linux-kernel - Re: [PATCH] virt: vbox: fix memory leak in hgcm_call_preprocess

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Date:   Sat, 28 Sep 2019 11:54:20 +0200
From:   Hans de Goede <hdegoede@...hat.com>
To:     Navid Emamdoost <navid.emamdoost@...il.com>
Cc:     emamd001@....edu, kjlu@....edu, smccaman@....edu,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] virt: vbox: fix memory leak in
 hgcm_call_preprocess_linaddr

Hi,

On 28-09-2019 01:04, Navid Emamdoost wrote:
> In hgcm_call_preprocess_linaddr memory is allocated for bounce_buf but
> is not released if copy_form_user fails. The release is added.
> 
> Fixes: 579db9d45cb4 ("virt: Add vboxguest VMMDEV communication code")
> 
> Signed-off-by: Navid Emamdoost <navid.emamdoost@...il.com>

Thank you for catching this, I agree this is a bug, but I think we
can fix it in a cleaner way (see below).

> ---
>   drivers/virt/vboxguest/vboxguest_utils.c | 4 +++-
>   1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/virt/vboxguest/vboxguest_utils.c b/drivers/virt/vboxguest/vboxguest_utils.c
> index 75fd140b02ff..7965885a50fa 100644
> --- a/drivers/virt/vboxguest/vboxguest_utils.c
> +++ b/drivers/virt/vboxguest/vboxguest_utils.c
> @@ -222,8 +222,10 @@ static int hgcm_call_preprocess_linaddr(
>   
>   	if (copy_in) {
>   		ret = copy_from_user(bounce_buf, (void __user *)buf, len);
> -		if (ret)
> +		if (ret) {
> +			kvfree(bounce_buf);
>   			return -EFAULT;
> +		}
>   	} else {
>   		memset(bounce_buf, 0, len);
>   	}
> 

First let me quote some more of the function, pre leak fix, for context:

	bounce_buf = kvmalloc(len, GFP_KERNEL);
	if (!bounce_buf)
		return -ENOMEM;

	if (copy_in) {
		ret = copy_from_user(bounce_buf, (void __user *)buf, len);
		if (ret)
			return -EFAULT;
	} else {
		memset(bounce_buf, 0, len);
	}

	*bounce_buf_ret = bounce_buf;

This function gets called repeatedly by hgcm_call_preprocess(), and the
caller of hgcm_call_preprocess() already takes care of freeing the
bounce bufs both on a (later) error and on success:

	ret = hgcm_call_preprocess(parms, parm_count, &bounce_bufs, &size);
	if (ret) {
		/* Even on error bounce bufs may still have been allocated */
		goto free_bounce_bufs;
	}

	...

free_bounce_bufs:
	if (bounce_bufs) {
		for (i = 0; i < parm_count; i++)
			kvfree(bounce_bufs[i]);
		kfree(bounce_bufs);
	}

So we are already taking care of freeing bounce-bufs allocated for previous
parameters to the call (which me must do anyways), so a cleaner fix would
be to store the allocated bounce_buf in the bounce_bufs array before
doing the copy_from_user, then if copy_from_user fails it will be cleaned
up by the code at the free_bounce_bufs label.

IOW I believe it is better to fix this by changing the part of
hgcm_call_preprocess_linaddr I quoted to:

	bounce_buf = kvmalloc(len, GFP_KERNEL);
	if (!bounce_buf)
		return -ENOMEM;

	*bounce_buf_ret = bounce_buf;

	if (copy_in) {
		ret = copy_from_user(bounce_buf, (void __user *)buf, len);
		if (ret)
			return -EFAULT;
	} else {
		memset(bounce_buf, 0, len);
	}

This should also fix the leak in IMHO is a clear way of doing so.

Regards,

Hans