linux-kernel - Re: [PATCH] staging: most: don't access hdm

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Date:   Mon, 28 Sep 2020 14:48:57 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Jing Xiangfeng <jingxiangfeng@...wei.com>
Cc:     gregkh@...uxfoundation.org, christian.gromm@...rochip.com,
        masahiroy@...nel.org, tglx@...utronix.de, keescook@...omium.org,
        devel@...verdev.osuosl.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] staging: most: don't access hdm_ch before checking it
 valid

On Mon, Sep 28, 2020 at 06:48:38PM +0800, Jing Xiangfeng wrote:
> In try_start_dim_transfer(), pointer hdm_ch is accessed before checking.
> This may lead to a potential null pointer dereference. Fix this by
> dereferencing hdm_ch after calling BUG_ON().
> 
> Signed-off-by: Jing Xiangfeng <jingxiangfeng@...wei.com>
> ---
>  drivers/staging/most/dim2/dim2.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/staging/most/dim2/dim2.c b/drivers/staging/most/dim2/dim2.c
> index 509c8012d20b..ccd7cc7545e4 100644
> --- a/drivers/staging/most/dim2/dim2.c
> +++ b/drivers/staging/most/dim2/dim2.c
> @@ -148,7 +148,7 @@ void dimcb_on_error(u8 error_id, const char *error_message)
>  static int try_start_dim_transfer(struct hdm_channel *hdm_ch)
>  {
>  	u16 buf_size;
> -	struct list_head *head = &hdm_ch->pending_list;

This is not a dereference, it's just pointer math.  In other words:

	struct list_head *head = hdm_ch + offsetof(struct hdm_channel, pending_list);

So the commit message is wrong because this cannot lead to a NULL
dereference.  It's better to just delete the BUG_ON().  We don't really
like BUG_ON().  Checkpatch will complain about them.  An Oops gives
basically the same information as a BUG_ON() without completely killing
the kernel so just dereferencing a NULL is preferable.  Finally, we can
see from the callers that "hdm_ch" is never NULL.

regards,
dan carpenter