The AI-generated review reported a potential DMA use-after-free issue
[1]. If netc_xmit_ntmp_cmd() times out and returns an error, the pending
command is not explicitly aborted, while ntmp_free_data_mem()
unconditionally frees the DMA buffer. If the buffer has already been
reallocated elsewhere, this may lead to silent memory corruption. Because
the hardware eventually processes the pending command and perform a DMA
write of the response to the physical address of the freed buffer.

To resolve this issue, this patch does the following modifications:

1. Convert cbdr->ring_lock from a spinlock to a mutex

The lock was originally a spinlock in case NTMP operations might be
invoked from atomic context. After downstream support for all NTMP
tables, no such usage has materialized. A mutex lock is now required
because the driver now needs to reclaim used BDs and release associated
DMA memory within the lock's context, while dma_free_coherent() might
sleep.

2. Introduce software command BD (struct netc_swcbd)

The hardware write-back overwrites the addr and len fields of the BD,
so the driver cannot rely on the hardware BD to free the associated DMA
memory. The driver now maintains a software shadow BD storing the DMA
buffer pointer, DMA address, and size. And netc_xmit_ntmp_cmd() only
reclaims older BDs when the number of used BDs reaches
NETC_CBDR_CLEAN_WORK (16). The software BD enables correct DMA memory
release. With this, struct ntmp_dma_buf and ntmp_free_data_mem() are no
longer needed and are removed.

3. Require callers to hold ring_lock across netc_xmit_ntmp_cmd()

netc_xmit_ntmp_cmd() releases the ring_lock before the caller finishes
consuming the response. At this point, if a concurrent thread submits
a new command, it may trigger ntmp_clean_cbdr() and free the DMA buffer
while it is still in use. Move ring_lock ownership to the caller to
ensure the response buffer cannot be reclaimed prematurely. So the
helpers ntmp_select_and_lock_cbdr() and ntmp_unlock_cbdr() are added.

These changes eliminate the DMA use-after-free condition and ensure safe
and consistent BD reclamation and DMA buffer lifecycle management.

Fixes: 4701073c3deb ("net: enetc: add initial netc-lib driver to support NTMP")
Link: https://lore.kernel.org/netdev/20260403011729.1795413-1-kuba@kernel.org/ # [1]
Signed-off-by: Wei Fang <wei.fang@nxp.com>
Link: https://patch.msgid.link/20260415060833.2303846-3-wei.fang@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Some NETC functionality is controlled using control messages sent to the
hardware using BD ring interface with 32B descriptor similar to transmit
BD ring used on ENETC. This BD ring interface is referred to as command
BD ring. It is used to configure functionality where the underlying
resources may be shared between different entities or being too large to
configure using direct registers. Therefore, a messaging protocol called
NETC Table Management Protocol (NTMP) is provided for exchanging
configuration and management information between the software and the
hardware using the command BD ring interface.

For the management protocol of LS1028A has been retroactively named NTMP
1.0, and its implementation is in enetc_cbdr.c and enetc_qos.c. However,
NTMP of i.MX95 has been upgraded to version 2.0, which is incompatible
with LS1028A, because the message formats have been changed. Therefore,
add the netc-lib driver to support NTMP 2.0 to operate various tables.
Note that, only MAC address filter table and RSS table are supported at
the moment. More tables will be supported in subsequent patches.

It is worth mentioning that the purpose of the netc-lib driver is to
provide some NTMP-based generic interfaces for ENETC and NETC Switch
drivers. Currently, it only supports the configurations of some tables.
Interfaces such as tc flower and debugfs will be added in the future.

Signed-off-by: Wei Fang <wei.fang@nxp.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://patch.msgid.link/20250506080735.3444381-2-wei.fang@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>