Re: [PATCH v4 1/1] of: unittest: overlay: ensure proper alignment of copied FDT

[Rob Herring]

On Thu, Apr 8, 2021 at 3:45 PM <frowand.list@xxxxxxxxx> wrote:
>
> From: Frank Rowand <frank.rowand@xxxxxxxx>
>
> The Devicetree standard specifies an 8 byte alignment of the FDT.
> Code in libfdt expects this alignment for an FDT image in memory.
> kmemdup() returns 4 byte alignment on openrisc.  Replace kmemdup()
> with kmalloc(), align pointer, memcpy() to get proper alignment.
>
> The 4 byte alignment exposed a related bug which triggered a crash
> on openrisc with:
> commit 79edff12060f ("scripts/dtc: Update to upstream version v1.6.0-51-g183df9e9c2b9")
> as reported in:
> https://lore.kernel.org/lkml/20210327224116.69309-1-linux@xxxxxxxxxxxx/
>
> Reported-by: Guenter Roeck <linux@xxxxxxxxxxxx>
> Signed-off-by: Frank Rowand <frank.rowand@xxxxxxxx>
>
> ---
>
> changes since version 1:
>   - use pointer from kmalloc() for kfree() instead of using pointer that
>     has been modified for FDT alignment
>
> changes since version 2:
>   - version 1 was a work in progress version, I failed to commit the following
>     final changes
>   - reorder first two arguments of of_overlay_apply()
>
> changes since version 3:
>   - size of memory allocation and size of copy after pointer alignment
>     differ, use separate variables with correct values for each case
>   - edit comment to more clearly describe that ovcs->fdt is the allocated
>     memory region, which may be different than where the aligned pointer points
>   - remove unused parameter from of_overlay_apply()
>
>  drivers/of/of_private.h |  2 ++
>  drivers/of/overlay.c    | 27 +++++++++++++++++----------
>  drivers/of/unittest.c   | 13 ++++++++++---
>  3 files changed, 29 insertions(+), 13 deletions(-)
>
> diff --git a/drivers/of/of_private.h b/drivers/of/of_private.h
> index d9e6a324de0a..d717efbd637d 100644
> --- a/drivers/of/of_private.h
> +++ b/drivers/of/of_private.h
> @@ -8,6 +8,8 @@
>   * Copyright (C) 1996-2005 Paul Mackerras.
>   */
>
> +#define FDT_ALIGN_SIZE 8
> +
>  /**
>   * struct alias_prop - Alias property in 'aliases' node
>   * @link:      List node to link the structure in aliases_lookup list
> diff --git a/drivers/of/overlay.c b/drivers/of/overlay.c
> index 50bbe0edf538..ecf967c57900 100644
> --- a/drivers/of/overlay.c
> +++ b/drivers/of/overlay.c
> @@ -57,7 +57,7 @@ struct fragment {
>   * struct overlay_changeset
>   * @id:                        changeset identifier
>   * @ovcs_list:         list on which we are located
> - * @fdt:               FDT that was unflattened to create @overlay_tree
> + * @fdt:               base of memory allocated to hold aligned FDT that was unflattened to create @overlay_tree
>   * @overlay_tree:      expanded device tree that contains the fragment nodes
>   * @count:             count of fragment structures
>   * @fragments:         fragment nodes in the overlay expanded device tree
> @@ -719,8 +719,8 @@ static struct device_node *find_target(struct device_node *info_node)
>  /**
>   * init_overlay_changeset() - initialize overlay changeset from overlay tree
>   * @ovcs:      Overlay changeset to build
> - * @fdt:       the FDT that was unflattened to create @tree
> - * @tree:      Contains all the overlay fragments and overlay fixup nodes
> + * @fdt:       base of memory allocated to hold aligned FDT that was unflattened to create @tree
> + * @tree:      Contains the overlay fragments and overlay fixup nodes
>   *
>   * Initialize @ovcs.  Populate @ovcs->fragments with node information from
>   * the top level of @tree.  The relevant top level nodes are the fragment
> @@ -873,7 +873,7 @@ static void free_overlay_changeset(struct overlay_changeset *ovcs)
>   * internal documentation
>   *
>   * of_overlay_apply() - Create and apply an overlay changeset
> - * @fdt:       the FDT that was unflattened to create @tree
> + * @fdt:       base of memory allocated to hold the aligned FDT
>   * @tree:      Expanded overlay device tree
>   * @ovcs_id:   Pointer to overlay changeset id
>   *
> @@ -913,7 +913,7 @@ static void free_overlay_changeset(struct overlay_changeset *ovcs)
>   */
>
>  static int of_overlay_apply(const void *fdt, struct device_node *tree,
> -               int *ovcs_id)
> +                           int *ovcs_id)
>  {
>         struct overlay_changeset *ovcs;
>         int ret = 0, ret_revert, ret_tmp;
> @@ -953,7 +953,9 @@ static int of_overlay_apply(const void *fdt, struct device_node *tree,
>         /*
>          * after overlay_notify(), ovcs->overlay_tree related pointers may have
>          * leaked to drivers, so can not kfree() tree, aka ovcs->overlay_tree;
> -        * and can not free fdt, aka ovcs->fdt
> +        * and can not free memory containing aligned fdt.  The aligned fdt
> +        * is contained within the memory at ovcs->fdt, possibly at an offset
> +        * from ovcs->fdt.
>          */
>         ret = overlay_notify(ovcs, OF_OVERLAY_PRE_APPLY);
>         if (ret) {
> @@ -1014,9 +1016,10 @@ static int of_overlay_apply(const void *fdt, struct device_node *tree,
>  int of_overlay_fdt_apply(const void *overlay_fdt, u32 overlay_fdt_size,
>                          int *ovcs_id)
>  {
> -       const void *new_fdt;
> +       void *new_fdt;
> +       void *new_fdt_align;
>         int ret;
> -       u32 size;
> +       u32 size, size_alloc;
>         struct device_node *overlay_root;
>
>         *ovcs_id = 0;
> @@ -1036,11 +1039,15 @@ int of_overlay_fdt_apply(const void *overlay_fdt, u32 overlay_fdt_size,
>          * Must create permanent copy of FDT because of_fdt_unflatten_tree()
>          * will create pointers to the passed in FDT in the unflattened tree.
>          */
> -       new_fdt = kmemdup(overlay_fdt, size, GFP_KERNEL);
> +       size_alloc = size + FDT_ALIGN_SIZE;
> +       new_fdt = kmalloc(size_alloc, GFP_KERNEL);

As size_alloc is only used once, you can just do:

new_fdt = kmalloc(size + FDT_ALIGN_SIZE, GFP_KERNEL);

Same for the unittest. I can fix up.

Applying now so this gets into linux-next this week.

Rob