[PATCH 2/3] mailbox: Avoid NULL dereference in mbox_chan_received_data
From: andre.przywara@arm.com (Andre Przywara)
Date: 2018-02-28 17:17:24
Also in:
linux-devicetree, lkml
Hi, On 28/02/18 02:27, Samuel Holland wrote:
quoted hunk ↗ jump to hunk
If a reception IRQ is pending when a mailbox channel is shut down (for example, if the controller uses threaded interrupts), it is possible for mbox_chan_received_data to be called while chan->cl is NULL. This was found while developing a mailbox controller driver for use with SCPI. The SCPI protocol driver frees its mailbox channel during probing if the SCP firmware does not respond within a specified timeout. In this case, if the SCP firmware takes slightly too long to respond, mbox_chan_received_data races with mbox_free_channel clearing chan->cl. Signed-off-by: Samuel Holland <samuel@sholland.org> --- drivers/mailbox/mailbox.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-)diff --git a/drivers/mailbox/mailbox.c b/drivers/mailbox/mailbox.c index 674b35f402f5..a0258d8672d5 100644 --- a/drivers/mailbox/mailbox.c +++ b/drivers/mailbox/mailbox.c@@ -152,9 +152,11 @@ static enum hrtimer_restart txdone_hrtimer(struct hrtimer *hrtimer) */ void mbox_chan_received_data(struct mbox_chan *chan, void *mssg) { + struct mbox_client *cl = READ_ONCE(chan->cl); + /* No buffering the received data */ - if (chan->cl->rx_callback) - chan->cl->rx_callback(chan->cl, mssg); + if (cl && cl->rx_callback) + cl->rx_callback(cl, mssg);
I don't think this is the proper fix. This sounds like we should have a lock here. If mbox_free_channel now frees or clears chan->cl between the READ_ONCE and the second part of the "&&", we will have a use-after-free. If it's being cleared after the comparison, we will end up with a NULL pointer dereference again, IIUC. Or am I missing something? Cheers, Andre.
} EXPORT_SYMBOL_GPL(mbox_chan_received_data);