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From: Mathias Nyman <>
To: Johan Hovold <>
Cc:, USB <>,
	Alan Stern <>
Subject: Re: [PATCH 8/8] xhci: Fix NULL pointer dereference in xhci_clear_tt_buffer_complete()
Date: Fri, 11 Oct 2019 15:47:46 +0300
Message-ID: <> (raw)
In-Reply-To: <>

On 8.10.2019 11.15, Mathias Nyman wrote:
> On 7.10.2019 17.02, Johan Hovold wrote:
>> 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.

I was able to reproduce the use-after-free issue by faking a endpoint halt,
and resetting the endpoint early in enumeration at Get device descriptor request.

To fix this I added the endpoint_disable() callback that will wait for
clear_tt_work to finish before returning, similar to the ehci solution.
It works in my case.

the endpoint_disable() callback is called by usb core both before dropping
xhci endpoints belonging to a interface, and separately for ep0 before udev is
freed during enumeration retry.

Both the hack that triggers the issue (LS/FS behind HS bug won't ever enumerate with this)
and the fix to prevent use after free can be found in  clear_tt_fix branch:

git:// clear_tt_fix

I'll send the fix out to the list as well, any chance you could try that out?


  reply index

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-04 11:59 [PATCH 0/8] xhci fixes for usb-linus Mathias Nyman
2019-10-04 11:59 ` [PATCH 1/8] xhci: Fix false warning message about wrong bounce buffer write length Mathias Nyman
2019-10-04 11:59 ` [PATCH 2/8] xhci: Prevent device initiated U1/U2 link pm if exit latency is too long Mathias Nyman
2019-10-04 11:59 ` [PATCH 3/8] xhci: Check all endpoints for LPM timeout Mathias Nyman
2019-10-04 11:59 ` [PATCH 4/8] xhci: Fix USB 3.1 capability detection on early xHCI 1.1 spec based hosts Mathias Nyman
2019-10-04 11:59 ` [PATCH 5/8] usb: xhci: wait for CNR controller not ready bit in xhci resume Mathias Nyman
2019-10-04 11:59 ` [PATCH 6/8] xhci: Prevent deadlock when xhci adapter breaks during init Mathias Nyman
2019-10-04 11:59 ` [PATCH 7/8] xhci: Increase STS_SAVE timeout in xhci_suspend() Mathias Nyman
     [not found]   ` <>
2019-10-07 18:51     ` Kai-Heng Feng
2019-10-04 11:59 ` [PATCH 8/8] xhci: Fix NULL pointer dereference in xhci_clear_tt_buffer_complete() Mathias Nyman
2019-10-07 14:02   ` Johan Hovold
2019-10-08  8:15     ` Mathias Nyman
2019-10-11 12:47       ` Mathias Nyman [this message]
2019-10-11 12:58         ` [RFT PATCH] xhci: Fix use-after-free regression in xhci clear hub TT implementation Mathias Nyman
2019-10-14 10:16           ` Johan Hovold
2019-10-14 13:18             ` Mathias Nyman

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