linux-kernel - Re: [PATCH] drm/ast: fix using freed memory

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Message-ID: <CAKwvOd=MviBd5iwSnhyt9aSZU+QTsBrBSGPcjsjt2eepfYC3KQ@mail.gmail.com>
Date:   Mon, 7 Feb 2022 11:26:06 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     trix@...hat.com
Cc:     airlied@...hat.com, tzimmermann@...e.de, airlied@...ux.ie,
        daniel@...ll.ch, nathan@...nel.org, maxime@...no.tech,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH] drm/ast: fix using freed memory

On Thu, Feb 3, 2022 at 7:23 AM <trix@...hat.com> wrote:
>
> From: Tom Rix <trix@...hat.com>
>
> clang static analysis reports this problem
> ast_mode.c:1235:3: warning: Use of memory after it is freed
>   drm_connector_update_edid_property(&ast_connector->base, edid);
>   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> The second condition on
>
>   if (!flags && ast_connector->i2c)
>
> Means that the edid is not always set.  If the previous block
> fails the freed edid value will be used.  So set edid to NULL
> after freeing.
>
> Fixes: 55dc449a7c60 ("drm/ast: Handle failed I2C initialization gracefully")
> Signed-off-by: Tom Rix <trix@...hat.com>
> ---
>  drivers/gpu/drm/ast/ast_mode.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/ast/ast_mode.c b/drivers/gpu/drm/ast/ast_mode.c
> index ab52efb15670e..9131dc8a1a2fc 100644
> --- a/drivers/gpu/drm/ast/ast_mode.c
> +++ b/drivers/gpu/drm/ast/ast_mode.c
> @@ -1224,10 +1224,12 @@ static int ast_get_modes(struct drm_connector *connector)
>                         return -ENOMEM;
>
>                 flags = ast_dp501_read_edid(connector->dev, (u8 *)edid);
> -               if (flags)
> +               if (flags) {
>                         ast->dp501_maxclk = ast_get_dp501_max_clk(connector->dev);
> -               else
> +               } else {
>                         kfree(edid);
> +                       edid = NULL;
> +               }

You probably didn't need to add the {} for the if, but it doesn't really matter.
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

>         }
>         if (!flags && ast_connector->i2c)
>                 edid = drm_get_edid(connector, &ast_connector->i2c->adapter);
> --
> 2.26.3
>


-- 
Thanks,
~Nick Desaulniers