clk: visconti: Fix undefined behavior bug and add bounds-checking coverage

This small series aims to fix an undefined behavior bug in struct visconti_pll_provider and add bounds-checking coverage at run-time for flexible-array member hws in struct clk_hw_onecell_data when accessed throught struct visconti_pll_provider.

Changes in v2:
- Mention -Wflex-array-member-not-at-end in the changelog text.

v1:
- Link: https://lore.kernel.org/linux-hardening/[email protected]/

Gustavo A. R. Silva (2):
clk: visconti: Fix undefined behavior bug in struct
visconti_pll_provider
clk: visconti: Add bounds-checking coverage for struct
visconti_pll_provider

 drivers/clk/visconti/pll.c | 6 +++---
 drivers/clk/visconti/pll.h | 4 +++-
 2 files changed, 6 insertions(+), 4 deletions(-)

2.34.1

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.

---
Changes in v2:
- Mention -Wflex-array-member-not-at-end in the changelog text.

v1:
- Link: https://lore.kernel.org/linux-hardening/0a59a721d54b557076cc94eabeb694d463773204.1697076650.git.gustavoars@kernel.org/

 drivers/clk/visconti/pll.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

index 01d07f1bf01b..c4bd40676da4 100644
@@ -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, \

2.34.1

Quoting Gustavo A. R. Silva (2023-10-16 15:05:27)

Applied to clk-next

Hi!

Here is another similar small series, in case you missed it:

https://lore.kernel.org/linux-hardening/[email protected]/

Thanks
--
Gustavo

Quoting Gustavo A. R. Silva (2023-10-23 19:48:00)

Please Cc linux-clk on clk patches.

In order to gain the bounds-checking coverage that __counted_by provides to flexible-array members at run-time via CONFIG_UBSAN_BOUNDS (for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family functions), we must make sure that the counter member, in this particular case num, is updated before the first access to the flex-array member, in this particular case array hws. See below:

commit f316cdff8d67 ("clk: Annotate struct clk_hw_onecell_data with __counted_by") introduced __counted_by for struct clk_hw_onecell_data together with changes to relocate some of assignments of counter num before hws is accessed:

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

However, this structure is used as a member in other structs, in this case in struct visconti_pll_provider:

drivers/clk/visconti/pll.h:
16 struct visconti_pll_provider {
17 void __iomem *reg_base;
18 struct device_node *node;
19
20 /* Must be last */
21 struct clk_hw_onecell_data clk_data;
22 };

Hence, we need to move the assignments to ctx->clk_data.num after allocation for struct visconti_pll_provider and before accessing the flexible array ctx->clk_data.hws. And, as assignments for all members in struct visconti_pll_provider are originally adjacent to each other, relocate all assignments together, so we don't split up ctx->clk_data.hws = nr_plls from the rest. :)

---
Changes in v2:
- None.

v1:
- Link: https://lore.kernel.org/linux-hardening/675633f7ae9893371d35b238f06fd02a3acffebb.1697076650.git.gustavoars@kernel.org/

 drivers/clk/visconti/pll.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

index 1f3234f22667..e9cd80e085dc 100644
@@ -329,12 +329,12 @@ struct visconti_pll_provider * __init visconti_init_pll(struct device_node *np,
 	if (!ctx)
 		return ERR_PTR(-ENOMEM);
 
-	for (i = 0; i < nr_plls; ++i)
-		ctx->clk_data.hws[i] = ERR_PTR(-ENOENT);
-
 	ctx->node = np;
 	ctx->reg_base = base;
 	ctx->clk_data.num = nr_plls;
 
+	for (i = 0; i < nr_plls; ++i)
+		ctx->clk_data.hws[i] = ERR_PTR(-ENOENT);
+
 	return ctx;
 }

2.34.1

Quoting Gustavo A. R. Silva (2023-10-16 15:06:16)

Applied to clk-next