[PATCH] spi: stm32: fix double free of SRAM buffer on MDMA fallback

[Pin-Hao Chen <billy920225@gmail.com>]

When the "rxm2m" MDMA channel cannot be obtained, the driver frees the
SRAM buffer and falls back to DMA-only mode. However, sram_rx_buf is
left pointing to the freed buffer.

As a result, stm32_spi_unprepare_msg() may clear the released SRAM
area, while stm32_spi_remove() or a later probe error path may free the
buffer again, triggering BUG_ON() in gen_pool_free_owner().

The probe error path also checks sram_pool instead of sram_rx_buf. If
the SRAM allocation failed and a later probe step fails,
gen_pool_free() may therefore be called with an unallocated buffer.

Clear sram_rx_buf after freeing it on the MDMA fallback path and use
the buffer pointer to determine whether cleanup is needed.

Fixes: d17dd2f1d8a1 ("spi: stm32: use STM32 DMA with STM32 MDMA to enhance DDR use")
Cc: stable@vger.kernel.org
Signed-off-by: Pin-Hao Chen <billy920225@gmail.com>
---
Found by code inspection. Compile-tested only (ARM multi_v7_defconfig,
W=1 and sparse); no hardware was available for runtime testing.

 drivers/spi/spi-stm32.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/spi/spi-stm32.c b/drivers/spi/spi-stm32.c
index be88e62075af..605b847d8773 100644
--- a/drivers/spi/spi-stm32.c
+++ b/drivers/spi/spi-stm32.c
@@ -2511,6 +2511,7 @@ static int stm32_spi_probe(struct platform_device *pdev)
 					gen_pool_free(spi->sram_pool,
 						      (unsigned long)spi->sram_rx_buf,
 						      spi->sram_rx_buf_size);
+					spi->sram_rx_buf = NULL;
 					dev_warn(&pdev->dev,
 						 "failed to request rx mdma channel, DMA only\n");
 				}
@@ -2548,7 +2549,7 @@ static int stm32_spi_probe(struct platform_device *pdev)
 	if (spi->mdma_rx)
 		dma_release_channel(spi->mdma_rx);
 err_pool_free:
-	if (spi->sram_pool)
+	if (spi->sram_rx_buf)
 		gen_pool_free(spi->sram_pool, (unsigned long)spi->sram_rx_buf,
 			      spi->sram_rx_buf_size);
 	if (spi->dma_rx)

base-commit: fd99864b0ac936e870c7ae28354c5cd3dad5efd0
-- 
2.53.0