Re: [PATCH] drm/i915: Fix potential context UAFs

[Andi Shyti]

Hi Rob,

On Tue, Jan 03, 2023 at 03:49:46PM -0800, Rob Clark wrote:
> From: Rob Clark <robdclark@xxxxxxxxxxxx>
> 
> 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@xxxxxxxxxxxx>

Reviewed-by: Andi Shyti <andi.shyti@xxxxxxxxxxxxxxx>

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