drm/prime: Always add exported buffers to the handle cache

... not only when the dma-buf is freshly created. In contrived
examples someone else could have exported/imported the dma-buf already
and handed us the gem object with a flink name. If such on object gets
reexported as a dma_buf we won't have it in the handle cache already,
which breaks the guarantee that for dma-buf imports we always hand
back an existing handle if there is one.

This is exercised by igt/prime_self_import/with_one_bo_two_files

Now if we extend the locked sections just a notch more we can also
plug th racy buf/handle cache setup in handle_to_fd:

If evil userspace races a concurrent gem close against a prime export
operation we can end up tearing down the gem handle before the dma buf
handle cache is set up. When handle_to_fd gets around to adding the
handle to the cache there will be no one left to clean it up,
effectily leaking the bo (and the dma-buf, since the handle cache
holds a ref on the dma-buf):

Thread A			Thread B

handle_to_fd:

lookup gem object from handle
creates new dma_buf

				gem_close on the same handle
				obj->dma_buf is set, but file priv buf
				handle cache has no entry

				obj->handle_count drops to 0

drm_prime_add_buf_handle sets up the handle cache

-> We have a dma-buf reference in the handle cache, but since the
handle_count of the gem object already dropped to 0 no on will clean
it up. When closing the drm device fd we'll hit the WARN_ON in
drm_prime_destroy_file_private.

The important change is to extend the critical section of the
filp->prime.lock to cover the gem handle lookup. This serializes with
a concurrent gem handle close.

This leak is exercised by igt/prime_self_import/export-vs-gem_close-race

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>

diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
index 0a5a0ca..1ce88c3 100644 (file)
--- a/drivers/gpu/drm/drm_gem.c
+++ b/drivers/gpu/drm/drm_gem.c
@@ -195,10 +195,12 @@ drm_gem_remove_prime_handles(struct drm_gem_object *obj, struct drm_file *filp)
         * Note: obj->dma_buf can't disappear as long as we still hold a
         * handle reference in obj->handle_count.
         */
+       mutex_lock(&filp->prime.lock);
        if (obj->dma_buf) {
-               drm_prime_remove_buf_handle(&filp->prime,
-                               obj->dma_buf);
+               drm_prime_remove_buf_handle_locked(&filp->prime,
+                                                  obj->dma_buf);
        }
+       mutex_unlock(&filp->prime.lock);
 }
 
 static void drm_gem_object_ref_bug(struct kref *list_kref)

diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
index ed1ea5c..7ae2bfc 100644 (file)
--- a/drivers/gpu/drm/drm_prime.c
+++ b/drivers/gpu/drm/drm_prime.c
@@ -83,6 +83,19 @@ static int drm_prime_add_buf_handle(struct drm_prime_file_private *prime_fpriv,
        return 0;
 }
 
+static struct dma_buf *drm_prime_lookup_buf_by_handle(struct drm_prime_file_private *prime_fpriv,
+                                                     uint32_t handle)
+{
+       struct drm_prime_member *member;
+
+       list_for_each_entry(member, &prime_fpriv->head, entry) {
+               if (member->handle == handle)
+                       return member->dma_buf;
+       }
+
+       return NULL;
+}
+
 static int drm_prime_lookup_buf_handle(struct drm_prime_file_private *prime_fpriv,
                                       struct dma_buf *dma_buf,
                                       uint32_t *handle)
@@ -146,9 +159,8 @@ static void drm_gem_map_detach(struct dma_buf *dma_buf,
        attach->priv = NULL;
 }
 
-static void drm_prime_remove_buf_handle_locked(
-               struct drm_prime_file_private *prime_fpriv,
-               struct dma_buf *dma_buf)
+void drm_prime_remove_buf_handle_locked(struct drm_prime_file_private *prime_fpriv,
+                                       struct dma_buf *dma_buf)
 {
        struct drm_prime_member *member, *safe;
 
@@ -337,6 +349,8 @@ static struct dma_buf *export_and_register_object(struct drm_device *dev,
         */
        obj->dma_buf = dmabuf;
        get_dma_buf(obj->dma_buf);
+       /* Grab a new ref since the callers is now used by the dma-buf */
+       drm_gem_object_reference(obj);
 
        return dmabuf;
 }
@@ -349,10 +363,20 @@ int drm_gem_prime_handle_to_fd(struct drm_device *dev,
        int ret = 0;
        struct dma_buf *dmabuf;
 
+       mutex_lock(&file_priv->prime.lock);
        obj = drm_gem_object_lookup(dev, file_priv, handle);
-       if (!obj)
-               return -ENOENT;
+       if (!obj)  {
+               ret = -ENOENT;
+               goto out_unlock;
+       }
+
+       dmabuf = drm_prime_lookup_buf_by_handle(&file_priv->prime, handle);
+       if (dmabuf) {
+               get_dma_buf(dmabuf);
+               goto out_have_handle;
+       }
 
