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.

https://git.kernel.org/stable/c/3d1561537237c6cc1db76155183d8bbdac2339f0

https://git.kernel.org/stable/c/55f2ea9ff83cc27a85526b14bc9b32f96a08d6ec

https://git.kernel.org/stable/c/9708eb50fd7343145b422be852f890212155d845

https://git.kernel.org/stable/c/9f0d45d509b434c54da10e01f4ef8086e4583401

https://git.kernel.org/stable/c/aad4337a21b9ad3ae8d668fa8678d05e26ecbaa8

https://git.kernel.org/stable/c/d54a221b0f3cd9e1f03f18104be34e02a8258fae

https://git.kernel.org/stable/c/dc4d5c57e012c2c669793deb1515a57bbc6bf5dd

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