clk: visconti: Fix undefined behavior bug in struct visconti_pll_provider

`struct clk_hw_onecell_data` is a flexible structure, which means that
it contains flexible-array member at the bottom, in this case array
`hws`:

include/linux/clk-provider.h:
1380 struct clk_hw_onecell_data {
1381         unsigned int num;
1382         struct clk_hw *hws[] __counted_by(num);
1383 };

This could potentially lead to an overwrite of the objects following
`clk_data` in `struct visconti_pll_provider`, in this case
`struct device_node *node;`, at run-time:

drivers/clk/visconti/pll.h:
 16 struct visconti_pll_provider {
 17         void __iomem *reg_base;
 18         struct clk_hw_onecell_data clk_data;
 19         struct device_node *node;
 20 };

Notice that a total of 56 bytes are allocated for flexible-array `hws`
at line 328. See below:

include/dt-bindings/clock/toshiba,tmpv770x.h:
 14 #define TMPV770X_NR_PLL		7

drivers/clk/visconti/pll-tmpv770x.c:
 69 ctx = visconti_init_pll(np, reg_base, TMPV770X_NR_PLL);

drivers/clk/visconti/pll.c:
321 struct visconti_pll_provider * __init visconti_init_pll(struct device_node *np,
322                                                         void __iomem *base,
323                                                         unsigned long nr_plls)
324 {
325         struct visconti_pll_provider *ctx;
...
328         ctx = kzalloc(struct_size(ctx, clk_data.hws, nr_plls), GFP_KERNEL);

`struct_size(ctx, clk_data.hws, nr_plls)` above translates to
sizeof(struct visconti_pll_provider) + sizeof(struct clk_hw *) * 7 ==
24 + 8 * 7 == 24 + 56
		  ^^^^
		   |
	allocated bytes for flex array `hws`

$ pahole -C visconti_pll_provider drivers/clk/visconti/pll.o
struct visconti_pll_provider {
	void *                     reg_base;             /*     0     8 */
	struct clk_hw_onecell_data clk_data;             /*     8     8 */
	struct device_node *       node;                 /*    16     8 */

	/* size: 24, cachelines: 1, members: 3 */
	/* last cacheline: 24 bytes */
};

And then, after the allocation, some data is written into all members
of `struct visconti_pll_provider`:

332         for (i = 0; i < nr_plls; ++i)
333                 ctx->clk_data.hws[i] = ERR_PTR(-ENOENT);
334
335         ctx->node = np;
336         ctx->reg_base = base;
337         ctx->clk_data.num = nr_plls;

Fix all these by placing the declaration of object `clk_data` at the
end of `struct visconti_pll_provider`. Also, add a comment to make it
clear that this object must always be last in the structure, and
prevent this bug from being introduced again in the future.

-Wflex-array-member-not-at-end is coming in GCC-14, and we are getting
ready to enable it globally.

Fixes: b4cbe60 ("clk: visconti: Add support common clock driver and reset driver")
Cc: [email protected]
Reviewed-by: Kees Cook <[email protected]>
Acked-by: Nobuhiro Iwamatsu <[email protected]>
Signed-off-by: Gustavo A. R. Silva <[email protected]>
Link: https://lore.kernel.org/r/57a831d94ee2b3889b11525d4ad500356f89576f.1697492890.git.gustavoars@kernel.org
Signed-off-by: Stephen Boyd <[email protected]>

Author: GustavoARSilva

drivers/clk/visconti/pll.h

@@ -15,8 +15,10 @@
 
 struct visconti_pll_provider {
 	void __iomem *reg_base;
-	struct clk_hw_onecell_data clk_data;
 	struct device_node *node;
+
+	/* Must be last */
+	struct clk_hw_onecell_data clk_data;
 };
 
 #define VISCONTI_PLL_RATE(_rate, _dacen, _dsmen, \