Re: [PATCH] net/9p/usbg: Fix use-after-free on usb9pfs_clear_tx

[Michael Grzeschik <mgr@kernel.org>]

On Fri, May 29, 2026 at 04:10:25PM +0800, Yizhou Zhao wrote:
> When p9_usbg_close() is called, it invokes usb9pfs_clear_tx() but does
> not clear usb9pfs->client nor usb9pfs->in_req->context afterwards.
> The 9p client core (p9_client_destroy) calls close() and then
> kfree(clnt), so after p9_usbg_close returns the client struct is freed.
> 
> If the USB gadget is later disabled (e.g., host disconnect),
> usb9pfs_disable calls usb9pfs_clear_tx again, which passes the dangling
> usb9pfs->client pointer to p9_client_cb. Inside p9_client_cb,
> p9_req_put(c, req) dereferences the freed client to access its reqs IDR,
> causing a use-after-free.
> 
> Additionally, usb9pfs_clear_tx does not clear usb9pfs->in_req->context
> after calling p9_client_cb, so a second invocation finds a stale request
> pointer and attempts double-completion, further compounding the UAF risk.
> 
> Fix by:
> 1. Clearing usb9pfs->in_req->context to NULL in usb9pfs_clear_tx()
>    before checking usb9pfs->client, so that even if the client is gone
>    the stale request pointer is consumed and cannot cause
>    double-completion.
> 2. Adding a NULL check on usb9pfs->client in usb9pfs_clear_tx() to skip
>    p9_client_cb() when the client has already been destroyed.
> 3. Setting usb9pfs->client = NULL in p9_usbg_close() after calling
>    usb9pfs_clear_tx(), to prevent any subsequent call from dereferencing
>    the freed client pointer.
> 
> Fixes: a3be076dc174 ("net/9p/usbg: Add new usb gadget function transport")
> Reported-by: Yizhou Zhao <zhaoyz24@mails.tsinghua.edu.cn>
> Reported-by: Yuxiang Yang <yangyx22@mails.tsinghua.edu.cn>
> Reported-by: Ao Wang <wangao@seu.edu.cn>
> Reported-by: Xuewei Feng <fengxw06@126.com>
> Reported-by: Qi Li <qli01@tsinghua.edu.cn>
> Reported-by: Ke Xu <xuke@tsinghua.edu.cn>
> Assisted-by: GLM:GLM-5.1
> Signed-off-by: Yizhou Zhao <zhaoyz24@mails.tsinghua.edu.cn>

Acked-by: Michael Grzeschik <mgr@kernel.org>

> ---
> diff --git a/net/9p/trans_usbg.c b/net/9p/trans_usbg.c
> index 1ce7033..c75de5c 100644
> --- a/net/9p/trans_usbg.c
> +++ b/net/9p/trans_usbg.c
> @@ -435,6 +435,11 @@ static void usb9pfs_clear_tx(struct f_usb9pfs *usb9pfs)
>  	if (!req)
>  		return;
>  
> +	usb9pfs->in_req->context = NULL;
> +
> +	if (!usb9pfs->client)
> +		return;
> +
>  	if (!req->t_err)
>  		req->t_err = -ECONNRESET;
>  
> @@ -457,6 +462,7 @@ static void p9_usbg_close(struct p9_client *client)
>  	client->status = Disconnected;
>  
>  	usb9pfs_clear_tx(usb9pfs);
> +	usb9pfs->client = NULL;
>  
>  	opts = container_of(usb9pfs->function.fi,
>  			    struct f_usb9pfs_opts, func_inst);
> 
> --
> 2.43.0
>