spi: bcm2835: Fix out-of-bounds access with more than 4 slaves

Commit 571e31fa ("spi: bcm2835: Cache CS register value for
->prepare_message()") limited the number of slaves to 3 at compile-time.
The limitation was necessitated by a statically-sized array prepare_cs[]
in the driver private data which contains a per-slave register value.

The commit sought to enforce the limitation at run-time by setting the
controller's num_chipselect to 3:  Slaves with a higher chipselect are
rejected by spi_add_device().

However the commit neglected that num_chipselect only limits the number
of *native* chipselects.  If GPIO chipselects are specified in the
device tree for more than 3 slaves, num_chipselect is silently raised by
of_spi_get_gpio_numbers() and the result are out-of-bounds accesses to
the statically-sized array prepare_cs[].

As a bandaid fix which is backportable to stable, raise the number of
allowed slaves to 24 (which "ought to be enough for anybody"), enforce
the limitation on slave ->setup and revert num_chipselect to 3 (which is
the number of native chipselects supported by the controller).
An upcoming for-next commit will allow an arbitrary number of slaves.

Fixes: 571e31fa ("spi: bcm2835: Cache CS register value for ->prepare_message()")
Reported-by: Joe Burmeister <joe.burmeister@devtank.co.uk>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Cc: stable@vger.kernel.org # v5.4+
Cc: Phil Elwell <phil@raspberrypi.com>
Link: https://lore.kernel.org/r/75854affc1923309fde05e47494263bde73e5592.1621703210.git.lukas@wunner.deSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-bcm2835.c

@@ -68,7 +68,7 @@
 #define BCM2835_SPI_FIFO_SIZE		64
 #define BCM2835_SPI_FIFO_SIZE_3_4	48
 #define BCM2835_SPI_DMA_MIN_LENGTH	96
-#define BCM2835_SPI_NUM_CS		4   /* raise as necessary */
+#define BCM2835_SPI_NUM_CS		24  /* raise as necessary */
 #define BCM2835_SPI_MODE_BITS	(SPI_CPOL | SPI_CPHA | SPI_CS_HIGH \
 				| SPI_NO_CS | SPI_3WIRE)
 
@@ -1195,6 +1195,12 @@ static int bcm2835_spi_setup(struct spi_device *spi)
 	struct gpio_chip *chip;
 	u32 cs;
 
+	if (spi->chip_select >= BCM2835_SPI_NUM_CS) {
+		dev_err(&spi->dev, "only %d chip-selects supported\n",
+			BCM2835_SPI_NUM_CS - 1);
+		return -EINVAL;
+	}
+
 	/*
 	 * Precalculate SPI slave's CS register value for ->prepare_message():
 	 * The driver always uses software-controlled GPIO chip select, hence
@@ -1288,7 +1294,7 @@ static int bcm2835_spi_probe(struct platform_device *pdev)
 	ctlr->use_gpio_descriptors = true;
 	ctlr->mode_bits = BCM2835_SPI_MODE_BITS;
 	ctlr->bits_per_word_mask = SPI_BPW_MASK(8);
-	ctlr->num_chipselect = BCM2835_SPI_NUM_CS;
+	ctlr->num_chipselect = 3;
 	ctlr->setup = bcm2835_spi_setup;
 	ctlr->transfer_one = bcm2835_spi_transfer_one;
 	ctlr->handle_err = bcm2835_spi_handle_err;