linux-kernel - Re: [PATCH][next] media: allegro: fix potential null dereference on header

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Message-ID: <20200722130025.GA21264@pengutronix.de>
Date:   Wed, 22 Jul 2020 15:00:25 +0200
From:   Michael Tretter <m.tretter@...gutronix.de>
To:     Colin King <colin.king@...onical.com>
Cc:     Pengutronix Kernel Team <kernel@...gutronix.de>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Hans Verkuil <hverkuil-cisco@...all.nl>,
        linux-media@...r.kernel.org, devel@...verdev.osuosl.org,
        kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH][next] media: allegro: fix potential null dereference on
 header

On Mon, 20 Jul 2020 17:38:04 +0100, Colin King wrote:
> From: Colin Ian King <colin.king@...onical.com>
> 
> The pointer header is an alias to msg and msg is being null checked.
> However, if msg is null then header is also null and this can lead to
> a null pointer dereference on the assignment type = header->type. Fix
> this by only dereferencing header after the null check on msg.
> 
> Addresses-Coverity: ("Dereference before null check")
> Fixes: 3de16839669f ("media: allegro: add explicit mail encoding and decoding")
> Signed-off-by: Colin Ian King <colin.king@...onical.com>
> ---
>  drivers/staging/media/allegro-dvt/allegro-mail.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/staging/media/allegro-dvt/allegro-mail.c b/drivers/staging/media/allegro-dvt/allegro-mail.c
> index 4ac65de12463..4496e2a4da5c 100644
> --- a/drivers/staging/media/allegro-dvt/allegro-mail.c
> +++ b/drivers/staging/media/allegro-dvt/allegro-mail.c
> @@ -462,12 +462,14 @@ allegro_dec_encode_frame(struct mcu_msg_encode_frame_response *msg, u32 *src)
>  ssize_t allegro_encode_mail(u32 *dst, void *msg)
>  {
>  	const struct mcu_msg_header *header = msg;
> -	enum mcu_msg_type type = header->type;
> +	enum mcu_msg_type type;
>  	ssize_t size;
>  
>  	if (!msg || !dst)
>  		return -EINVAL;
>  
> +	type = header->type;

type is only used in the switch statement right below the assignment. Thus,
you could remove the local variable and directly use header->type in the
switch.

Michael

> +
>  	switch (type) {
>  	case MCU_MSG_TYPE_INIT:
>  		size = allegro_enc_init(&dst[1], msg);
> -- 
> 2.27.0
> 
>