Date:   Tue, 7 Feb 2023 11:59:25 +0000
From:   John Keeping <john@metanate.com>
To:     Alan Stern <stern@rowland.harvard.edu>
Cc:     Xin Zhao <xnzhao@google.com>, gregkh@linuxfoundation.org,
        jakobkoschel@gmail.com, rdunlap@infradead.org, ira.weiny@intel.com,
        linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] usb: gadget: dummyhcd: Fix use-after-free in
 dummy_free_request
Message-ID: <Y+I9HcPvrm1TzUCw@donbot>
References: <20230206225258.2302954-1-xnzhao@google.com>
 <Y+GcoFKiAkrCoAsv@rowland.harvard.edu>
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On Mon, Feb 06, 2023 at 07:34:40PM -0500, Alan Stern wrote:
> On Mon, Feb 06, 2023 at 10:52:58PM +0000, Xin Zhao wrote:
> > DummyHCD assume when dummy_free_request is called, the request
> > is already detached from request queues. It is correct in most
> > cases.
> > But when DummyHCD is detached from gadget configfs with pending
> > requests and some requests are still in pending queue,
> > dummy_free_request would free them directly.
> > Later on, dummy_udc_stop would iterate pending queue to release
> > the requests again.
> > 
> > Stacktrace for dummy_free_reqeust
> > ```
> > kfree(const void * x) (slub.c:4200)
> > dummy_free_request(struct usb_ep * _ep, struct usb_request * _req) (dummy_hcd.c:691)
> > usb_ep_free_request(struct usb_ep * ep, struct usb_request * req) (core.c:201)
> > functionfs_unbind(struct ffs_data * ffs) (f_fs.c:1894)
> 
> That's the bug right there.  The kerneldoc for usb_ep_free_request() 
> says "Caller guarantees the request is not queued".  So it looks like 
> the real solution is to fix functionfs_unbind().

This is commit ce405d561b02 ("usb: gadget: f_fs: Ensure ep0req is
dequeued before free_request") IIUC.

Xin, are you able to test a version with that commit?