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commitf3d12ec847upstream. DbC may add 1024 bogus bytes to the beginneing of the receiving endpoint if DbC hw triggers a STALL event before any Transfer Blocks (TRBs) for incoming data are queued, but driver handles the event after it queued the TRBs. This is possible as xHCI DbC hardware may trigger spurious STALL transfer events even if endpoint is empty. The STALL event contains a pointer to the stalled TRB, and "remaining" untransferred data length. As there are no TRBs queued yet the STALL event will just point to first TRB position of the empty ring, with '0' bytes remaining untransferred. DbC driver is polling for events, and may not handle the STALL event before /dev/ttyDBC0 is opened and incoming data TRBs are queued. The DbC event handler will now assume the first queued TRB (length 1024) has stalled with '0' bytes remaining untransferred, and copies the data This race situation can be practically mitigated by making sure the event handler handles all pending transfer events when DbC reaches configured state, and only then create dev/ttyDbC0, and start queueing transfers. The event handler can this way detect the STALL events on empty rings and discard them before any transfers are queued. This does in practice solve the issue, but still leaves a small possible gap for the race to trigger. We still need a way to distinguish spurious STALLs on empty rings with '0' bytes remaing, from actual STALL events with all bytes transmitted. Cc: stable <stable@kernel.org> Fixes:dfba2174dc("usb: xhci: Add DbC support in xHCI driver") Tested-by: Łukasz Bartosik <ukaszb@chromium.org> Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>