diff mbox series

drm/i915: Fix potential context UAFs

Message ID 20230103234948.1218393-1-robdclark@gmail.com
State New
Headers
Series drm/i915: Fix potential context UAFs |

Commit Message

Rob Clark Jan. 3, 2023, 11:49 p.m. UTC 
  From: Rob Clark <robdclark@chromium.org>

gem_context_register() makes the context visible to userspace, and which
point a separate thread can trigger the I915_GEM_CONTEXT_DESTROY ioctl.
So we need to ensure that nothing uses the ctx ptr after this.  And we
need to ensure that adding the ctx to the xarray is the *last* thing
that gem_context_register() does with the ctx pointer.

Signed-off-by: Rob Clark <robdclark@chromium.org>
---
 drivers/gpu/drm/i915/gem/i915_gem_context.c | 24 +++++++++++++++------
 1 file changed, 18 insertions(+), 6 deletions(-)

Comments

Tvrtko Ursulin Jan. 4, 2023, 9:33 a.m. UTC | #1 
On 03/01/2023 23:49, Rob Clark wrote:
> From: Rob Clark <robdclark@chromium.org>
> 
> gem_context_register() makes the context visible to userspace, and which
> point a separate thread can trigger the I915_GEM_CONTEXT_DESTROY ioctl.
> So we need to ensure that nothing uses the ctx ptr after this.  And we
> need to ensure that adding the ctx to the xarray is the *last* thing
> that gem_context_register() does with the ctx pointer.

Any backtraces from oopses or notes on how it was found to record in the commit message?

> Signed-off-by: Rob Clark <robdclark@chromium.org>

Fixes: a4c1cdd34e2c ("drm/i915/gem: Delay context creation (v3)")
References: 3aa9945a528e ("drm/i915: Separate GEM context construction and registration to userspace")
Cc: <stable@vger.kernel.org> # v5.15+

> ---
>   drivers/gpu/drm/i915/gem/i915_gem_context.c | 24 +++++++++++++++------
>   1 file changed, 18 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c b/drivers/gpu/drm/i915/gem/i915_gem_context.c
> index 7f2831efc798..6250de9b9196 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_context.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c
> @@ -1688,6 +1688,10 @@ void i915_gem_init__contexts(struct drm_i915_private *i915)
>   	init_contexts(&i915->gem.contexts);
>   }
>   
> +/*
> + * Note that this implicitly consumes the ctx reference, by placing
> + * the ctx in the context_xa.
> + */
>   static void gem_context_register(struct i915_gem_context *ctx,
>   				 struct drm_i915_file_private *fpriv,
>   				 u32 id)
> @@ -1703,10 +1707,6 @@ static void gem_context_register(struct i915_gem_context *ctx,
>   	snprintf(ctx->name, sizeof(ctx->name), "%s[%d]",
>   		 current->comm, pid_nr(ctx->pid));
>   
> -	/* And finally expose ourselves to userspace via the idr */
> -	old = xa_store(&fpriv->context_xa, id, ctx, GFP_KERNEL);
> -	WARN_ON(old);
> -
>   	spin_lock(&ctx->client->ctx_lock);
>   	list_add_tail_rcu(&ctx->client_link, &ctx->client->ctx_list);
>   	spin_unlock(&ctx->client->ctx_lock);
> @@ -1714,6 +1714,10 @@ static void gem_context_register(struct i915_gem_context *ctx,
>   	spin_lock(&i915->gem.contexts.lock);
>   	list_add_tail(&ctx->link, &i915->gem.contexts.list);
>   	spin_unlock(&i915->gem.contexts.lock);
> +
> +	/* And finally expose ourselves to userspace via the idr */
> +	old = xa_store(&fpriv->context_xa, id, ctx, GFP_KERNEL);
> +	WARN_ON(old);

Have you seen that this hunk is needed or just moving it for a good measure? To be clear, it is probably best to move it even if the current placement cannot cause any problems, I am just double-checking if you had any concrete observations here while mulling over easier stable backports if we would omit it.

