[PATCH v2 1/2][next] clk: visconti: Fix undefined behavior bug in struct visconti_pll_provider

[Stephen Boyd]

Quoting Gustavo A. R. Silva (2023-10-16 15:05:27)
> `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: b4cbe606dc36 ("clk: visconti: Add support common clock driver and reset driver")
> Cc: stable at vger.kernel.org
> Reviewed-by: Kees Cook <keescook at chromium.org>
> Acked-by: Nobuhiro Iwamatsu <nobuhiro1.iwamatsu at toshiba.co.jp>
> Signed-off-by: Gustavo A. R. Silva <gustavoars at kernel.org>
> ---

Applied to clk-next