[PATCHv2 3/4] coresight: etm4x: Add support to exclude kernel mode tracing

[Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>]

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.

Tested-by: Denis Nikitin <denik@chromium.org>
Signed-off-by: Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>
---
 drivers/hwtracing/coresight/coresight-etm4x-core.c  | 6 +++++-
 drivers/hwtracing/coresight/coresight-etm4x-sysfs.c | 6 ++++++
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/drivers/hwtracing/coresight/coresight-etm4x-core.c b/drivers/hwtracing/coresight/coresight-etm4x-core.c
index 15016f757828..dbd6b96f3a78 100644
--- a/drivers/hwtracing/coresight/coresight-etm4x-core.c
+++ b/drivers/hwtracing/coresight/coresight-etm4x-core.c
@@ -1235,12 +1235,16 @@ static void etm4_set_default(struct etmv4_config *config)
 		return;
 
 	/*
-	 * Make default initialisation trace everything
+	 * Make default initialisation trace everything when
+	 * CONFIG_EXCLUDE_KERNEL_PMU_TRACE is disabled.
 	 *
 	 * This is done by a minimum default config sufficient to enable
 	 * full instruction trace - with a default filter for trace all
 	 * achieved by having no filtering.
 	 */
+	if (IS_ENABLED(CONFIG_EXCLUDE_KERNEL_PMU_TRACE))
+		config->mode |= ETM_MODE_EXCL_KERN;
+
 	etm4_set_default_config(config);
 	etm4_set_default_filter(config);
 }
diff --git a/drivers/hwtracing/coresight/coresight-etm4x-sysfs.c b/drivers/hwtracing/coresight/coresight-etm4x-sysfs.c
index 0995a10790f4..78ba2a0888fc 100644
--- a/drivers/hwtracing/coresight/coresight-etm4x-sysfs.c
+++ b/drivers/hwtracing/coresight/coresight-etm4x-sysfs.c
@@ -296,6 +296,12 @@ static ssize_t mode_store(struct device *dev,
 	if (kstrtoul(buf, 16, &val))
 		return -EINVAL;
 
+	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 & ETMv4_MODE_ALL;
 
-- 
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