Re: [PATCH] mtd: onenand_base: Avoid fall-through warnings

[Kees Cook <keescook@chromium.org>]

Sorry for being late to speaking up on this. I missed something in the
code the first time I read the thread, that now stood out to me. Notes
below...

On Wed, May 22, 2019 at 01:04:46PM -0500, Gustavo A. R. Silva wrote:
> diff --git a/drivers/mtd/nand/onenand/onenand_base.c b/drivers/mtd/nand/onenand/onenand_base.c
> index f41d76248550..6cf4df9f8c01 100644
> --- a/drivers/mtd/nand/onenand/onenand_base.c
> +++ b/drivers/mtd/nand/onenand/onenand_base.c
> @@ -3280,12 +3280,14 @@ static void onenand_check_features(struct mtd_info *mtd)

Reverse-order review, second hunk first:

>  	case ONENAND_DEVICE_DENSITY_2Gb:
>  		/* 2Gb DDP does not have 2 plane */
>  		if (!ONENAND_IS_DDP(this))
>  			this->options |= ONENAND_HAS_2PLANE;
>  		this->options |= ONENAND_HAS_UNLOCK_ALL;
> +		/* Fall through - ? */
>  
>  	case ONENAND_DEVICE_DENSITY_1Gb:
>  		/* A-Die has all block unlock */

So, I think the ONENAND_DEVICE_DENSITY_2Gb should be a "break". Though,
actually, it doesn't matter:

        case ONENAND_DEVICE_DENSITY_2Gb:
                /* 2Gb DDP does not have 2 plane */
                if (!ONENAND_IS_DDP(this))
                        this->options |= ONENAND_HAS_2PLANE;
                this->options |= ONENAND_HAS_UNLOCK_ALL;

        case ONENAND_DEVICE_DENSITY_1Gb:
                /* A-Die has all block unlock */
                if (process)
                        this->options |= ONENAND_HAS_UNLOCK_ALL;
                break;

Falling through from ONENAND_DEVICE_DENSITY_2Gb to
ONENAND_DEVICE_DENSITY_1Gb will actually have no side-effects:
ONENAND_HAS_UNLOCK_ALL was unconditionally set in ..._2Gb, so there is
no reason to fall through to ..._1Gb. (But falling through is harmless.)

Now the first hunk:

>  			if ((this->version_id & 0xf) == 0xe)
>  				this->options |= ONENAND_HAS_NOP_1;
>  		}
> +		/* Fall through - ? */
>  

        case ONENAND_DEVICE_DENSITY_4Gb:
                if (ONENAND_IS_DDP(this))
                        this->options |= ONENAND_HAS_2PLANE;
                else if (numbufs == 1) {
                        this->options |= ONENAND_HAS_4KB_PAGE;
                        this->options |= ONENAND_HAS_CACHE_PROGRAM;
                        /*
                         * There are two different 4KiB pagesize chips
                         * and no way to detect it by H/W config values.
                         *
                         * To detect the correct NOP for each chips,
                         * It should check the version ID as workaround.
                         *
                         * Now it has as following
                         * KFM4G16Q4M has NOP 4 with version ID 0x0131
                         * KFM4G16Q5M has NOP 1 with versoin ID 0x013e
                         */
                        if ((this->version_id & 0xf) == 0xe)
                                this->options |= ONENAND_HAS_NOP_1;
                }

Falling through from ONENAND_DEVICE_DENSITY_4Gb to
ONENAND_DEVICE_DENSITY_2Gb looks like it would mean that
ONENAND_HAS_2PLANE would be unconditionally set for ...4Gb, which seems
very strange to expect:

                if (ONENAND_IS_DDP(this))
                        this->options |= ONENAND_HAS_2PLANE;
...
                if (!ONENAND_IS_DDP(this))
                        this->options |= ONENAND_HAS_2PLANE;

However! This happens later:

        if (ONENAND_IS_4KB_PAGE(this))
                this->options &= ~ONENAND_HAS_2PLANE;

i.e. falling through to ...2Gb (which sets ONENAND_HAS_2PLANE) has no
effect because when ONENAND_HAS_2PLANE isn't set (numbufs == 1), it gets
_cleared_ by the above code due to ONENAND_HAS_4KB_PAGE getting set:

#define ONENAND_IS_4KB_PAGE(this) \
        (this->options & ONENAND_HAS_4KB_PAGE)


Unfortunately, though, it's less clear about ONENAND_HAS_UNLOCK_ALL,
which is getting set unconditionally for ...4Gb currently (due to the
fallthrough to ...2Gb). However, this happens later:

        if (FLEXONENAND(this)) {
                this->options &= ~ONENAND_HAS_CONT_LOCK;
                this->options |= ONENAND_HAS_UNLOCK_ALL;
        }
...
#define FLEXONENAND(this) \
        (this->device_id & DEVICE_IS_FLEXONENAND)

So it's possible this fall through has no effect (are all 4Gb density
devices also FLEXONENAND devices?)

Setting a "break" after 4Gb may remove ONENAND_HAS_UNLOCK_ALL in the
!FLEXONENAND(this) case. Does anyone have real hardware to test with?

Thoughts?

-- 
Kees Cook

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