Re: [PATCH] fbdev: atyfb: fix array overflow

On Thursday, June 23, 2016 10:50:04 AM CEST Geert Uytterhoeven wrote:

Hi Arnd,

On Wed, Jun 22, 2016 at 2:37 PM, Arnd Bergmann [off-list ref] wrote:

quoted

When building with CONFIG_UBSAN_SANITIZE_ALL on ARM, I get this
gcc warning for atyfb:

drivers/video/fbdev/aty/atyfb_base.c: In function 'aty_bl_update_status':
drivers/video/fbdev/aty/atyfb_base.c:167:33: warning: array subscript is above array bounds [-Warray-bounds]
drivers/video/fbdev/aty/atyfb_base.c:152:26: warning: array subscript is above array bounds [-Warray-bounds]

Apparently the warning is correct and there is indeed an overflow,
which was never caught. I could only reproduce this on ARM and
have opened a bug against the compiler for the lack of warning.

This patch makes the array larger, so we cover all possible
registers in the LCD controller without an overflow.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://bugs.linaro.org/show_bug.cgi?id#49
---
 drivers/video/fbdev/aty/atyfb_base.c | 2 +-
 include/video/mach64.h               | 1 +
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/video/fbdev/aty/atyfb_base.c b/drivers/video/fbdev/aty/atyfb_base.c
index 001d3d871800..36ffba152eab 100644
--- a/drivers/video/fbdev/aty/atyfb_base.c
+++ b/drivers/video/fbdev/aty/atyfb_base.c

@@ -134,7 +134,7 @@

 #if defined(CONFIG_PM) || defined(CONFIG_PMAC_BACKLIGHT) || \
 defined (CONFIG_FB_ATY_GENERIC_LCD) || defined(CONFIG_FB_ATY_BACKLIGHT)
-static const u32 lt_lcd_regs[] = {
+static const u32 lt_lcd_regs[LCD_REG_NUM] = {
        CNFG_PANEL_LG,
        LCD_GEN_CNTL_LG,
        DSTN_CONTROL_LG,

diff --git a/include/video/mach64.h b/include/video/mach64.h
index 89e91c0cb737..9f74e9e0aeb8 100644
--- a/include/video/mach64.h
+++ b/include/video/mach64.h

@@ -1299,6 +1299,7 @@
 #define APC_LUT_KL             0x38
 #define APC_LUT_MN             0x39
 #define APC_LUT_OP             0x3A
+#define LCD_REG_NUM            0x3B /* total number */

 /* Values in LCD_GEN_CTRL */
 #define CRT_ON                          0x00000001ul

This doesn't look like the right fix to me.

Before, aty_st_lcd(LCD_MISC_CNTL, reg, par) in aty_bl_update_status()
wrote into an arbitrary register.
With your fix, it will write to register 0, which is IMHO also not OK.

Ok, I see now. I thought it array was for initializing the registers and
caching the contents as some other drivers do it, but it's really used
for some indirect addressing method.

I think aty_st_lcd() and aty_ld_lcd() should check whether the index is
out of range, perhaps even with a WARN_ON()?

Just guessing what the right behavior would be, maybe something like
this? That would assume that the LCD_MISC_CNTL is accessible
through LCD_INDEX/LCD_DATA but not through a direct register.

	Arnd

diff --git a/drivers/video/fbdev/aty/atyfb_base.c b/drivers/video/fbdev/aty/atyfb_base.c
index 36ffba152eab..c67d4b767e9a 100644
--- a/drivers/video/fbdev/aty/atyfb_base.c
+++ b/drivers/video/fbdev/aty/atyfb_base.c

@@ -148,7 +148,7 @@ static const u32 lt_lcd_regs[LCD_REG_NUM] = {
 
 void aty_st_lcd(int index, u32 val, const struct atyfb_par *par)
 {
-	if (M64_HAS(LT_LCD_REGS)) {
+	if (M64_HAS(LT_LCD_REGS) && index < ARRAY_SIZE(lt_lcd_regs)) {
 		aty_st_le32(lt_lcd_regs[index], val, par);
 	} else {
 		unsigned long temp;

@@ -163,7 +163,7 @@ void aty_st_lcd(int index, u32 val, const struct atyfb_par *par)
 
 u32 aty_ld_lcd(int index, const struct atyfb_par *par)
 {
-	if (M64_HAS(LT_LCD_REGS)) {
+	if (M64_HAS(LT_LCD_REGS) && index < ARRAY_SIZE(lt_lcd_regs)) {
 		return aty_ld_le32(lt_lcd_regs[index], par);
 	} else {
 		unsigned long temp;