Re: [Intel-gfx] [PATCH] drm/i915: Fix global state use-after-frees with a refcount

["Lisovskiy, Stanislav" <stanislav.lisovskiy@intel.com>]

On Thu, May 28, 2020 at 10:38:52PM +0300, Lisovskiy, Stanislav wrote:
> On Wed, May 27, 2020 at 11:02:45PM +0300, Ville Syrjala wrote:
> > From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > 
> > While the current locking/serialization of the global state
> > suffices for protecting the obj->state access and the actual
> > hardware reprogramming, we do have a problem with accessing
> > the old/new states during nonblocking commits.
> > 
> > The state computation and swap will be protected by the crtc
> > locks, but the commit_tails can finish out of order, thus also
> > causing the atomic states to be cleaned up out of order. This
> > would mean the commit that started first but finished last has
> > had its new state freed as the no-longer-needed old state by the
> > other commit.
> > 
> > To fix this let's just refcount the states. obj->state amounts
> > to one reference, and the intel_atomic_state holds extra references
> > to both its new and old global obj states.
> > 
> > Fixes: 0ef1905ecf2e ("drm/i915: Introduce better global state handling")
> > Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > ---
> >  .../gpu/drm/i915/display/intel_global_state.c | 45 ++++++++++++++++---
> >  .../gpu/drm/i915/display/intel_global_state.h |  3 ++
> >  2 files changed, 42 insertions(+), 6 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/display/intel_global_state.c b/drivers/gpu/drm/i915/display/intel_global_state.c
> > index 212d4ee68205..7a19215ad844 100644
> > --- a/drivers/gpu/drm/i915/display/intel_global_state.c
> > +++ b/drivers/gpu/drm/i915/display/intel_global_state.c
> > @@ -10,6 +10,28 @@
> >  #include "intel_display_types.h"
> >  #include "intel_global_state.h"
> >  
> > +static void __intel_atomic_global_state_free(struct kref *kref)
> > +{
> > +	struct intel_global_state *obj_state =
> > +		container_of(kref, struct intel_global_state, ref);
> > +	struct intel_global_obj *obj = obj_state->obj;
> > +
> > +	obj->funcs->atomic_destroy_state(obj, obj_state);
> > +}
> > +
> > +static void intel_atomic_global_state_put(struct intel_global_state *obj_state)
> > +{
> > +	kref_put(&obj_state->ref, __intel_atomic_global_state_free);
> > +}
> > +
> > +static struct intel_global_state *
> > +intel_atomic_global_state_get(struct intel_global_state *obj_state)
> > +{
> > +	kref_get(&obj_state->ref);
> > +
> > +	return obj_state;
> > +}
> > +
> >  void intel_atomic_global_obj_init(struct drm_i915_private *dev_priv,
> >  				  struct intel_global_obj *obj,
> >  				  struct intel_global_state *state,
> > @@ -17,6 +39,10 @@ void intel_atomic_global_obj_init(struct drm_i915_private *dev_priv,
> >  {
> >  	memset(obj, 0, sizeof(*obj));
> >  
> > +	state->obj = obj;
> > +
> > +	kref_init(&state->ref);
> > +
> >  	obj->state = state;
> >  	obj->funcs = funcs;
> >  	list_add_tail(&obj->head, &dev_priv->global_obj_list);
> > @@ -28,7 +54,9 @@ void intel_atomic_global_obj_cleanup(struct drm_i915_private *dev_priv)
> >  
> >  	list_for_each_entry_safe(obj, next, &dev_priv->global_obj_list, head) {
> >  		list_del(&obj->head);
> > -		obj->funcs->atomic_destroy_state(obj, obj->state);
> > +
> > +		drm_WARN_ON(&dev_priv->drm, kref_read(&obj->state->ref) != 1);
> > +		intel_atomic_global_state_put(obj->state);
> >  	}
> >  }
> >  
> > @@ -97,10 +125,14 @@ intel_atomic_get_global_obj_state(struct intel_atomic_state *state,
> >  	if (!obj_state)
> >  		return ERR_PTR(-ENOMEM);
> >  
> > +	obj_state->obj = obj;
> >  	obj_state->changed = false;
> >  
> > +	kref_init(&obj_state->ref);
> > +
> >  	state->global_objs[index].state = obj_state;
> > -	state->global_objs[index].old_state = obj->state;
> > +	state->global_objs[index].old_state =
> > +		intel_atomic_global_state_get(obj->state);
> >  	state->global_objs[index].new_state = obj_state;
> >  	state->global_objs[index].ptr = obj;
> >  	obj_state->state = state;
> > @@ -163,7 +195,9 @@ void intel_atomic_swap_global_state(struct intel_atomic_state *state)
> >  		new_obj_state->state = NULL;
> >  
> >  		state->global_objs[i].state = old_obj_state;
> > -		obj->state = new_obj_state;
> > +
> > +		intel_atomic_global_state_put(obj->state);
> > +		obj->state = intel_atomic_global_state_get(new_obj_state);
> >  	}
> >  }
> >  
> > @@ -172,10 +206,9 @@ void intel_atomic_clear_global_state(struct intel_atomic_state *state)
> >  	int i;
> >  
> >  	for (i = 0; i < state->num_global_objs; i++) {
> > -		struct intel_global_obj *obj = state->global_objs[i].ptr;
> > +		intel_atomic_global_state_put(state->global_objs[i].old_state);
> > +		intel_atomic_global_state_put(state->global_objs[i].new_state);
> 
> Shouldn't we clean old_state only? 
> 
> As I understand in absence of any transaction you now have a pool of
> global_obj each has a state with single kref taken.
> 
> So when we are going to get a new state, we do +1 kref to old_state(which is current global obj->state)
> in order to prevent it being cleared by competing commit.
> However the new state doesn't have any kref taken by that moment.
> Then you swap do -1 kref for the old state and do +1 kref for new state, 
> which means that when you -1 kref again for old state in atomic_clear also, 
> it will be destroyed, however regarding the new state, as I understand
> it still has only single kref grabbed when it was swapped, 
> so isn't it going to be now removed? unless we are lucky and somebody
> haven't grabbed it already as an old_state in the next commit?
> 
> Stan

