[Intel-gfx] [PATCH] drm/i915: Fix a use after free, and unbalanced refcounting

[Daniel, Thomas]

> -----Original Message-----
> From: Intel-gfx [mailto:intel-gfx-bounces at lists.freedesktop.org] On Behalf Of
> Nick Hoath
> Sent: Thursday, February 19, 2015 4:31 PM
> To: intel-gfx at lists.freedesktop.org
> Subject: [Intel-gfx] [PATCH] drm/i915: Fix a use after free, and unbalanced
> refcounting
> 
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=88652
> 
> When converting from implicitly tracked execlist queue items to ref counted
> requests, not all frees of requests were replaced with unrefs, and extraneous
> refs/unrefs of contexts were added.
> Correct the unbalanced refcount & replace the frees.
> Remove a noisy warning when hitting the request creation path.
> 
> drm_i915_gem_request and intel_context are both kref reference counted
> structures. Upon allocation, drm_i915_gem_request's ref count should be
> bumped using kref_init. When a context is assigned to the request,
> the context's reference count should be bumped using
> i915_gem_context_reference.
> i915_gem_request_reference will reduce the context reference count when
> the request is freed.
> 
> Problem introduced in
> commit 6d3d8274bc45de4babb62d64562d92af984dd238
> Author:     Nick Hoath <nicholas.hoath at intel.com>
> AuthorDate: Thu Jan 15 13:10:39 2015 +0000
> 
>      drm/i915: Subsume intel_ctx_submit_request in to drm_i915_gem_request
> 
> v2: Added comments explaining how the ctx pointer and the request object
> should
> be ref-counted. Removed noisy warning.
> 
> v3: Cleaned up the language used in the commit & the header
> description (Thanks David Gordon)
> 
> Signed-off-by: Nick Hoath <nicholas.hoath at intel.com>
> ---
>  drivers/gpu/drm/i915/i915_drv.h  | 11 ++++++++++-
>  drivers/gpu/drm/i915/i915_gem.c  |  3 +--
>  drivers/gpu/drm/i915/intel_lrc.c |  8 ++++----
>  4 files changed, 16 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index 2dedd43..956fe26 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -2121,6 +2121,9 @@ void i915_gem_track_fb(struct drm_i915_gem_object
> *old,
>   * number comparisons on buffer last_read|write_seqno. It also allows an
>   * emission time to be associated with the request for tracking how far ahead
>   * of the GPU the submission is.
> + *
> + * The requests are reference counted, so upon creation they should have an
> + * initial reference taken using kref_init
>   */
>  struct drm_i915_gem_request {
>  	struct kref ref;
> @@ -2144,7 +2147,16 @@ struct drm_i915_gem_request {
>  	/** Position in the ringbuffer of the end of the whole request */
>  	u32 tail;
> 
> -	/** Context related to this request */
> +	/**
> +	 * Context related to this request
> +	 * Contexts are refcounted, so when this request is associated with a
> +	 * context, we must increment the context's refcount, to guarantee that
> +	 * it persists while any request is linked to it. Requests themselves
> +	 * are also refcounted, so the request will only be freed when the last
> +	 * reference to it is dismissed, and the code in
> +	 * i915_gem_request_free() will then decrement the refcount on the
> +	 * context.
> +	 */
>  	struct intel_context *ctx;
> 
>  	/** Batch buffer related to this request if any */
> diff --git a/drivers/gpu/drm/i915/i915_gem.c
> b/drivers/gpu/drm/i915/i915_gem.c
> index 61134ab..996f60f 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -2664,8 +2664,7 @@ static void i915_gem_reset_ring_cleanup(struct
> drm_i915_private *dev_priv,
>  		if (submit_req->ctx != ring->default_context)
>  			intel_lr_context_unpin(ring, submit_req->ctx);
> 
> -		i915_gem_context_unreference(submit_req->ctx);
> -		kfree(submit_req);
> +		i915_gem_request_unreference(submit_req);
>  	}
> 
>  	/*
> diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
> index aafcef3..62a2b2a 100644
> --- a/drivers/gpu/drm/i915/intel_lrc.c
> +++ b/drivers/gpu/drm/i915/intel_lrc.c
> @@ -512,18 +512,19 @@ static int execlists_context_queue(struct
> intel_engine_cs *ring,
>  		 * If there isn't a request associated with this submission,
>  		 * create one as a temporary holder.
>  		 */
> -		WARN(1, "execlist context submission without request");
>  		request = kzalloc(sizeof(*request), GFP_KERNEL);
>  		if (request == NULL)
>  			return -ENOMEM;
>  		request->ring = ring;
>  		request->ctx = to;
> +		kref_init(&request->ref);
> +		request->uniq = dev_priv->request_uniq++;
> +		i915_gem_context_reference(request->ctx);
>  	} else {
> +		i915_gem_request_reference(request);
>  		WARN_ON(to != request->ctx);
>  	}
>  	request->tail = tail;
> -	i915_gem_request_reference(request);
> -	i915_gem_context_reference(request->ctx);
> 
>  	intel_runtime_pm_get(dev_priv);
> 
> @@ -740,7 +741,6 @@ void intel_execlists_retire_requests(struct
> intel_engine_cs *ring)
>  		if (ctx_obj && (ctx != ring->default_context))
>  			intel_lr_context_unpin(ring, ctx);
>  		intel_runtime_pm_put(dev_priv);
> -		i915_gem_context_unreference(ctx);
>  		list_del(&req->execlist_link);
>  		i915_gem_request_unreference(req);
>  	}
> --
> 2.1.1
Reviewed-by: Thomas Daniel <thomas.daniel at intel.com>