linux-kernel - Re: [PATCH] firmware: xilinx: fix debugfs write handler

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Message-ID: <0cb22dd7-0bb2-5181-4019-ad8561ccdefb@xilinx.com>
Date:   Mon, 18 Mar 2019 13:44:49 +0100
From:   Michal Simek <michal.simek@...inx.com>
To:     Jann Horn <jannh@...gle.com>,
        Michal Simek <michal.simek@...inx.com>
CC:     Rajan Vaja <rajanv@...inx.com>, Jolly Shah <jollys@...inx.com>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] firmware: xilinx: fix debugfs write handler

On 18. 02. 19 22:43, Jann Horn wrote:
>  - Userspace wants to write a string with `len` bytes, not counting the
>    terminating NULL, so we should allocate `len+1` bytes. It looks like the
>    current code relied on having a nullbyte directly behind `kern_buff`,
>    which happens to work reliably as long as `len` isn't one of the kmalloc
>    size classes.
>  - strncpy_from_user() is completely wrong here; userspace is giving us a
>    (not necessarily null-terminated) buffer and its length.
>    strncpy_from_user() is for cases in which we don't know the length.
>  - Don't let broken userspace allocate arbitrarily big kmalloc allocations.
> 
> Just use memdup_user_nul(), which is designed precisely for things like
> this.
> 
> Signed-off-by: Jann Horn <jannh@...gle.com>
> ---
> WARNING: completely untested patch
> 
> drivers/firmware/xilinx/zynqmp-debug.c | 15 ++++-----------
>  1 file changed, 4 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/firmware/xilinx/zynqmp-debug.c b/drivers/firmware/xilinx/zynqmp-debug.c
> index 2771df6df379..90b66cdbfd58 100644
> --- a/drivers/firmware/xilinx/zynqmp-debug.c
> +++ b/drivers/firmware/xilinx/zynqmp-debug.c
> @@ -163,21 +163,14 @@ static ssize_t zynqmp_pm_debugfs_api_write(struct file *file,
>  
>  	strcpy(debugfs_buf, "");
>  
> -	if (*off != 0 || len == 0)
> +	if (*off != 0 || len <= 1 || len > PAGE_SIZE - 1)
>  		return -EINVAL;
>  
> -	kern_buff = kzalloc(len, GFP_KERNEL);
> -	if (!kern_buff)
> -		return -ENOMEM;
> -
> +	kern_buff = memdup_user_nul(ptr, len);
> +	if (IS_ERR(kern_buff))
> +		return PTR_ERR(kern_buff);
>  	tmp_buff = kern_buff;
>  
> -	ret = strncpy_from_user(kern_buff, ptr, len);
> -	if (ret < 0) {
> -		ret = -EFAULT;
> -		goto err;
> -	}
> -
>  	/* Read the API name from a user request */
>  	pm_api_req = strsep(&kern_buff, " ");
>  
> 

applied with Jolly's ack.

Thanks,
Michal