[PATCHv2 4/4] coresight: etm3x: Add support to exclude kernel mode tracing

Sai Prakash Ranjan
2021-03-01T19:11:33Z

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. Perf subsystem interface uses the newly
introduced kernel config CONFIG_EXCLUDE_KERNEL_PMU_TRACE to exclude
kernel mode tracing, but there is an additional interface
via sysfs for ETMs which also needs to be handled to exclude kernel
mode tracing. So we use this same generic kernel config to handle
the sysfs mode of tracing. This config is disabled by default and
would not affect the current configuration which has both kernel and
userspace tracing enabled by default.

Signed-off-by: Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>
---
drivers/hwtracing/coresight/coresight-etm3x-core.c | 3 +++
drivers/hwtracing/coresight/coresight-etm3x-sysfs.c | 6 ++++++
2 files changed, 9 insertions(+)

diff --git a/drivers/hwtracing/coresight/coresight-etm3x-core.c b/drivers/hwtracing/coresight/coresight-etm3x-core.c
index cf64ce73a741..d94f6b01ca09 100644
--- a/drivers/hwtracing/coresight/coresight-etm3x-core.c
+++ b/drivers/hwtracing/coresight/coresight-etm3x-core.c
@@ -195,6 +195,9 @@ void etm_set_default(struct etm_config *config)
if (WARN_ON_ONCE(!config))
return;

+ if (IS_ENABLED(CONFIG_EXCLUDE_KERNEL_PMU_TRACE))
+ config->mode |= ETM_MODE_EXCL_KERN;
+
/*
* Taken verbatim from the TRM:
*
diff --git a/drivers/hwtracing/coresight/coresight-etm3x-sysfs.c b/drivers/hwtracing/coresight/coresight-etm3x-sysfs.c
index e8c7649f123e..f522fc2e01b3 100644
--- a/drivers/hwtracing/coresight/coresight-etm3x-sysfs.c
+++ b/drivers/hwtracing/coresight/coresight-etm3x-sysfs.c
@@ -116,6 +116,12 @@ static ssize_t mode_store(struct device *dev,
if (ret)
return ret;

+ if (IS_ENABLED(CONFIG_EXCLUDE_KERNEL_PMU_TRACE) && (!(val & ETM_MODE_EXCL_KERN))) {
+ dev_warn(dev,
+ "Kernel mode tracing is not allowed, check your kernel config\n");
+ return -EACCES;
+ }
+
spin_lock(&drvdata->spinlock);
config->mode = val & ETM_MODE_ALL;

--
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