Re: [PATCH 6/8] drm/i915/gt: Fix memory leaks in per-gt sysfs

[Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx>]

On 10/05/2022 11:41, Andrzej Hajda wrote:
> On 10.05.2022 11:48, Tvrtko Ursulin wrote:
> > On 10/05/2022 10:39, Andrzej Hajda wrote:
> > > On 10.05.2022 10:18, Tvrtko Ursulin wrote:
> > > > On 10/05/2022 08:58, Andrzej Hajda wrote:
> > > > > Hi Tvrtko,
> > > > >
> > > > > On 10.05.2022 09:28, Tvrtko Ursulin wrote:
> > > > > > On 29/04/2022 20:56, Ashutosh Dixit wrote:
> > > > > > > All kmalloc'd kobjects need a kobject_put() to free memory. For 
> > > > > > > example in
> > > > > > > previous code, kobj_gt_release() never gets called. The 
> > > > > > > requirement of
> > > > > > > kobject_put() now results in a slightly different code organization.
> > > > > > >
> > > > > > > v2: s/gtn/gt/ (Andi)
> > > > > > >
> > > > > > > Cc: Andi Shyti <andi.shyti@xxxxxxxxx>
> > > > > > > Cc: Andrzej Hajda <andrzej.hajda@xxxxxxxxx>
> > > > > > > Fixes: b770bcfae9ad ("drm/i915/gt: create per-tile sysfs interface")
> > > > > > > Signed-off-by: Ashutosh Dixit <ashutosh.dixit@xxxxxxxxx>
> > > > > > > ---
> > > > > > >   drivers/gpu/drm/i915/gt/intel_gt.c       |  1 +
> > > > > > >   drivers/gpu/drm/i915/gt/intel_gt_sysfs.c | 29 
> > > > > > > ++++++++++--------------
> > > > > > >   drivers/gpu/drm/i915/gt/intel_gt_sysfs.h |  6 +----
> > > > > > >   drivers/gpu/drm/i915/gt/intel_gt_types.h |  3 +++
> > > > > > >   drivers/gpu/drm/i915/i915_sysfs.c        |  2 ++
> > > > > > >   5 files changed, 19 insertions(+), 22 deletions(-)
> > > > > > >
> > > > > > > diff --git a/drivers/gpu/drm/i915/gt/intel_gt.c 
> > > > > > > b/drivers/gpu/drm/i915/gt/intel_gt.c
> > > > > > > index 92394f13b42f..9aede288eb86 100644
> > > > > > > --- a/drivers/gpu/drm/i915/gt/intel_gt.c
> > > > > > > +++ b/drivers/gpu/drm/i915/gt/intel_gt.c
> > > > > > > @@ -785,6 +785,7 @@ void intel_gt_driver_unregister(struct 
> > > > > > > intel_gt *gt)
> > > > > > >   {
> > > > > > >       intel_wakeref_t wakeref;
> > > > > > >   +    intel_gt_sysfs_unregister(gt);
> > > > > > >       intel_rps_driver_unregister(&gt->rps);
> > > > > > >       intel_gsc_fini(&gt->gsc);
> > > > > > >   diff --git a/drivers/gpu/drm/i915/gt/intel_gt_sysfs.c 
> > > > > > > b/drivers/gpu/drm/i915/gt/intel_gt_sysfs.c
> > > > > > > index 8ec8bc660c8c..9e4ebf53379b 100644
> > > > > > > --- a/drivers/gpu/drm/i915/gt/intel_gt_sysfs.c
> > > > > > > +++ b/drivers/gpu/drm/i915/gt/intel_gt_sysfs.c
> > > > > > > @@ -24,7 +24,7 @@ bool is_object_gt(struct kobject *kobj)
> > > > > > >     static struct intel_gt *kobj_to_gt(struct kobject *kobj)
> > > > > > >   {
> > > > > > > -    return container_of(kobj, struct kobj_gt, base)->gt;
> > > > > > > +    return container_of(kobj, struct intel_gt, sysfs_gt);
> > > > > > >   }
> > > > > > >     struct intel_gt *intel_gt_sysfs_get_drvdata(struct device *dev,
> > > > > > > @@ -72,9 +72,9 @@ static struct attribute *id_attrs[] = {
> > > > > > >   };
> > > > > > >   ATTRIBUTE_GROUPS(id);
> > > > > > >   +/* A kobject needs a release() method even if it does nothing */
