Re: [PATCH 09/12] of: overlay: avoid race condition between applying multiple overlays

[Rob Herring]

On Mon, Oct 2, 2017 at 10:53 PM,  <frowand.list@xxxxxxxxx> wrote:
> From: Frank Rowand <frank.rowand@xxxxxxxx>
>
> The process of applying an overlay consists of:
>   - unflatten an overlay FDT (flattened device tree) into an
>     EDT (expanded device tree)
>   - fixup the phandle values in the overlay EDT to fit in a
>     range above the phandle values in the live device tree
>   - create the overlay changeset to reflect the contents of
>     the overlay EDT
>   - apply the overlay changeset, to modify the live device tree,
>     potentially changing the maximum phandle value in the live
>     device tree
>
> There is currently no protection against two overlay applies
> concurrently determining what range of phandle values are in use
> in the live device tree, and subsequently changing that range.
> Add a mutex to prevent multiple overlay applies from occurring
> simultaneously.
>
> Ignoring 2 checkpatch warnings: Prefer using '"%s...", __func__'
> so that the WARN() string will be more easily grepped.
>
> Signed-off-by: Frank Rowand <frank.rowand@xxxxxxxx>
> ---
>  drivers/gpu/drm/tilcdc/tilcdc_slave_compat.c |  7 +++++++
>  drivers/of/overlay.c                         | 22 ++++++++++++++++++++++
>  drivers/of/unittest.c                        | 21 +++++++++++++++++++++
>  include/linux/of.h                           | 19 +++++++++++++++++++
>  4 files changed, 69 insertions(+)
>
> diff --git a/drivers/gpu/drm/tilcdc/tilcdc_slave_compat.c b/drivers/gpu/drm/tilcdc/tilcdc_slave_compat.c
> index 7a7be0515bfd..c99f7924b1c6 100644
> --- a/drivers/gpu/drm/tilcdc/tilcdc_slave_compat.c
> +++ b/drivers/gpu/drm/tilcdc/tilcdc_slave_compat.c
> @@ -221,6 +221,11 @@ static void __init tilcdc_convert_slave_node(void)
>                 goto out;
>         }
>
> +       /*
> +        * protect from of_resolve_phandles() through of_overlay_apply()
> +        */
> +       of_overlay_mutex_lock();
> +

We can't be relying on callers to get the locking right...

>         overlay = tilcdc_get_overlay(&kft);
>         if (!overlay)
>                 goto out;
> @@ -256,6 +261,8 @@ static void __init tilcdc_convert_slave_node(void)
>                 pr_info("%s: ti,tilcdc,slave node successfully converted\n",
>                         __func__);
>  out:
> +       of_overlay_mutex_unlock();
> +
>         kfree_table_free(&kft);
>         of_node_put(i2c);
>         of_node_put(slave);
> diff --git a/drivers/of/overlay.c b/drivers/of/overlay.c
> index a0d3222febdc..4ed372af6ce7 100644
> --- a/drivers/of/overlay.c
> +++ b/drivers/of/overlay.c
> @@ -71,6 +71,28 @@ static int build_changeset_next_level(struct overlay_changeset *ovcs,
>                 const struct device_node *overlay_node,
>                 bool is_symbols_node);
>
> +/*
> + * of_resolve_phandles() finds the largest phandle in the live tree.
> + * of_overlay_apply() may add a larger phandle to the live tree.
> + * Do not allow race between two overlays being applied simultaneously:
> + *    mutex_lock(&of_overlay_phandle_mutex)
> + *    of_resolve_phandles()
> + *    of_overlay_apply()
> + *    mutex_unlock(&of_overlay_phandle_mutex)

Why do these need to be separate functions? I think I mentioned it
before, but essentially overlay_data_add() should be part of the
overlay API. We may need to allow for callers to do each step, but
generally I think the interface should just be "apply this fdt blob".

Rob