- a/drivers/gpu/drm/i915/i915_dma.c
+++ b/drivers/gpu/drm/i915/i915_dma.c
@@ -1454,6 +1454,14 @@ 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);
@@ -1484,6 +1492,8 @@ 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;
 }
 
diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 8ba88cfc36d..39a72f642b3 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -713,6 +713,7 @@ 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;