>   }
>   
>   int i915_gem_context_open(struct drm_i915_private *i915,
> @@ -2199,14 +2203,22 @@ finalize_create_context_locked(struct drm_i915_file_private *file_priv,
>   	if (IS_ERR(ctx))
>   		return ctx;
>   
> +	/*
> +	 * One for the xarray and one for the caller.  We need to grab
> +	 * the reference *prior* to making the ctx visble to userspace
> +	 * in gem_context_register(), as at any point after that
> +	 * userspace can try to race us with another thread destroying
> +	 * the context under our feet.
> +	 */
> +	i915_gem_context_get(ctx);
> +
>   	gem_context_register(ctx, file_priv, id);
>   
>   	old = xa_erase(&file_priv->proto_context_xa, id);
>   	GEM_BUG_ON(old != pc);
>   	proto_context_close(file_priv->dev_priv, pc);
>   
> -	/* One for the xarray and one for the caller */
> -	return i915_gem_context_get(ctx);
> +	return ctx;

Otherwise userspace can look up a context which hasn't had it's reference count increased yep. I can add the Fixes: and Stable: tags while merging if no complaints.

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

Regards,

Tvrtko

>   }
>   
>   struct i915_gem_context *
Rob Clark Jan. 4, 2023, 4:01 p.m. UTC | #2 
On Wed, Jan 4, 2023 at 1:34 AM Tvrtko Ursulin
<tvrtko.ursulin@linux.intel.com> wrote:
>
>
> On 03/01/2023 23:49, Rob Clark wrote:
> > From: Rob Clark <robdclark@chromium.org>
> >
> > gem_context_register() makes the context visible to userspace, and which
> > point a separate thread can trigger the I915_GEM_CONTEXT_DESTROY ioctl.
> > So we need to ensure that nothing uses the ctx ptr after this.  And we
> > need to ensure that adding the ctx to the xarray is the *last* thing
> > that gem_context_register() does with the ctx pointer.
>
> Any backtraces from oopses or notes on how it was found to record in the commit message?

It was a UAF bug that was reported to us

https://bugs.chromium.org/p/chromium/issues/detail?id=1401594 (but I
guess security bugs are not going to be visible)

>
> > Signed-off-by: Rob Clark <robdclark@chromium.org>
>
> Fixes: a4c1cdd34e2c ("drm/i915/gem: Delay context creation (v3)")
> References: 3aa9945a528e ("drm/i915: Separate GEM context construction and registration to userspace")
> Cc: <stable@vger.kernel.org> # v5.15+
>
> > ---
> >   drivers/gpu/drm/i915/gem/i915_gem_context.c | 24 +++++++++++++++------
> >   1 file changed, 18 insertions(+), 6 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c b/drivers/gpu/drm/i915/gem/i915_gem_context.c
> > index 7f2831efc798..6250de9b9196 100644
> > --- a/drivers/gpu/drm/i915/gem/i915_gem_context.c
> > +++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c
> > @@ -1688,6 +1688,10 @@ void i915_gem_init__contexts(struct drm_i915_private *i915)
> >       init_contexts(&i915->gem.contexts);
> >   }
> >
> > +/*
> > + * Note that this implicitly consumes the ctx reference, by placing
> > + * the ctx in the context_xa.
> > + */
> >   static void gem_context_register(struct i915_gem_context *ctx,
> >                                struct drm_i915_file_private *fpriv,
> >                                u32 id)
> > @@ -1703,10 +1707,6 @@ static void gem_context_register(struct i915_gem_context *ctx,
> >       snprintf(ctx->name, sizeof(ctx->name), "%s[%d]",
> >                current->comm, pid_nr(ctx->pid));
> >
> > -     /* And finally expose ourselves to userspace via the idr */
> > -     old = xa_store(&fpriv->context_xa, id, ctx, GFP_KERNEL);
> > -     WARN_ON(old);
> > -
> >       spin_lock(&ctx->client->ctx_lock);
> >       list_add_tail_rcu(&ctx->client_link, &ctx->client->ctx_list);
> >       spin_unlock(&ctx->client->ctx_lock);
> > @@ -1714,6 +1714,10 @@ static void gem_context_register(struct i915_gem_context *ctx,
> >       spin_lock(&i915->gem.contexts.lock);
> >       list_add_tail(&ctx->link, &i915->gem.contexts.list);
> >       spin_unlock(&i915->gem.contexts.lock);
> > +
> > +     /* And finally expose ourselves to userspace via the idr */
> > +     old = xa_store(&fpriv->context_xa, id, ctx, GFP_KERNEL);
> > +     WARN_ON(old);
>
> Have you seen that this hunk is needed or just moving it for a good measure? To be clear, it is probably best to move it even if the current placement cannot cause any problems, I am just double-checking if you had any concrete observations here while mulling over easier stable backports if we would omit it.
>

This was actually the originally reported issue, the
finalize_create_context_locked() part was something I found when the
original report prompted me to audit gem_context_register() call
paths.


