From 4ed0b577457eb6aeb7cdc7e7316576e63d15abb2 Mon Sep 17 00:00:00 2001 From: Eugeni Dodonov Date: Thu, 10 Nov 2011 13:55:15 -0200 Subject: [PATCH] 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 , who proposed a similar patch. Reported-by: Konstantin Belousov Acked-by: Jesse Barnes Acked-by: Keith Packard Reviewed-by: Chris Wilson Cc: Signed-off-by: Eugeni Dodonov Signed-off-by: Keith Packard --- drivers/gpu/drm/i915/i915_dma.c | 10 ++++++++++ drivers/gpu/drm/i915/i915_drv.h | 1 + 2 files changed, 11 insertions(+) diff --git a/drivers/gpu/drm/i915/i915_dma.c b/drivers/gpu/drm/i915/i915_dma.c index a9533c54c93c..a9ae374861e7 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 8ba88cfc36de..39a72f642b33 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; -- 2.34.1