Re: [PATCH] tpm_tis_spi: Account for SPI header when allocating TPM SPI xfer buffer

[Jarkko Sakkinen]

On Tue May 21, 2024 at 6:40 PM EEST, Matthew R. Ochs wrote:
> 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 MAX_SPI_BUFSIZE 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 <mochs@xxxxxxxxxx>
> Tested-by: Carol Soto <csoto@xxxxxxxxxx>
> ---
>  drivers/char/tpm/tpm_tis_spi_main.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/char/tpm/tpm_tis_spi_main.c b/drivers/char/tpm/tpm_tis_spi_main.c
> index 3f9eaf27b41b..ba50eaead9d8 100644
> --- a/drivers/char/tpm/tpm_tis_spi_main.c
> +++ b/drivers/char/tpm/tpm_tis_spi_main.c
> @@ -37,6 +37,8 @@
>  #include "tpm_tis_spi.h"
>  
>  #define MAX_SPI_FRAMESIZE 64
> +#define MAX_SPI_HDRSIZE 4
> +#define MAX_SPI_BUFSIZE (MAX_SPI_HDRSIZE + MAX_SPI_FRAMESIZE)

"MAX_" prefix does not make sense in the new entries.

>  
>  /*
>   * TCG SPI flow control is documented in section 6.4 of the spec[1]. In short,
> @@ -247,7 +249,7 @@ static int tpm_tis_spi_write_bytes(struct tpm_tis_data *data, u32 addr,
>  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, MAX_SPI_BUFSIZE, GFP_KERNEL);

It would better to open code here "SPI_HDRSIZE + MAX_SPI_FRAMESIZE".

I.e. less cross-referencing and documents better what is going on at
the call site.

>  	if (!phy->iobuf)
>  		return -ENOMEM;
>  

BR, Jarkko