linux-kernel - Re: [PATCH] thunderbolt: Fix a double put in tb_cfg_read

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Date:   Tue, 23 Mar 2021 16:06:47 +0200
From:   Mika Westerberg <mika.westerberg@...ux.intel.com>
To:     Lv Yunlong <lyl2019@...l.ustc.edu.cn>
Cc:     andreas.noever@...il.com, michael.jamet@...el.com,
        YehezkelShB@...il.com, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] thunderbolt: Fix a double put in tb_cfg_read_raw

Hi,

On Mon, Mar 22, 2021 at 08:15:12PM -0700, Lv Yunlong wrote:
> In tb_cfg_read_raw, req is allocated by tb_cfg_request_alloc()
> with an initial reference. Before calling tb_cfg_request_sync(),
> there is no refcount inc operation. tb_cfg_request_sync()
> calls tb_cfg_request(..,req,..) and if the callee failed,
> the initial reference of req is dropped and req is freed.
> 
> Later in tb_cfg_read_raw before the err check,
> tb_cfg_request_put(req) is called again. It may cause error
> in race.

Hmm, tb_cfg_request() does tb_cfg_request_get() too and in case of error
it does tb_cfg_request_put(). So the refcount should be fine. What am I
missing?

> 
> My patch puts tb_cfg_request_put(req) after the err check
> finished to avoid unexpected result.
> 
> Signed-off-by: Lv Yunlong <lyl2019@...l.ustc.edu.cn>
> ---
>  drivers/thunderbolt/ctl.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/thunderbolt/ctl.c b/drivers/thunderbolt/ctl.c
> index f1aeaff9f368..bb60269c89ab 100644
> --- a/drivers/thunderbolt/ctl.c
> +++ b/drivers/thunderbolt/ctl.c
> @@ -890,11 +890,11 @@ struct tb_cfg_result tb_cfg_read_raw(struct tb_ctl *ctl, void *buffer,
>  
>  		res = tb_cfg_request_sync(ctl, req, timeout_msec);
>  
> -		tb_cfg_request_put(req);
> -
>  		if (res.err != -ETIMEDOUT)
>  			break;
>  
> +		tb_cfg_request_put(req);
> +
>  		/* Wait a bit (arbitrary time) until we send a retry */
>  		usleep_range(10, 100);
>  	}
> -- 
> 2.25.1
>