Re: [PATCH v2] tpm: infineon: add bounds check in tpm_inf_recv

[Jarkko Sakkinen <jarkko@kernel.org>]

On Fri, Oct 10, 2025 at 06:52:55AM +0000, Shahriyar Jalayeri wrote:
> Add two buffer size validations to prevent buffer overflows in
> tpm_inf_recv():
> 
> 1. Validate that the provided buffer can hold at least the 4-byte header
>    before attempting to read it.
> 2. Validate that the buffer is large enough to hold the data size reported
>    by the TPM before reading the payload.
> 
> Without these checks, a malicious or malfunctioning TPM could cause buffer
> overflows by reporting data sizes larger than the provided buffer, leading
> to memory corruption.
> 
> Fixes: ebb81fdb3dd0 ("[PATCH] tpm: Support for Infineon TPM")
> Signed-off-by: Shahriyar Jalayeri <shahriyar@posteo.de>
> ---
>  drivers/char/tpm/tpm_infineon.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/drivers/char/tpm/tpm_infineon.c b/drivers/char/tpm/tpm_infineon.c
> index 7638b65b8..0fe4193a3 100644
> --- a/drivers/char/tpm/tpm_infineon.c
> +++ b/drivers/char/tpm/tpm_infineon.c
> @@ -250,6 +250,10 @@ static int tpm_inf_recv(struct tpm_chip *chip, u8 * buf, size_t count)
>  	number_of_wtx = 0;
>  
>  recv_begin:
> +    /* expect at least 1-byte VL header, 1-byte ctrl-tag, 2-byte data size */

This is definitely good enough :-)

But is that comment misaligned? Does VL come from "VLAN"?

> +	if (count < 4)
> +		return -EIO;
> +
>  	/* start receiving header */
>  	for (i = 0; i < 4; i++) {
>  		ret = wait(chip, STAT_RDA);
> @@ -268,6 +272,9 @@ static int tpm_inf_recv(struct tpm_chip *chip, u8 * buf, size_t count)
>  		/* size of the data received */

I'd delete the above comment.

>  		size = ((buf[2] << 8) | buf[3]);

And use here:

https://elixir.bootlin.com/linux/v6.17.1/source/include/linux/byteorder/generic.h#L108

Not exactly in scope but it would be good convention and make
the check after it more readable.

>  
> +		if (size + 6 > count)
> +			return -EIO;
> +
>  		for (i = 0; i < size; i++) {
>  			wait(chip, STAT_RDA);
>  			buf[i] = tpm_data_in(RDFIFO);
> -- 
> 2.43.0
> 

BR, Jarkko