Hardware assisted tracing families such as ARM Coresight, Intel PT provides rich tracing capabilities including instruction level tracing and accurate timestamps which are very useful for profiling and also pose a significant security risk. One such example of security risk is when kernel mode tracing is not excluded and these hardware assisted tracing can be used to analyze cryptographic code execution. In this case, even the root user must not be able to infer anything.
To explain it more clearly, here is the quote from a member of the security team (credits: Mattias Nissler),
"Consider a system where disk contents are encrypted and the encryption key is set up by the user when mounting the file system. From that point on the encryption key resides in the kernel. It seems reasonable to expect that the disk encryption key be protected from exfiltration even if the system later suffers a root compromise (or even against insiders that have root access), at least as long as the attacker doesn't manage to compromise the kernel."
Here the idea is to protect such important information from all users including root users since root privileges does not have to mean full control over the kernel [1] and root compromise does not have to be the end of the world.
Currently we can exclude kernel mode tracing via perf_event_paranoid sysctl but it has following limitations,
* It is applicable to all PMUs and not just the ones supporting instruction tracing. * No option to restrict kernel mode instruction tracing by the root user. * Not possible to restrict kernel mode instruction tracing when the hardware assisted tracing IPs like ARM Coresight ETMs use an additional interface via sysfs for tracing in addition to perf interface.
So introduce a new config CONFIG_EXCLUDE_KERNEL_HW_ITRACE to exclude kernel mode instruction tracing which will be generic and applicable to all hardware tracing families and which can also be used with other interfaces like sysfs in case of ETMs.
Patch 1 adds this new config and the support in perf core to exclude kernel mode tracing for PMUs which support instruction mode tracing.
Patch 2 adds the perf evsel warning message when the perf tool users attempt to perform a kernel mode instruction trace with the config enabled to exclude the kernel mode tracing.
Patch 3 and Patch 4 adds the support for excluding kernel mode for ARM Coresight ETM{4,3}XX sysfs mode using the newly introduced generic config.
[1] https://lwn.net/Articles/796866/
Sai Prakash Ranjan (4): perf/core: Add support to exclude kernel mode instruction tracing perf evsel: Print warning for excluding kernel mode instruction tracing coresight: etm4x: Add support to exclude kernel mode tracing coresight: etm3x: Add support to exclude kernel mode tracing
drivers/hwtracing/coresight/coresight-etm3x-core.c | 11 +++++++++++ .../hwtracing/coresight/coresight-etm3x-sysfs.c | 3 ++- drivers/hwtracing/coresight/coresight-etm4x-core.c | 14 +++++++++++++- .../hwtracing/coresight/coresight-etm4x-sysfs.c | 3 ++- init/Kconfig | 12 ++++++++++++ kernel/events/core.c | 6 ++++++ tools/perf/util/evsel.c | 3 ++- 7 files changed, 48 insertions(+), 4 deletions(-)