Commit fc96ec826bce ("spi: fsl-cpm: Use 16 bit mode for large transfers
with even size") failed to make sure that the size is really even
before switching to 16 bit mode. Until recently the problem went
unnoticed because kernfs uses a pre-allocated bounce buffer of size
PAGE_SIZE for reading EEPROM.

But commit 8ad6249c51d0 ("eeprom: at25: convert to spi-mem API")
introduced an additional dynamically allocated bounce buffer whose size
is exactly the size of the transfer, leading to a buffer overrun in
the fsl-cpm driver when that size is odd.

Add the missing length parity verification and remain in 8 bit mode
when the length is not even.

Fixes: fc96ec826bce ("spi: fsl-cpm: Use 16 bit mode for large transfers with even size")
Cc: stable@vger.kernel.org
Closes: https://lore.kernel.org/all/638496dd-ec60-4e53-bad7-eb657f67d580@csgroup.eu/
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Sverdlin Alexander <alexander.sverdlin@siemens.com>
---
v2: Updated with comments from Alexander
---
 drivers/spi/spi-fsl-spi.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/spi/spi-fsl-spi.c b/drivers/spi/spi-fsl-spi.c
index 2f2082652a1a..481a7b28aacd 100644
--- a/drivers/spi/spi-fsl-spi.c
+++ b/drivers/spi/spi-fsl-spi.c
@@ -335,7 +335,7 @@ static int fsl_spi_prepare_message(struct spi_controller *ctlr,
 			if (t->bits_per_word == 16 || t->bits_per_word == 32)
 				t->bits_per_word = 8; /* pretend its 8 bits */
 			if (t->bits_per_word == 8 && t->len >= 256 &&
-			    (mpc8xxx_spi->flags & SPI_CPM1))
+			    !(t->len & 1) && (mpc8xxx_spi->flags & SPI_CPM1))
 				t->bits_per_word = 16;
 		}
 	}
-- 
2.49.0

On Thu, 20 Nov 2025 09:34:49 +0100, Christophe Leroy wrote:
> Commit fc96ec826bce ("spi: fsl-cpm: Use 16 bit mode for large transfers
> with even size") failed to make sure that the size is really even
> before switching to 16 bit mode. Until recently the problem went
> unnoticed because kernfs uses a pre-allocated bounce buffer of size
> PAGE_SIZE for reading EEPROM.
> 
> But commit 8ad6249c51d0 ("eeprom: at25: convert to spi-mem API")
> introduced an additional dynamically allocated bounce buffer whose size
> is exactly the size of the transfer, leading to a buffer overrun in
> the fsl-cpm driver when that size is odd.
> 
> [...]

Applied to

   https://git.kernel.org/pub/scm/linux/kernel/git/broonie/spi.git for-next

Thanks!

[1/1] spi: fsl-cpm: Check length parity before switching to 16 bit mode
      commit: 1417927df8049a0194933861e9b098669a95c762

All being well this means that it will be integrated into the linux-next
tree (usually sometime in the next 24 hours) and sent to Linus during
the next merge window (or sooner if it is a bug fix), however if
problems are discovered then the patch may be dropped or reverted.

You may get further e-mails resulting from automated or manual testing
and review of the tree, please engage with people reporting problems and
send followup patches addressing any issues that are reported if needed.

If any updates are required or you are submitting further changes they
should be sent as incremental updates against current git, existing
patches will not be replaced.

Please add any relevant lists and maintainers to the CCs when replying
to this mail.

Thanks,
Mark