> >   }
> >
> >   int i915_gem_context_open(struct drm_i915_private *i915,
> > @@ -2199,14 +2203,22 @@ finalize_create_context_locked(struct drm_i915_file_private *file_priv,
> >       if (IS_ERR(ctx))
> >               return ctx;
> >
> > +     /*
> > +      * One for the xarray and one for the caller.  We need to grab
> > +      * the reference *prior* to making the ctx visble to userspace
> > +      * in gem_context_register(), as at any point after that
> > +      * userspace can try to race us with another thread destroying
> > +      * the context under our feet.
> > +      */
> > +     i915_gem_context_get(ctx);
> > +
> >       gem_context_register(ctx, file_priv, id);
> >
> >       old = xa_erase(&file_priv->proto_context_xa, id);
> >       GEM_BUG_ON(old != pc);
> >       proto_context_close(file_priv->dev_priv, pc);
> >
> > -     /* One for the xarray and one for the caller */
> > -     return i915_gem_context_get(ctx);
> > +     return ctx;
>
> Otherwise userspace can look up a context which hasn't had it's reference count increased yep. I can add the Fixes: and Stable: tags while merging if no complaints.
>
> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

Thanks

BR,
-R

>
> Regards,
>
> Tvrtko
>
> >   }
> >
> >   struct i915_gem_context *
Andi Shyti Jan. 5, 2023, 3:52 p.m. UTC | #3 
Hi Rob,

On Tue, Jan 03, 2023 at 03:49:46PM -0800, Rob Clark wrote:
> From: Rob Clark <robdclark@chromium.org>
> 
> gem_context_register() makes the context visible to userspace, and which
> point a separate thread can trigger the I915_GEM_CONTEXT_DESTROY ioctl.
> So we need to ensure that nothing uses the ctx ptr after this.  And we
> need to ensure that adding the ctx to the xarray is the *last* thing
> that gem_context_register() does with the ctx pointer.
> 
> Signed-off-by: Rob Clark <robdclark@chromium.org>

Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>

I also agree with Tvrtko that we should add Stable: and Fixes:.

One little thing, "user after free" is clearer that UAF :)

Thanks,
Andi

> ---
>  drivers/gpu/drm/i915/gem/i915_gem_context.c | 24 +++++++++++++++------
>  1 file changed, 18 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c b/drivers/gpu/drm/i915/gem/i915_gem_context.c
> index 7f2831efc798..6250de9b9196 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_context.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c
> @@ -1688,6 +1688,10 @@ void i915_gem_init__contexts(struct drm_i915_private *i915)
>  	init_contexts(&i915->gem.contexts);
>  }
>  
> +/*
> + * Note that this implicitly consumes the ctx reference, by placing
> + * the ctx in the context_xa.
> + */
>  static void gem_context_register(struct i915_gem_context *ctx,
>  				 struct drm_i915_file_private *fpriv,
>  				 u32 id)
> @@ -1703,10 +1707,6 @@ static void gem_context_register(struct i915_gem_context *ctx,
>  	snprintf(ctx->name, sizeof(ctx->name), "%s[%d]",
>  		 current->comm, pid_nr(ctx->pid));
>  
> -	/* And finally expose ourselves to userspace via the idr */
> -	old = xa_store(&fpriv->context_xa, id, ctx, GFP_KERNEL);
> -	WARN_ON(old);
> -
>  	spin_lock(&ctx->client->ctx_lock);
>  	list_add_tail_rcu(&ctx->client_link, &ctx->client->ctx_list);
>  	spin_unlock(&ctx->client->ctx_lock);
> @@ -1714,6 +1714,10 @@ static void gem_context_register(struct i915_gem_context *ctx,
>  	spin_lock(&i915->gem.contexts.lock);
>  	list_add_tail(&ctx->link, &i915->gem.contexts.list);
>  	spin_unlock(&i915->gem.contexts.lock);
> +
> +	/* And finally expose ourselves to userspace via the idr */
> +	old = xa_store(&fpriv->context_xa, id, ctx, GFP_KERNEL);
> +	WARN_ON(old);
>  }
>  
>  int i915_gem_context_open(struct drm_i915_private *i915,
> @@ -2199,14 +2203,22 @@ finalize_create_context_locked(struct drm_i915_file_private *file_priv,
>  	if (IS_ERR(ctx))
>  		return ctx;
>  
> +	/*
> +	 * One for the xarray and one for the caller.  We need to grab
> +	 * the reference *prior* to making the ctx visble to userspace
> +	 * in gem_context_register(), as at any point after that
> +	 * userspace can try to race us with another thread destroying
> +	 * the context under our feet.
> +	 */
> +	i915_gem_context_get(ctx);
> +
>  	gem_context_register(ctx, file_priv, id);
>  
>  	old = xa_erase(&file_priv->proto_context_xa, id);
>  	GEM_BUG_ON(old != pc);
>  	proto_context_close(file_priv->dev_priv, pc);
>  
> -	/* One for the xarray and one for the caller */
> -	return i915_gem_context_get(ctx);
> +	return ctx;
>  }
>  
>  struct i915_gem_context *
> -- 
> 2.38.1
Tvrtko Ursulin Jan. 5, 2023, 4 p.m. UTC | #4 
On 05/01/2023 15:52, Andi Shyti wrote:
> Hi Rob,
> 
> On Tue, Jan 03, 2023 at 03:49:46PM -0800, Rob Clark wrote:
>> From: Rob Clark <robdclark@chromium.org>
>>
>> gem_context_register() makes the context visible to userspace, and which
>> point a separate thread can trigger the I915_GEM_CONTEXT_DESTROY ioctl.
>> So we need to ensure that nothing uses the ctx ptr after this.  And we
>> need to ensure that adding the ctx to the xarray is the *last* thing
>> that gem_context_register() does with the ctx pointer.
>>
>> Signed-off-by: Rob Clark <robdclark@chromium.org>
> 
> Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
> 
> I also agree with Tvrtko that we should add Stable: and Fixes:.

