• Vladimir Oltean's avatar
    spi: spi-fsl-dspi: Fix race condition in TCFQ/EOQ interrupt · e3273649
    Vladimir Oltean authored and Mark Brown's avatar Mark Brown committed
    When the driver is working in TCFQ/EOQ mode (i.e. interacts with the SPI
    controller's FIFOs directly) the following sequence of operations
    happens:
    
    - The first byte of the tx buffer gets pushed to the TX FIFO (dspi->len
      gets decremented). This triggers the train of interrupts that handle
      the rest of the bytes.
    
    - The dspi_interrupt handles a TX confirmation event. It reads the newly
      available byte from the RX FIFO, checks the dspi->len exit condition,
      and if there's more to be done, it kicks off the next interrupt in the
      train by writing the next byte to the TX FIFO.
    
    Now the problem is that the wait queue is woken up one byte too early,
    because dspi->len becomes 0 as soon as the byte has been pushed into the
    TX FIFO. Its interrupt has not yet been processed and the RX byte has
    not been put from the FIFO into the buffer.
    
    Depending on the timing of the wait queue wakeup vs the handling of the
    last dspi_interrupt, it can happen that the main SPI message pump thread
    has already returned back into the spi_device driver. When the rx buffer
    is on stack (which it can be, because in this mode, the DSPI doesn't do
    DMA), the last interrupt will perform a memory write into an rx buffer
    that has been freed. This manifests as stack corruption.
    
    The solution is to only wake up the wait queue when dspi_rxtx says so,
    i.e. after it has processed the last TX confirmation interrupt and
    collected the last RX byte.
    
    Fixes: c55be305
    
     ("spi: spi-fsl-dspi: Use poll mode in case the platform IRQ is missing")
    Signed-off-by: default avatarVladimir Oltean <olteanv@gmail.com>
    Link: https://lore.kernel.org/r/20190903105708.32273-1-olteanv@gmail.com
    
    Signed-off-by: Mark Brown's avatarMark Brown <broonie@kernel.org>
    e3273649