linux-kernel - Re: [PATCH net v2] 9p/xen : Fix use after free bug in xen_9pfs_front

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Date:   Mon, 13 Mar 2023 14:54:20 +0100
From:   Michal Swiatkowski <michal.swiatkowski@...ux.intel.com>
To:     Zheng Wang <zyytlz.wz@....com>
Cc:     ericvh@...il.com, lucho@...kov.net, asmadeus@...ewreck.org,
        linux_oss@...debyte.com, davem@...emloft.net, edumazet@...gle.com,
        kuba@...nel.org, pabeni@...hat.com,
        v9fs-developer@...ts.sourceforge.net, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, hackerzheng666@...il.com,
        1395428693sheep@...il.com, alex000young@...il.com
Subject: Re: [PATCH net v2] 9p/xen : Fix use after free bug in
 xen_9pfs_front_remove due  to race condition

On Mon, Mar 13, 2023 at 05:00:02PM +0800, Zheng Wang wrote:
> In xen_9pfs_front_probe, it calls xen_9pfs_front_alloc_dataring
> to init priv->rings and bound &ring->work with p9_xen_response.
> 
> When it calls xen_9pfs_front_event_handler to handle IRQ requests,
> it will finally call schedule_work to start the work.
> 
> When we call xen_9pfs_front_remove to remove the driver, there
> may be a sequence as follows:
> 
> Fix it by finishing the work before cleanup in xen_9pfs_front_free.
> 
> Note that, this bug is found by static analysis, which might be
> false positive.
> 
> CPU0                  CPU1
> 
>                      |p9_xen_response
> xen_9pfs_front_remove|
>   xen_9pfs_front_free|
> kfree(priv)          |
> //free priv          |
>                      |p9_tag_lookup
>                      |//use priv->client
> 
> Fixes: 71ebd71921e4 ("xen/9pfs: connect to the backend")
> Signed-off-by: Zheng Wang <zyytlz.wz@....com>
> ---
> v2:
> - fix type error of ring found by kernel test robot
> ---
>  net/9p/trans_xen.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/net/9p/trans_xen.c b/net/9p/trans_xen.c
> index c64050e839ac..83764431c066 100644
> --- a/net/9p/trans_xen.c
> +++ b/net/9p/trans_xen.c
> @@ -274,12 +274,17 @@ static const struct xenbus_device_id xen_9pfs_front_ids[] = {
>  static void xen_9pfs_front_free(struct xen_9pfs_front_priv *priv)
>  {
>  	int i, j;
> +	struct xen_9pfs_dataring *ring = NULL;
Move it before int i, j to have RCT.

>  
>  	write_lock(&xen_9pfs_lock);
>  	list_del(&priv->list);
>  	write_unlock(&xen_9pfs_lock);
>  
>  	for (i = 0; i < priv->num_rings; i++) {
> +		/*cancel work*/
It isn't needed I think, the function cancel_work_sync() tells everything
here.

> +		ring = &priv->rings[i];
> +		cancel_work_sync(&ring->work);
> +
>  		if (!priv->rings[i].intf)
>  			break;
>  		if (priv->rings[i].irq > 0)
> -- 
> 2.25.1