+       mutex_lock(&dev->object_name_lock);
        /* re-export the original imported object */
        if (obj->import_attach) {
                dmabuf = obj->import_attach->dmabuf;
@@ -360,45 +384,45 @@ int drm_gem_prime_handle_to_fd(struct drm_device *dev,
                goto out_have_obj;
        }
 
-       mutex_lock(&dev->object_name_lock);
        if (obj->dma_buf) {
                get_dma_buf(obj->dma_buf);
                dmabuf = obj->dma_buf;
-               mutex_unlock(&dev->object_name_lock);
                goto out_have_obj;
        }
 
        dmabuf = export_and_register_object(dev, obj, flags);
-       mutex_unlock(&dev->object_name_lock);
        if (IS_ERR(dmabuf)) {
                /* normally the created dma-buf takes ownership of the ref,
                 * but if that fails then drop the ref
                 */
                ret = PTR_ERR(dmabuf);
+               mutex_unlock(&dev->object_name_lock);
                goto out;
        }
 
-       mutex_lock(&file_priv->prime.lock);
-       /* if we've exported this buffer the cheat and add it to the import list
-        * so we get the correct handle back
+out_have_obj:
+       /*
+        * If we've exported this buffer then cheat and add it to the import list
+        * so we get the correct handle back. We must do this under the
+        * protection of dev->object_name_lock to ensure that a racing gem close
+        * ioctl doesn't miss to remove this buffer handle from the cache.
         */
        ret = drm_prime_add_buf_handle(&file_priv->prime,
                                       dmabuf, handle);
+       mutex_unlock(&dev->object_name_lock);
        if (ret)
                goto fail_put_dmabuf;
 
+out_have_handle:
        ret = dma_buf_fd(dmabuf, flags);
-       if (ret < 0)
-               goto fail_rm_handle;
-
-       *prime_fd = ret;
-       mutex_unlock(&file_priv->prime.lock);
-       return 0;
-
-out_have_obj:
-       ret = dma_buf_fd(dmabuf, flags);
+       /*
+        * We must _not_ remove the buffer from the handle cache since the newly
+        * created dma buf is already linked in the global obj->dma_buf pointer,
+        * and that is invariant as long as a userspace gem handle exists.
+        * Closing the handle will clean out the cache anyway, so we don't leak.
+        */
        if (ret < 0) {
-               dma_buf_put(dmabuf);
+               goto fail_put_dmabuf;
        } else {
                *prime_fd = ret;
                ret = 0;
@@ -406,14 +430,13 @@ out_have_obj:
 
        goto out;
 
-fail_rm_handle:
-       drm_prime_remove_buf_handle_locked(&file_priv->prime,
-                                          dmabuf);
-       mutex_unlock(&file_priv->prime.lock);
 fail_put_dmabuf:
        dma_buf_put(dmabuf);
 out:
        drm_gem_object_unreference_unlocked(obj);
+out_unlock:
+       mutex_unlock(&file_priv->prime.lock);
+
        return ret;
 }
 EXPORT_SYMBOL(drm_gem_prime_handle_to_fd);
@@ -669,11 +692,3 @@ void drm_prime_destroy_file_private(struct drm_prime_file_private *prime_fpriv)
        WARN_ON(!list_empty(&prime_fpriv->head));
 }
 EXPORT_SYMBOL(drm_prime_destroy_file_private);
-
-void drm_prime_remove_buf_handle(struct drm_prime_file_private *prime_fpriv, struct dma_buf *dma_buf)
-{
-       mutex_lock(&prime_fpriv->lock);
-       drm_prime_remove_buf_handle_locked(prime_fpriv, dma_buf);
-       mutex_unlock(&prime_fpriv->lock);
-}
-EXPORT_SYMBOL(drm_prime_remove_buf_handle);

diff --git a/include/drm/drmP.h b/include/drm/drmP.h
index 5914cc5..90833dc 100644 (file)
--- a/include/drm/drmP.h
+++ b/include/drm/drmP.h
@@ -1508,7 +1508,7 @@ int drm_gem_dumb_destroy(struct drm_file *file,
 
 void drm_prime_init_file_private(struct drm_prime_file_private *prime_fpriv);
 void drm_prime_destroy_file_private(struct drm_prime_file_private *prime_fpriv);
-void drm_prime_remove_buf_handle(struct drm_prime_file_private *prime_fpriv, struct dma_buf *dma_buf);
+void drm_prime_remove_buf_handle_locked(struct drm_prime_file_private *prime_fpriv, struct dma_buf *dma_buf);
 
 #if DRM_DEBUG_CODE
 extern int drm_vma_info(struct seq_file *m, void *data);