linux-kernel - Re: [PATCH 3/3] drm/nouveau: clean up all clients on device removal

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Date:   Wed, 25 Nov 2020 13:44:33 -0500
From:   Lyude Paul <lyude@...hat.com>
To:     Jeremy Cline <jcline@...hat.com>, Ben Skeggs <bskeggs@...hat.com>
Cc:     Karol Herbst <kherbst@...hat.com>, David Airlie <airlied@...ux.ie>,
        nouveau@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] drm/nouveau: clean up all clients on device removal

On Tue, 2020-11-03 at 14:49 -0500, Jeremy Cline wrote:
> The postclose handler can run after the device has been removed (or the
> driver has been unbound) since userspace clients are free to hold the
> file open the file as long as they want. Because the device removal
            ^
         typo

> callback frees the entire nouveau_drm structure, any reference to it in
> the postclose handler will result in a use-after-free.
> 
> To reproduce this, one must simply open the device file, unbind the
> driver (or physically remove the device), and then close the device
> file. This was found and can be reproduced easily with the IGT
> core_hotunplug tests.
> 
> To avoid this, all clients are cleaned up in the device finialization
> rather than deferring it to the postclose handler, and the postclose
> handler is protected by a critical section which ensures the
> drm_dev_unplug() and the postclose handler won't race.
> 
> This is not an ideal fix, since as I understand the proposed plan for
> the kernel<->userspace interface for hotplug support, destroying the
> client before the file is closed will cause problems. However, I believe
> to properly fix this issue, the lifetime of the nouveau_drm structure
> needs to be extended to match the drm_device, and this proved to be a
> rather invasive change. Thus, I've broken this out so the fix can be
> easily backported.

JFYI - if the intent is for this to be backported, you should add:

Cc: stable@...r.kernel.org 
> Signed-off-by: Jeremy Cline <jcline@...hat.com>
> ---
>  drivers/gpu/drm/nouveau/nouveau_drm.c | 30 +++++++++++++++++++++++++++
>  1 file changed, 30 insertions(+)
> 
> diff --git a/drivers/gpu/drm/nouveau/nouveau_drm.c
> b/drivers/gpu/drm/nouveau/nouveau_drm.c
> index d182b877258a..74fab777f4d0 100644
> --- a/drivers/gpu/drm/nouveau/nouveau_drm.c
> +++ b/drivers/gpu/drm/nouveau/nouveau_drm.c
> @@ -628,6 +628,7 @@ nouveau_drm_device_init(struct drm_device *dev)
>  static void
>  nouveau_drm_device_fini(struct drm_device *dev)
>  {
> +       struct nouveau_cli *cli, *temp_cli;
>         struct nouveau_drm *drm = nouveau_drm(dev);
>  
>         if (nouveau_pmops_runtime()) {
> @@ -652,6 +653,24 @@ nouveau_drm_device_fini(struct drm_device *dev)
>         nouveau_ttm_fini(drm);
>         nouveau_vga_fini(drm);
>  
> +       /*
> +        * There may be existing clients from as-yet unclosed files. For now,
> +        * clean them up here rather than deferring until the file is closed,
> +        * but this likely not correct if we want to support hot-unplugging
> +        * properly.
> +        */
> +       mutex_lock(&drm->clients_lock);
> +       list_for_each_entry_safe(cli, temp_cli, &drm->clients, head) {
> +               list_del(&cli->head);
> +               mutex_lock(&cli->mutex);
> +               if (cli->abi16)
> +                       nouveau_abi16_fini(cli->abi16);
> +               mutex_unlock(&cli->mutex);
> +               nouveau_cli_fini(cli);
> +               kfree(cli);
> +       }
> +       mutex_unlock(&drm->clients_lock);
> +
>         nouveau_cli_fini(&drm->client);
>         nouveau_cli_fini(&drm->master);
>         nvif_parent_dtor(&drm->parent);
> @@ -1110,6 +1129,16 @@ nouveau_drm_postclose(struct drm_device *dev, struct
> drm_file *fpriv)
>  {
>         struct nouveau_cli *cli = nouveau_cli(fpriv);
>         struct nouveau_drm *drm = nouveau_drm(dev);
> +       int dev_index;
> +
> +       /*
> +        * The device is gone, and as it currently stands all clients are
> +        * cleaned up in the removal codepath. In the future this may change
> +        * so that we can support hot-unplugging, but for now we immediately
> +        * return to avoid a double-free situation.
> +        */
> +       if (!drm_dev_enter(dev, &dev_index))
> +               return;
>  
>         pm_runtime_get_sync(dev->dev);
>  
> @@ -1126,6 +1155,7 @@ nouveau_drm_postclose(struct drm_device *dev, struct
> drm_file *fpriv)
>         kfree(cli);
>         pm_runtime_mark_last_busy(dev->dev);
>         pm_runtime_put_autosuspend(dev->dev);
> +       drm_dev_exit(dev_index);
>  }
>  
>  static const struct drm_ioctl_desc

-- 
Sincerely,
   Lyude Paul (she/her)
   Software Engineer at Red Hat
   
Note: I deal with a lot of emails and have a lot of bugs on my plate. If you've
asked me a question, are waiting for a review/merge on a patch, etc. and I
haven't responded in a while, please feel free to send me another email to check
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