[PATCH 2/2] serial: tegra: fix TX DMA descriptor use-after-free

[Austin via B4 Relay <devnull+austin.schlegel.arthrex.com@kernel.org>]

From: Austin <austin.schlegel@arthrex.com>

tegra_uart_stop_tx() calls dmaengine_terminate_all() and then
async_tx_ack(tup->tx_dma_desc) on the descriptor that was just
terminated. With the GPC DMA driver, dmaengine_terminate_all() frees
the active descriptor immediately, so the ack call that follows
touches freed memory, the same use-after-free pattern fixed for the
RX path in tegra_uart_terminate_rx_dma() ("serial: tegra: fix RX DMA
descriptor use-after-free"). dmaengine clients must not touch a
descriptor once dmaengine_terminate_all() has returned.

Move the ack before dmaengine_terminate_all(), while the descriptor
is still owned by the driver. tegra_uart_tx_dma_complete() already
acks inside the completion callback, where the descriptor is valid,
and is unaffected.

Found by code inspection while fixing the analogous RX bug; not
reproduced on hardware, since triggering it requires stopping an
in-flight TX DMA transfer (e.g. via a modem control line or flush)
at the right moment. The RX and TX paths share the same
dmaengine_terminate_all()-then-ack structure and the same root cause.

Signed-off-by: Austin <austin.schlegel@arthrex.com>
---
 drivers/tty/serial/serial-tegra.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/tty/serial/serial-tegra.c b/drivers/tty/serial/serial-tegra.c
index c9ec633e7164..0d1a2ebe2a59 100644
--- a/drivers/tty/serial/serial-tegra.c
+++ b/drivers/tty/serial/serial-tegra.c
@@ -625,9 +625,9 @@ static void tegra_uart_stop_tx(struct uart_port *u)
 
 	dmaengine_pause(tup->tx_dma_chan);
 	dmaengine_tx_status(tup->tx_dma_chan, tup->tx_cookie, &state);
+	async_tx_ack(tup->tx_dma_desc);
 	dmaengine_terminate_all(tup->tx_dma_chan);
 	count = tup->tx_bytes_requested - state.residue;
-	async_tx_ack(tup->tx_dma_desc);
 	uart_xmit_advance(&tup->uport, count);
 	tup->tx_in_progress = 0;
 }

-- 
2.53.0