Re: [PATCH] coresight: etm4x: Add config to exclude kernel mode tracing

Mathieu Poirier
2020-10-15T16:03:05Z

On Thu, Oct 15, 2020 at 06:15:22PM +0530, Sai Prakash Ranjan wrote:
> On production systems with ETMs enabled, it is preferred to
> exclude kernel mode(NS EL1) tracing for security concerns and
> support only userspace(NS EL0) tracing. So provide an option
> via kconfig to exclude kernel mode tracing if it is required.
> This config is disabled by default and would not affect the
> current configuration which has both kernel and userspace
> tracing enabled by default.
>

One requires root access (or be part of a special trace group) to be able to use
the cs_etm PMU. With this kind of elevated access restricting tracing at EL1
provides little in terms of security.

Thanks,
Mathieu

> Signed-off-by: Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>
> ---
> drivers/hwtracing/coresight/Kconfig | 9 +++++++++
> drivers/hwtracing/coresight/coresight-etm4x-core.c | 6 +++++-
> 2 files changed, 14 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/hwtracing/coresight/Kconfig b/drivers/hwtracing/coresight/Kconfig
> index c1198245461d..52435de8824c 100644
> --- a/drivers/hwtracing/coresight/Kconfig
> +++ b/drivers/hwtracing/coresight/Kconfig
> @@ -110,6 +110,15 @@ config CORESIGHT_SOURCE_ETM4X
> To compile this driver as a module, choose M here: the
> module will be called coresight-etm4x.
>
> +config CORESIGHT_ETM4X_EXCL_KERN
> + bool "Coresight ETM 4.x exclude kernel mode tracing"
> + depends on CORESIGHT_SOURCE_ETM4X
> + help
> + This will exclude kernel mode(NS EL1) tracing if enabled. This option
> + will be useful to provide more flexible options on production systems
> + where only userspace(NS EL0) tracing might be preferred for security
> + reasons.
> +
> config CORESIGHT_STM
> tristate "CoreSight System Trace Macrocell driver"
> depends on (ARM && !(CPU_32v3 || CPU_32v4 || CPU_32v4T)) || ARM64
> diff --git a/drivers/hwtracing/coresight/coresight-etm4x-core.c b/drivers/hwtracing/coresight/coresight-etm4x-core.c
> index abd706b216ac..7e5669e5cd1f 100644
> --- a/drivers/hwtracing/coresight/coresight-etm4x-core.c
> +++ b/drivers/hwtracing/coresight/coresight-etm4x-core.c
> @@ -832,6 +832,9 @@ static u64 etm4_get_ns_access_type(struct etmv4_config *config)
> {
> u64 access_type = 0;
>
> + if (IS_ENABLED(CONFIG_CORESIGHT_ETM4X_EXCL_KERN))
> + config->mode |= ETM_MODE_EXCL_KERN;
> +
> /*
> * EXLEVEL_NS, bits[15:12]
> * The Exception levels are:
> @@ -849,7 +852,8 @@ static u64 etm4_get_ns_access_type(struct etmv4_config *config)
> access_type = ETM_EXLEVEL_NS_HYP;
> }
>
> - if (config->mode & ETM_MODE_EXCL_USER)
> + if (config->mode & ETM_MODE_EXCL_USER &&
> + !IS_ENABLED(CONFIG_CORESIGHT_ETM4X_EXCL_KERN))
> access_type |= ETM_EXLEVEL_NS_APP;
>
> return access_type;
>
> base-commit: 3477326277451000bc667dfcc4fd0774c039184c
> --
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>