linux-kernel - Re: [PATCH] fbdev: atyfb: fix array overflow

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Date:	Thu, 23 Jun 2016 11:22:20 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:	Tomi Valkeinen <tomi.valkeinen@...com>,
	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	Ingo Molnar <mingo@...nel.org>,
	"Luis R. Rodriguez" <mcgrof@...e.com>,
	Borislav Petkov <bp@...e.de>,
	Linux Fbdev development list <linux-fbdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: 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 <arnd@...db.de> wrote:
> > 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@...db.de>
> > Link: https://bugs.linaro.org/show_bug.cgi?id=2349
> > ---
> >  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;