Re: [PATCH v4] tpm: Prevent local DOS via tpm/tpm0/ppi/*operations

[Jarkko Sakkinen <jarkko@kernel.org>]

On Tue, Sep 02, 2025 at 10:24:30AM -0400, Denis Aleksandrov wrote:
> Reads on tpm/tpm0/ppi/*operations can become very long on
> misconfigured systems. Reading the TPM is a blocking operation,
> thus a user could effectively trigger a DOS.
> 
> Resolve this by caching the results and avoiding the blocking
> operations after the first read.
> 
> Reported-by: Jan Stancek <jstancek@redhat.com>
> Signed-off-by: Denis Aleksandrov <daleksan@redhat.com>
> Suggested-by: Jarkko Sakkinen <jarkko@kernel.org>
> Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>
> ---
> 
> Changes in v4:
> 	- Removes empty lines.
> 	- Reorders vars to reverse christmas tree.
> 
>  drivers/char/tpm/tpm_ppi.c | 85 +++++++++++++++++++++++++++-----------
>  1 file changed, 62 insertions(+), 23 deletions(-)
> 
> diff --git a/drivers/char/tpm/tpm_ppi.c b/drivers/char/tpm/tpm_ppi.c
> index d53fce1c9d6f..df34b215440d 100644
> --- a/drivers/char/tpm/tpm_ppi.c
> +++ b/drivers/char/tpm/tpm_ppi.c
> @@ -33,6 +33,20 @@ static const guid_t tpm_ppi_guid =
>  	GUID_INIT(0x3DDDFAA6, 0x361B, 0x4EB4,
>  		  0xA4, 0x24, 0x8D, 0x10, 0x08, 0x9D, 0x16, 0x53);
>  
> +static const char * const tpm_ppi_info[] = {
> +	"Not implemented",
> +	"BIOS only",
> +	"Blocked for OS by system firmware",
> +	"User required",
> +	"User not required",
> +};
> +
> +/* A spinlock to protect access to the cache from concurrent reads */
> +static DEFINE_SPINLOCK(tpm_ppi_lock);
> +
> +static u32 ppi_operations_cache[PPI_VS_REQ_END + 1];
> +static bool ppi_cache_populated;
> +
>  static bool tpm_ppi_req_has_parameter(u64 req)
>  {
>  	return req == 23;
> @@ -277,8 +291,7 @@ static ssize_t tpm_show_ppi_response(struct device *dev,
>  	return status;
>  }
>  
> -static ssize_t show_ppi_operations(acpi_handle dev_handle, char *buf, u32 start,
> -				   u32 end)
> +static ssize_t cache_ppi_operations(acpi_handle dev_handle, char *buf)
>  {
>  	int i;
>  	u32 ret;
> @@ -286,34 +299,22 @@ static ssize_t show_ppi_operations(acpi_handle dev_handle, char *buf, u32 start,
>  	union acpi_object *obj, tmp;
>  	union acpi_object argv = ACPI_INIT_DSM_ARGV4(1, &tmp);
>  
> -	static char *info[] = {
> -		"Not implemented",
> -		"BIOS only",
> -		"Blocked for OS by BIOS",
> -		"User required",
> -		"User not required",
> -	};
> -
>  	if (!acpi_check_dsm(dev_handle, &tpm_ppi_guid, TPM_PPI_REVISION_ID_1,
>  			    1 << TPM_PPI_FN_GETOPR))
>  		return -EPERM;
>  
>  	tmp.integer.type = ACPI_TYPE_INTEGER;
> -	for (i = start; i <= end; i++) {
> +	for (i = 0; i <= PPI_VS_REQ_END; i++) {
>  		tmp.integer.value = i;
>  		obj = tpm_eval_dsm(dev_handle, TPM_PPI_FN_GETOPR,
>  				   ACPI_TYPE_INTEGER, &argv,
>  				   TPM_PPI_REVISION_ID_1);
> -		if (!obj) {
> +		if (!obj)
>  			return -ENOMEM;
> -		} else {
> -			ret = obj->integer.value;
> -			ACPI_FREE(obj);
> -		}
>  
> -		if (ret > 0 && ret < ARRAY_SIZE(info))
> -			len += sysfs_emit_at(buf, len, "%d %d: %s\n",
> -					     i, ret, info[ret]);
> +		ret = obj->integer.value;
> +		ppi_operations_cache[i] = ret;
> +		ACPI_FREE(obj);
>  	}
>  
>  	return len;
> @@ -324,9 +325,28 @@ static ssize_t tpm_show_ppi_tcg_operations(struct device *dev,
>  					   char *buf)
>  {
>  	struct tpm_chip *chip = to_tpm_chip(dev);
> +	ssize_t len = 0;
> +	u32 ret;
> +	int i;
> +
> +	spin_lock(&tpm_ppi_lock);
> +	if (!ppi_cache_populated) {
> +		len = cache_ppi_operations(chip->acpi_dev_handle, buf);
> +		if (len < 0)
> +			return len;
>  
> -	return show_ppi_operations(chip->acpi_dev_handle, buf, 0,
> -				   PPI_TPM_REQ_MAX);
> +		ppi_cache_populated = true;
> +	}
> +
> +	for (i = 0; i <= PPI_TPM_REQ_MAX; i++) {
> +		ret = ppi_operations_cache[i];
> +		if (ret >= 0 && ret < ARRAY_SIZE(tpm_ppi_info))
> +			len += sysfs_emit_at(buf, len, "%d %d: %s\n",
> +							i, ret, tpm_ppi_info[ret]);
> +	}
> +	spin_unlock(&tpm_ppi_lock);
> +
> +	return len;
>  }
>  
>  static ssize_t tpm_show_ppi_vs_operations(struct device *dev,
> @@ -334,9 +354,28 @@ static ssize_t tpm_show_ppi_vs_operations(struct device *dev,
>  					  char *buf)
>  {
>  	struct tpm_chip *chip = to_tpm_chip(dev);
> +	ssize_t len = 0;
> +	u32 ret;
> +	int i;
>  
> -	return show_ppi_operations(chip->acpi_dev_handle, buf, PPI_VS_REQ_START,
> -				   PPI_VS_REQ_END);
> +	spin_lock(&tpm_ppi_lock);
> +	if (!ppi_cache_populated) {
> +		len = cache_ppi_operations(chip->acpi_dev_handle, buf);
> +		if (len < 0)
> +			return len;
> +
> +		ppi_cache_populated = true;
> +	}
> +
> +	for (i = PPI_VS_REQ_START; i <= PPI_VS_REQ_END; i++) {
> +		ret = ppi_operations_cache[i];
> +		if (ret >= 0 && ret < ARRAY_SIZE(tpm_ppi_info))
> +			len += sysfs_emit_at(buf, len, "%d %d: %s\n",
> +							i, ret, tpm_ppi_info[ret]);
> +	}
> +	spin_unlock(&tpm_ppi_lock);
> +
> +	return len;
>  }
>  
>  static DEVICE_ATTR(version, S_IRUGO, tpm_show_ppi_version, NULL);
> -- 
> 2.48.1
> 

I don't know how I messed up the patch in my Git but now it is good
(I think):

https://git.kernel.org/pub/scm/linux/kernel/git/jarkko/linux-tpmdd.git/commit/?h=next

Please check before I move forward with the PR.

BR, Jarkko