> > > > > > >   static void kobj_gt_release(struct kobject *kobj)
> > > > > > >   {
> > > > > > > -    kfree(kobj);
> > > > > > >   }
> > > > > > >     static struct kobj_type kobj_gt_type = {
> > > > > > > @@ -85,8 +85,6 @@ static struct kobj_type kobj_gt_type = {
> > > > > > >     void intel_gt_sysfs_register(struct intel_gt *gt)
> > > > > > >   {
> > > > > > > -    struct kobj_gt *kg;
> > > > > > > -
> > > > > > >       /*
> > > > > > >        * We need to make things right with the
> > > > > > >        * ABI compatibility. The files were originally
> > > > > > > @@ -98,25 +96,22 @@ void intel_gt_sysfs_register(struct intel_gt 
> > > > > > > *gt)
> > > > > > >       if (gt_is_root(gt))
> > > > > > >           intel_gt_sysfs_pm_init(gt, gt_get_parent_obj(gt));
> > > > > > >   -    kg = kzalloc(sizeof(*kg), GFP_KERNEL);
> > > > > > > -    if (!kg)
> > > > > > > +    /* init and xfer ownership to sysfs tree */
> > > > > > > +    if (kobject_init_and_add(&gt->sysfs_gt, &kobj_gt_type,
> > > > > > > +                 gt->i915->sysfs_gt, "gt%d", gt->info.id))
> > > > > >
> > > > > > Was there closure/agreement on the matter of whether or not there 
> > > > > > is a potential race between "kfree(gt)" and sysfs access (last put 
> > > > > > from sysfs that is)? I've noticed Andrzej and Ashutosh were 
> > > > > > discussing it but did not read all the details.
> > > > >
> > > > > Not really :)
> > > > > IMO docs are against this practice, Ashutosh shows examples of this 
> > > > > practice in code and according to his analysis it is safe.
> > > > > I gave up looking for contradictions :) Either it is OK, kobject is 
> > > > > not fully shared object, docs are obsolete and needs update, either 
> > > > > the patch is wrong.
> > > > > Anyway finally I tend to accept this solution, I failed to prove it 
> > > > > is wrong :)
> > > >
> > > > Like a question of whether hotunplug can be triggered while 
> > > > userspace is sitting in a sysfs hook? Final kfree then has to be 
> > > > delayed until userspace exists.
> > > >
> > > > Btw where is the "kfree(gt)" for the tiles on the PCI remove path? I 
> > > > can't find it.. Do we have a leak?
> > >
> > > intel_gt_tile_cleanup ?
> >
> > Called from intel_gt_release_all, whose only caller is the failure 
> > path of i915_driver_probe. Feels like something is missing?
>
> This is final proof this patch is safe - no kfree, no UAF :)
>
> Apparently it is broken in internal branch as well.
> Should I take care of it?

Don't know - can you see with Andi?

I *think* even though the patch which added this code carries my name, 
it is probably quite far from what I originally wrote. (I alluded to 
that in a1a70e75-2068-fa69-e307-456d031b25b1@xxxxxxxxxxxxxxx, maybe I 
should have been more explicit that I don't think it should have 
preserved my authorship.) At least I checked that my late 2019. version 
and it did not seem to have the gt leak issue. If it did I would have 
felt responsible to fix it. :) As it stands init/de-init paths are 
always tricky and need more time to look into than I have at the moment.

Regards,

Tvrtko