In the Linux kernel, the following vulnerability has been resolved: slimbus: qcom-ngd-ctrl:...

In the Linux kernel, the following vulnerability has been resolved:

slimbus: qcom-ngd-ctrl: Avoid ABBA on tx_lock/ctrl->lock

During the SSR/PDR down notification the tx_lock is taken with the intent to provide synchronization with active DMA transfers.

But during this period qcom_slim_ngd_down() is invoked, which ends up in slim_report_absent(), which takes the slim_controller lock. In multiple other codepaths these two locks are taken in the opposite order (i.e. slim_controller then tx_lock).

The result is a lockdep splat, and a possible deadlock:

rprocctl/449 is trying to acquire lock: ffff00009793e620 (&ctrl->lock){+.+.}-{4:4}, at: slim_report_absent (drivers/slimbus/core.c:322) slimbus

but task is already holding lock: ffff00009793fb50 (&ctrl->tx_lock){+.+.}-{4:4}, at: qcom_slim_ngd_ssr_pdr_notify (drivers/slimbus/qcom-ngd-ctrl.c:1475) slim_qcom_ngd_ctrl

which lock already depends on the new lock.

Possible unsafe locking scenario:

    CPU0                    CPU1
    ----                    ----

lock(&ctrl->tx_lock); lock(&ctrl->lock); lock(&ctrl->tx_lock); lock(&ctrl->lock);

The assumption is that the comment refers to the desire to not call qcom_slim_ngd_exit_dma() while we have an ongoing DMA TX transaction. But any such transaction is initiated and completed within a single qcom_slim_ngd_xfer_msg().

Prior to calling qcom_slim_ngd_exit_dma() the slim_controller is torn down, all child devices are notified that the slimbus is gone and the child devices are removed.

Stop taking the tx_lock in qcom_slim_ngd_ssr_pdr_notify() to avoid the deadlock.