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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: 4701073c3d ("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>
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