Re: [PATCH 8/8] xhci: Fix NULL pointer dereference in xhci_clear_tt_buffer_complete()

From: Mathias Nyman <mathias.nyman@linux.intel.com>
To: Johan Hovold <johan@kernel.org>
Cc: gregkh@linuxfoundation.org, linux-usb@vger.kernel.org,
	"# v5 . 3" <stable@vger.kernel.org>,
	Alan Stern <stern@rowland.harvard.edu>
Subject: Re: [PATCH 8/8] xhci: Fix NULL pointer dereference in xhci_clear_tt_buffer_complete()
Date: Tue, 8 Oct 2019 11:15:58 +0300	[thread overview]
Message-ID: <c0b1f81f-db1a-8f12-6880-a686cb9c35a7@linux.intel.com> (raw)
In-Reply-To: <20191007140245.GD13531@localhost>

On 7.10.2019 17.02, Johan Hovold wrote:
> [ +CC: Alan ]
> 
> On Fri, Oct 04, 2019 at 02:59:33PM +0300, Mathias Nyman wrote:
>> udev stored in ep->hcpriv might be NULL if tt buffer is cleared
>> due to a halted control endpoint during device enumeration
>>
>> xhci_clear_tt_buffer_complete is called by hub_tt_work() once it's
>> scheduled,  and by then usb core might have freed and allocated a
>> new udev for the next enumeration attempt.
>>
>> Fixes: ef513be0a905 ("usb: xhci: Add Clear_TT_Buffer")
>> Cc: <stable@vger.kernel.org> # v5.3
>> Reported-by: Johan Hovold <johan@kernel.org>
>> Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
>> ---
>>   drivers/usb/host/xhci.c | 8 ++++++++
>>   1 file changed, 8 insertions(+)
>>
>> diff --git a/drivers/usb/host/xhci.c b/drivers/usb/host/xhci.c
>> index 00f3804f7aa7..517ec3206f6e 100644
>> --- a/drivers/usb/host/xhci.c
>> +++ b/drivers/usb/host/xhci.c
>> @@ -5238,8 +5238,16 @@ static void xhci_clear_tt_buffer_complete(struct usb_hcd *hcd,
>>   	unsigned int ep_index;
>>   	unsigned long flags;
>>   
>> +	/*
>> +	 * udev might be NULL if tt buffer is cleared during a failed device
>> +	 * enumeration due to a halted control endpoint. Usb core might
>> +	 * have allocated a new udev for the next enumeration attempt.
>> +	 */
>> +
>>   	xhci = hcd_to_xhci(hcd);
>>   	udev = (struct usb_device *)ep->hcpriv;
>> +	if (!udev)
>> +		return;
> 
> I didn't have time to look into this myself last week, or comment on the
> patch before Greg picked it up, but this clearly isn't the right fix.
> 
> As your comment suggests, ep->hcpriv may indeed be NULL here if USB core
> have allocated a new udev. But this only happens after USB has freed the
> old usb_device and the new one happens to get the same address.
> 

You're right, that fix doesn't solve the actual issue, it avoids a few specific
null pointer dereference cases, but leaves both root cause and several other
use-after-free cases open.

> Note that even the usb_host_endpoint itself (ep) has then been freed and
> reallocated since it is member of struct usb_device, and it is the
> use-after-free that needs fixing.
> 
> I've even been able to trigger another NULL-deref in this function
> before a new udev has been allocated, due to the virt dev having been
> freed by xhci_free_dev as part of usb_release_dev:
> 
> It seems the xhci clear-tt implementation was incomplete since it did
> not take care to wait for any ongoing work before disabling the
> endpoint. EHCI does this in ehci_endpoint_disable(), but xhci doesn't
> even implement that callback.
> 

So it seems, it might be possible to remove pending clear_tt work for
most endpoints in the .drop_endpoint callbacks, but ep0 is different,
it isn't dropped, we might need to implement the endpoint_disable()
callback for this.

> As this may be something you could end up hitting in other paths as
> well, perhaps we should even consider reverting the offending commit
> pending a more complete implementation?

Possibly, if we can't find a working solution early enough in this release cycle

-Mathias