Ah actually I got it - forgot that kref is init as 1. 
But then you probably don't even need to increment kref for new state 
when swapping.
Before assigning new obj->state you release one kref in swap(which makes sense)
Then you just do only intel_atomic_global_state_put(old_state) in atomic_clear
and then no need in doing intel_atomic_global_state_get(new_state) during
swap. 
I.e we always call intel_atomic_global_state_get/put only regarding "old" 
obj->state and each new_state will be disposed when it becomes old_state.

Stan

> >  
> > -		obj->funcs->atomic_destroy_state(obj,
> > -						 state->global_objs[i].state);
> >  		state->global_objs[i].ptr = NULL;
> >  		state->global_objs[i].state = NULL;
> >  		state->global_objs[i].old_state = NULL;
> > diff --git a/drivers/gpu/drm/i915/display/intel_global_state.h b/drivers/gpu/drm/i915/display/intel_global_state.h
> > index e6163a469029..1f16fa3073c9 100644
> > --- a/drivers/gpu/drm/i915/display/intel_global_state.h
> > +++ b/drivers/gpu/drm/i915/display/intel_global_state.h
> > @@ -6,6 +6,7 @@
> >  #ifndef __INTEL_GLOBAL_STATE_H__
> >  #define __INTEL_GLOBAL_STATE_H__
> >  
> > +#include <linux/kref.h>
> >  #include <linux/list.h>
> >  
> >  struct drm_i915_private;
> > @@ -54,7 +55,9 @@ struct intel_global_obj {
> >  		for_each_if(obj)
> >  
> >  struct intel_global_state {
> > +	struct intel_global_obj *obj;
> >  	struct intel_atomic_state *state;
> > +	struct kref ref;
> >  	bool changed;
> >  };
> >  
> > -- 
> > 2.26.2
> > 
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