linux-hardening - Re: [PATCH] mtd: physmap: physmap-bt1-rom: Fix unintentional stack access

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Date:   Fri, 12 Feb 2021 15:12:16 +0100
From:   Miquel Raynal <miquel.raynal@...tlin.com>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc:     Richard Weinberger <richard@....at>,
        Vignesh Raghavendra <vigneshr@...com>,
        Serge Semin <Sergey.Semin@...kalelectronics.ru>,
        linux-mtd@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Kees Cook <keescook@...omium.org>,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH] mtd: physmap: physmap-bt1-rom: Fix unintentional stack
 access

Hi Gustavo,

"Gustavo A. R. Silva" <gustavoars@...nel.org> wrote on Fri, 12 Feb 2021
04:40:22 -0600:

> Cast &data to (char *) in order to avoid unintentionally accessing
> the stack.
> 
> Notice that data is of type u32, so any increment to &data
> will be in the order of 4-byte chunks, and this piece of code
> is actually intended to be a byte offset.

I don't have the same reading. I don't say that Coverity report is
wrong, but let's discuss this a bit further.

Given that &data is of type u32 *, you say that "&data + shift"
produces increments of 4-bytes, ie. we would access "&data + 4 *
shift"? Because I don't think this is the case (again, I may be wrong).

I'm sure this would be the case if we dereferenced data like
data[shift] though, but in this case I don't see what this cast is
fixing. Can you enlighten me?

Could the out-of-bounds warning come from the fact that shift
might be bigger than the data array spread?

> Fixes: b3e79e7682e0 ("mtd: physmap: Add Baikal-T1 physically mapped ROM support")
> Addresses-Coverity-ID: 1497765 ("Out-of-bounds access")
> Cc: stable@...r.kernel.org
> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
> ---
>  drivers/mtd/maps/physmap-bt1-rom.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/mtd/maps/physmap-bt1-rom.c b/drivers/mtd/maps/physmap-bt1-rom.c
> index a35450002284..58782cfaf71c 100644
> --- a/drivers/mtd/maps/physmap-bt1-rom.c
> +++ b/drivers/mtd/maps/physmap-bt1-rom.c
> @@ -79,7 +79,7 @@ static void __xipram bt1_rom_map_copy_from(struct map_info *map,
>  	if (shift) {
>  		chunk = min_t(ssize_t, 4 - shift, len);
>  		data = readl_relaxed(src - shift);
> -		memcpy(to, &data + shift, chunk);
> +		memcpy(to, (char *)&data + shift, chunk);
>  		src += chunk;
>  		to += chunk;
>  		len -= chunk;

Thanks,
Miquèl