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

Hi Mathieu,

I think one of the use cases could be VMs. Is there isolation between EL1 guest kernels which we can control from perf in a system wide mode?

Thanks, Denis

On Thu, Oct 15, 2020 at 9:03 AM Mathieu Poirier mathieu.poirier@linaro.org wrote:

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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