tpm_tis_spi: Account for SPI header when allocating TPM SPI xfer buffer

The TPM SPI transfer mechanism uses MAX_SPI_FRAMESIZE for computing the
maximum transfer length and the size of the transfer buffer. As such, it
does not account for the 4 bytes of header that prepends the SPI data
frame. This can result in out-of-bounds accesses and was confirmed with
KASAN.

Introduce SPI_HDRSIZE to account for the header and use to allocate the
transfer buffer.

Fixes: a86a42ac2bd6 ("tpm_tis_spi: Add hardware wait polling")
Signed-off-by: Matthew R. Ochs <[e-mail]>
Tested-by: Carol Soto <[e-mail]>
Reviewed-by: Jarkko Sakkinen <[e-mail]>
Signed-off-by: Jarkko Sakkinen <[e-mail]>

drivers/char/tpm/tpm_tis_spi_main.c

···
 #include "tpm_tis_spi.h"
 
 #define MAX_SPI_FRAMESIZE 64
+#define SPI_HDRSIZE 4
 
 /*
  * TCG SPI flow control is documented in section 6.4 of the spec[1]. In short,
···
 int tpm_tis_spi_init(struct spi_device *spi, struct tpm_tis_spi_phy *phy,
 		     int irq, const struct tpm_tis_phy_ops *phy_ops)
 {
-	phy->iobuf = devm_kmalloc(&spi->dev, MAX_SPI_FRAMESIZE, GFP_KERNEL);
+	phy->iobuf = devm_kmalloc(&spi->dev, SPI_HDRSIZE + MAX_SPI_FRAMESIZE, GFP_KERNEL);
 	if (!phy->iobuf)
 		return -ENOMEM;