Yeah I'll add them all when merging. Just waiting for full CI results. It will be like this:

Fixes: eb4dedae920a ("drm/i915/gem: Delay tracking the GEM context until it is registered")
Fixes: a4c1cdd34e2c ("drm/i915/gem: Delay context creation (v3)")
Fixes: 49bd54b390c2 ("drm/i915: Track all user contexts per client")
Cc: <stable@vger.kernel.org> # v5.10+

Regards,

Tvrtko
  
> One little thing, "user after free" is clearer that UAF :)
> 
> Thanks,
> Andi
> 
>> ---
>>   drivers/gpu/drm/i915/gem/i915_gem_context.c | 24 +++++++++++++++------
>>   1 file changed, 18 insertions(+), 6 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c b/drivers/gpu/drm/i915/gem/i915_gem_context.c
>> index 7f2831efc798..6250de9b9196 100644
>> --- a/drivers/gpu/drm/i915/gem/i915_gem_context.c
>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c
>> @@ -1688,6 +1688,10 @@ void i915_gem_init__contexts(struct drm_i915_private *i915)
>>   	init_contexts(&i915->gem.contexts);
>>   }
>>   
>> +/*
>> + * Note that this implicitly consumes the ctx reference, by placing
>> + * the ctx in the context_xa.
>> + */
>>   static void gem_context_register(struct i915_gem_context *ctx,
>>   				 struct drm_i915_file_private *fpriv,
>>   				 u32 id)
>> @@ -1703,10 +1707,6 @@ static void gem_context_register(struct i915_gem_context *ctx,
>>   	snprintf(ctx->name, sizeof(ctx->name), "%s[%d]",
>>   		 current->comm, pid_nr(ctx->pid));
>>   
>> -	/* And finally expose ourselves to userspace via the idr */
>> -	old = xa_store(&fpriv->context_xa, id, ctx, GFP_KERNEL);
>> -	WARN_ON(old);
>> -
>>   	spin_lock(&ctx->client->ctx_lock);
>>   	list_add_tail_rcu(&ctx->client_link, &ctx->client->ctx_list);
>>   	spin_unlock(&ctx->client->ctx_lock);
>> @@ -1714,6 +1714,10 @@ static void gem_context_register(struct i915_gem_context *ctx,
>>   	spin_lock(&i915->gem.contexts.lock);
>>   	list_add_tail(&ctx->link, &i915->gem.contexts.list);
>>   	spin_unlock(&i915->gem.contexts.lock);
>> +
>> +	/* And finally expose ourselves to userspace via the idr */
>> +	old = xa_store(&fpriv->context_xa, id, ctx, GFP_KERNEL);
>> +	WARN_ON(old);
>>   }
>>   
>>   int i915_gem_context_open(struct drm_i915_private *i915,
>> @@ -2199,14 +2203,22 @@ finalize_create_context_locked(struct drm_i915_file_private *file_priv,
>>   	if (IS_ERR(ctx))
>>   		return ctx;
>>   
>> +	/*
>> +	 * One for the xarray and one for the caller.  We need to grab
>> +	 * the reference *prior* to making the ctx visble to userspace
>> +	 * in gem_context_register(), as at any point after that
>> +	 * userspace can try to race us with another thread destroying
>> +	 * the context under our feet.
>> +	 */
>> +	i915_gem_context_get(ctx);
>> +
>>   	gem_context_register(ctx, file_priv, id);
>>   
>>   	old = xa_erase(&file_priv->proto_context_xa, id);
>>   	GEM_BUG_ON(old != pc);
>>   	proto_context_close(file_priv->dev_priv, pc);
>>   
>> -	/* One for the xarray and one for the caller */
>> -	return i915_gem_context_get(ctx);
>> +	return ctx;
>>   }
>>   
>>   struct i915_gem_context *
>> -- 
>> 2.38.1
Tvrtko Ursulin Jan. 6, 2023, 10:15 a.m. UTC | #5 
On 05/01/2023 16:00, Tvrtko Ursulin wrote:
> 
> On 05/01/2023 15:52, Andi Shyti wrote:
>> Hi Rob,
>>
>> On Tue, Jan 03, 2023 at 03:49:46PM -0800, Rob Clark wrote:
>>> From: Rob Clark <robdclark@chromium.org>
>>>
>>> gem_context_register() makes the context visible to userspace, and which
>>> point a separate thread can trigger the I915_GEM_CONTEXT_DESTROY ioctl.
>>> So we need to ensure that nothing uses the ctx ptr after this.  And we
>>> need to ensure that adding the ctx to the xarray is the *last* thing
