linux-kernel - Re: [PATCH] drm/ast: Avoid possible NULL dereference

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Date:   Mon, 21 Aug 2023 09:04:36 +0200
From:   Christophe JAILLET <christophe.jaillet@...adoo.fr>
To:     Su Hui <suhui@...china.com>, airlied@...hat.com,
        tzimmermann@...e.de, airlied@...il.com, daniel@...ll.ch
Cc:     eich@...e.de, tiwai@...e.de, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] drm/ast: Avoid possible NULL dereference

Le 21/08/2023 à 08:22, Su Hui a écrit :
> smatch error:
> drivers/gpu/drm/ast/ast_dp501.c:227 ast_launch_m68k() error:
> we previously assumed 'ast->dp501_fw' could be null (see line 223)
> 
> when "!ast->dp501_fw" and "ast_load_dp501_microcode(dev) >= 0" is true,
> there will be a NULL dereference of 'ast->dp501_fw'.
> 
> Fixes: 12f8030e05c6 ("drm/ast: Actually load DP501 firmware when required")
> Signed-off-by: Su Hui <suhui@...china.com>
> ---
>   drivers/gpu/drm/ast/ast_dp501.c | 6 ++++--
>   1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/ast/ast_dp501.c b/drivers/gpu/drm/ast/ast_dp501.c
> index 1bc35a992369..d9f3a0786a6f 100644
> --- a/drivers/gpu/drm/ast/ast_dp501.c
> +++ b/drivers/gpu/drm/ast/ast_dp501.c
> @@ -224,8 +224,10 @@ static bool ast_launch_m68k(struct drm_device *dev)
>   			    ast_load_dp501_microcode(dev) < 0)
>   				return false;
>   
> -			fw_addr = (u8 *)ast->dp501_fw->data;
> -			len = ast->dp501_fw->size;
> +			if (ast->dp501_fw) {
> +				fw_addr = (u8 *)ast->dp501_fw->data;
> +				len = ast->dp501_fw->size;
> +			}
>   		}
>   		/* Get BootAddress */
>   		ast_moutdwm(ast, 0x1e6e2000, 0x1688a8a8);

Hi,

this looks like a false positive.

If "!ast->dp501_fw", and ast_load_dp501_microcode()>=0, then 
ast_load_dp501_microcode() will set a valid value in ast->dp501_fw.

CJ