drm/gem: fix up flink name create race

This is the 2nd attempt, I've always been a bit dissatisified with the
tricky nature of the first one:

http://lists.freedesktop.org/archives/dri-devel/2012-July/025451.html

The issue is that the flink ioctl can race with calling gem_close on
the last gem handle. In that case we'll end up with a zero handle
count, but an flink name (and it's corresponding reference). Which
results in a neat space leak.

In my first attempt I've solved this by rechecking the handle count.
But fundamentally the issue is that ->handle_count isn't your usual
refcount - it can be resurrected from 0 among other things.

For those special beasts atomic_t often suggest way more ordering that
it actually guarantees. To prevent being tricked by those hairy
semantics take the easy way out and simply protect the handle with the
existing dev->object_name_lock.

With that change implemented it's dead easy to fix the flink vs. gem
close reace: When we try to create the name we simply have to check
whether there's still officially a gem handle around and if not refuse
to create the flink name. Since the handle count decrement and flink
name destruction is now also protected by that lock the reace is gone
and we can't ever leak the flink reference again.

Outside of the drm core only the exynos driver looks at the handle
count, and tbh I have no idea why (it's just for debug dmesg output
luckily).

I've considered inlining the drm_gem_object_handle_free, but I plan to
add more name-like things (like the exported dma_buf) to this scheme,
so it's clearer to leave the handle freeing in its own function.

This is exercised by the new gem_flink_race i-g-t testcase, which on
my snb leaks gem objects at a rate of roughly 1k objects/s.

v2: Fix up the error path handling in handle_create and make it more
robust by simply calling object_handle_unreference.

v3: Fix up the handle_unreference logic bug - atomic_dec_and_test
retursn 1 for 0. Oops.

v4: Squash in inlining of drm_gem_object_handle_reference as suggested
by Dave Airlie and add a note that we now have a testcase.

Cc: Dave Airlie <airlied@gmail.com>
Cc: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>

3 files changed, 22 insertions, 13 deletions

diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
index dcbd2f559e39..b8a8132becef 100644
--- a/drivers/gpu/drm/drm_gem.c
+++ b/drivers/gpu/drm/drm_gem.c
@@ -154,7 +154,7 @@ void drm_gem_private_object_init(struct drm_device *dev,
         obj->filp = NULL;
 
         kref_init(&obj->refcount);
-        atomic_set(&obj->handle_count, 0);
+        obj->handle_count = 0;
         obj->size = size;
 }
 EXPORT_SYMBOL(drm_gem_private_object_init);
@@ -218,11 +218,9 @@ static void drm_gem_object_handle_free(struct drm_gem_object *obj)
         struct drm_device *dev = obj->dev;
 
         /* Remove any name for this object */
-        spin_lock(&dev->object_name_lock);
         if (obj->name) {
                 idr_remove(&dev->object_name_idr, obj->name);
                 obj->name = 0;
-                spin_unlock(&dev->object_name_lock);
                 /*
                  * The object name held a reference to this object, drop
                  * that now.
@@ -230,15 +228,13 @@ static void drm_gem_object_handle_free(struct drm_gem_object *obj)
                 * This cannot be the last reference, since the handle holds one too.
                  */
                 kref_put(&obj->refcount, drm_gem_object_ref_bug);
-        } else
-                spin_unlock(&dev->object_name_lock);
-
+        }
 }
 
 void
 drm_gem_object_handle_unreference_unlocked(struct drm_gem_object *obj)
 {
-        if (WARN_ON(atomic_read(&obj->handle_count) == 0))
+        if (WARN_ON(obj->handle_count == 0))
                 return;
 
         /*
@@ -247,8 +243,11 @@ drm_gem_object_handle_unreference_unlocked(struct drm_gem_object *obj)
         * checked for a name
         */
 
-        if (atomic_dec_and_test(&obj->handle_count))
+        spin_lock(&obj->dev->object_name_lock);
+        if (--obj->handle_count == 0)
                 drm_gem_object_handle_free(obj);
+        spin_unlock(&obj->dev->object_name_lock);
+
         drm_gem_object_unreference_unlocked(obj);
 }
 
@@ -326,17 +325,21 @@ drm_gem_handle_create(struct drm_file *file_priv,
          * allocation under our spinlock.
          */
         idr_preload(GFP_KERNEL);
+        spin_lock(&dev->object_name_lock);
         spin_lock(&file_priv->table_lock);
 
         ret = idr_alloc(&file_priv->object_idr, obj, 1, 0, GFP_NOWAIT);
-
+        drm_gem_object_reference(obj);
+        obj->handle_count++;
         spin_unlock(&file_priv->table_lock);
+        spin_unlock(&dev->object_name_lock);
         idr_preload_end();
-        if (ret < 0)
+        if (ret < 0) {
+                drm_gem_object_handle_unreference_unlocked(obj);
                 return ret;
+        }
         *handlep = ret;
 
-        drm_gem_object_handle_reference(obj);
 
         if (dev->driver->gem_open_object) {
                 ret = dev->driver->gem_open_object(obj, file_priv);
@@ -577,6 +580,12 @@ drm_gem_flink_ioctl(struct drm_device *dev, void *data,
 
         idr_preload(GFP_KERNEL);
         spin_lock(&dev->object_name_lock);
+        /* prevent races with concurrent gem_close. */
+        if (obj->handle_count == 0) {
+                ret = -ENOENT;
+                goto err;
+        }
+
         if (!obj->name) {
                 ret = idr_alloc(&dev->object_name_idr, obj, 1, 0, GFP_NOWAIT);
                 if (ret < 0)
diff --git a/drivers/gpu/drm/drm_info.c b/drivers/gpu/drm/drm_info.c
index 9f8fc4c328c9..5351e811c421 100644
--- a/drivers/gpu/drm/drm_info.c
+++ b/drivers/gpu/drm/drm_info.c
@@ -207,7 +207,7 @@ static int drm_gem_one_name_info(int id, void *ptr, void *data)
 
         seq_printf(m, "%6d %8zd %7d %8d\n",
                    obj->name, obj->size,
-                   atomic_read(&obj->handle_count),
+                   obj->handle_count,
                    atomic_read(&obj->refcount.refcount));
         return 0;
 }
diff --git a/drivers/gpu/drm/exynos/exynos_drm_gem.c b/drivers/gpu/drm/exynos/exynos_drm_gem.c
index b904633863e8..f3c6f40666e1 100644
--- a/drivers/gpu/drm/exynos/exynos_drm_gem.c
+++ b/drivers/gpu/drm/exynos/exynos_drm_gem.c
@@ -136,7 +136,7 @@ void exynos_drm_gem_destroy(struct exynos_drm_gem_obj *exynos_gem_obj)
         obj = &exynos_gem_obj->base;
         buf = exynos_gem_obj->buffer;
 
-        DRM_DEBUG_KMS("handle count = %d\n", atomic_read(&obj->handle_count));
+        DRM_DEBUG_KMS("handle count = %d\n", obj->handle_count);
 
         /*
          * do not release memory region from exporter.