>>> that gem_context_register() does with the ctx pointer.
>>>
>>> Signed-off-by: Rob Clark <robdclark@chromium.org>
>>
>> Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
>>
>> I also agree with Tvrtko that we should add Stable: and Fixes:.
> 
> Yeah I'll add them all when merging. Just waiting for full CI results. 
> It will be like this:
> 
> Fixes: eb4dedae920a ("drm/i915/gem: Delay tracking the GEM context until 
> it is registered")
> Fixes: a4c1cdd34e2c ("drm/i915/gem: Delay context creation (v3)")
> Fixes: 49bd54b390c2 ("drm/i915: Track all user contexts per client")
> Cc: <stable@vger.kernel.org> # v5.10+

Pushed to drm-intel-gt-next - thanks for the fix and reviews.

Regards,

Tvrtko

> 
> Regards,
> 
> Tvrtko
> 
>> One little thing, "user after free" is clearer that UAF :)
>>
>> Thanks,
>> Andi
>>
>>> ---
>>>   drivers/gpu/drm/i915/gem/i915_gem_context.c | 24 +++++++++++++++------
>>>   1 file changed, 18 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c 
>>> b/drivers/gpu/drm/i915/gem/i915_gem_context.c
>>> index 7f2831efc798..6250de9b9196 100644
>>> --- a/drivers/gpu/drm/i915/gem/i915_gem_context.c
>>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c
>>> @@ -1688,6 +1688,10 @@ void i915_gem_init__contexts(struct 
>>> drm_i915_private *i915)
>>>       init_contexts(&i915->gem.contexts);
>>>   }
>>> +/*
>>> + * Note that this implicitly consumes the ctx reference, by placing
>>> + * the ctx in the context_xa.
>>> + */
>>>   static void gem_context_register(struct i915_gem_context *ctx,
>>>                    struct drm_i915_file_private *fpriv,
>>>                    u32 id)
>>> @@ -1703,10 +1707,6 @@ static void gem_context_register(struct 
>>> i915_gem_context *ctx,
>>>       snprintf(ctx->name, sizeof(ctx->name), "%s[%d]",
>>>            current->comm, pid_nr(ctx->pid));
>>> -    /* And finally expose ourselves to userspace via the idr */
>>> -    old = xa_store(&fpriv->context_xa, id, ctx, GFP_KERNEL);
>>> -    WARN_ON(old);
>>> -
>>>       spin_lock(&ctx->client->ctx_lock);
>>>       list_add_tail_rcu(&ctx->client_link, &ctx->client->ctx_list);
>>>       spin_unlock(&ctx->client->ctx_lock);
>>> @@ -1714,6 +1714,10 @@ static void gem_context_register(struct 
>>> i915_gem_context *ctx,
>>>       spin_lock(&i915->gem.contexts.lock);
>>>       list_add_tail(&ctx->link, &i915->gem.contexts.list);
>>>       spin_unlock(&i915->gem.contexts.lock);
>>> +
>>> +    /* And finally expose ourselves to userspace via the idr */
>>> +    old = xa_store(&fpriv->context_xa, id, ctx, GFP_KERNEL);
>>> +    WARN_ON(old);
>>>   }
>>>   int i915_gem_context_open(struct drm_i915_private *i915,
>>> @@ -2199,14 +2203,22 @@ finalize_create_context_locked(struct 
>>> drm_i915_file_private *file_priv,
>>>       if (IS_ERR(ctx))
>>>           return ctx;
>>> +    /*
>>> +     * One for the xarray and one for the caller.  We need to grab
>>> +     * the reference *prior* to making the ctx visble to userspace
>>> +     * in gem_context_register(), as at any point after that
>>> +     * userspace can try to race us with another thread destroying
>>> +     * the context under our feet.
>>> +     */
>>> +    i915_gem_context_get(ctx);
>>> +
>>>       gem_context_register(ctx, file_priv, id);
>>>       old = xa_erase(&file_priv->proto_context_xa, id);
>>>       GEM_BUG_ON(old != pc);
>>>       proto_context_close(file_priv->dev_priv, pc);
>>> -    /* One for the xarray and one for the caller */
>>> -    return i915_gem_context_get(ctx);
>>> +    return ctx;
>>>   }
>>>   struct i915_gem_context *
>>> -- 
>>> 2.38.1
diff mbox series

