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: <sta...@vger.kernel.org> # v5.3
Reported-by: Johan Hovold <jo...@kernel.org>
Signed-off-by: Mathias Nyman <mathias.ny...@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 
        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


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