linux-hardening - Re: [PATCH 2/3] spi: axi-spi-engine: use __counted

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Date: Fri, 1 Mar 2024 12:41:59 -0800
From: Kees Cook <keescook@...omium.org>
To: David Lechner <dlechner@...libre.com>
Cc: Mark Brown <broonie@...nel.org>,
	Michael Hennerich <michael.hennerich@...log.com>,
	Nuno Sá <nuno.sa@...log.com>,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>,
	linux-spi@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH 2/3] spi: axi-spi-engine: use __counted_by() attribute

On Fri, Mar 01, 2024 at 02:25:19PM -0600, David Lechner wrote:
> This adds the __counted_by() attribute to the flex array at the end of
> struct spi_engine_program in the AXI SPI Engine controller driver.
> 
> Suggested-by: Nuno Sá <nuno.sa@...log.com>
> Signed-off-by: David Lechner <dlechner@...libre.com>
> ---
>  drivers/spi/spi-axi-spi-engine.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/spi/spi-axi-spi-engine.c b/drivers/spi/spi-axi-spi-engine.c
> index d89f75170c9e..e801eed820df 100644
> --- a/drivers/spi/spi-axi-spi-engine.c
> +++ b/drivers/spi/spi-axi-spi-engine.c
> @@ -75,7 +75,7 @@
>  
>  struct spi_engine_program {
>  	unsigned int length;
> -	uint16_t instructions[];
> +	uint16_t instructions[] __counted_by(length);
>  };

You'll also need to change places where you deal with changes to
"length", as now accesses to "instructions" will be bounds-checked
by the compiler. For example, this change:

static void spi_engine_program_add_cmd(struct spi_engine_program *p,
         bool dry, uint16_t cmd)
{
         p->length++;
         if (!dry)
                 p->instructions[p->length - 1] = cmd;
}

-- 
Kees Cook