Patch

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c b/drivers/gpu/drm/i915/gem/i915_gem_context.c
index 7f2831efc798..6250de9b9196 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_context.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c
@@ -1688,6 +1688,10 @@  void i915_gem_init__contexts(struct drm_i915_private *i915)
 	init_contexts(&i915->gem.contexts);
 }
 
+/*
+ * Note that this implicitly consumes the ctx reference, by placing
+ * the ctx in the context_xa.
+ */
 static void gem_context_register(struct i915_gem_context *ctx,
 				 struct drm_i915_file_private *fpriv,
 				 u32 id)
@@ -1703,10 +1707,6 @@  static void gem_context_register(struct i915_gem_context *ctx,
 	snprintf(ctx->name, sizeof(ctx->name), "%s[%d]",
 		 current->comm, pid_nr(ctx->pid));
 
-	/* And finally expose ourselves to userspace via the idr */
-	old = xa_store(&fpriv->context_xa, id, ctx, GFP_KERNEL);
-	WARN_ON(old);
-
 	spin_lock(&ctx->client->ctx_lock);
 	list_add_tail_rcu(&ctx->client_link, &ctx->client->ctx_list);
 	spin_unlock(&ctx->client->ctx_lock);
@@ -1714,6 +1714,10 @@  static void gem_context_register(struct i915_gem_context *ctx,
 	spin_lock(&i915->gem.contexts.lock);
 	list_add_tail(&ctx->link, &i915->gem.contexts.list);
 	spin_unlock(&i915->gem.contexts.lock);
+
+	/* And finally expose ourselves to userspace via the idr */
+	old = xa_store(&fpriv->context_xa, id, ctx, GFP_KERNEL);
+	WARN_ON(old);
 }
 
 int i915_gem_context_open(struct drm_i915_private *i915,
@@ -2199,14 +2203,22 @@  finalize_create_context_locked(struct drm_i915_file_private *file_priv,
 	if (IS_ERR(ctx))
 		return ctx;
 
+	/*
+	 * One for the xarray and one for the caller.  We need to grab
+	 * the reference *prior* to making the ctx visble to userspace
+	 * in gem_context_register(), as at any point after that
+	 * userspace can try to race us with another thread destroying
+	 * the context under our feet.
+	 */
+	i915_gem_context_get(ctx);
+
 	gem_context_register(ctx, file_priv, id);
 
 	old = xa_erase(&file_priv->proto_context_xa, id);
 	GEM_BUG_ON(old != pc);
 	proto_context_close(file_priv->dev_priv, pc);
 
-	/* One for the xarray and one for the caller */
-	return i915_gem_context_get(ctx);
+	return ctx;
 }
 
 struct i915_gem_context *