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 in the words of a security team member (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.
[1] https://lwn.net/Articles/796866/
Suggested-by: Suzuki K Poulose suzuki.poulose@arm.com Suggested-by: Al Grant al.grant@arm.com Tested-by: Denis Nikitin denik@chromium.org Link: https://lore.kernel.org/lkml/20201015124522.1876-1-saiprakash.ranjan@codeaur... Signed-off-by: Sai Prakash Ranjan saiprakash.ranjan@codeaurora.org --- init/Kconfig | 12 ++++++++++++ kernel/events/core.c | 6 ++++++ 2 files changed, 18 insertions(+)
diff --git a/init/Kconfig b/init/Kconfig index af454a51f3c5..31b4d1f26bce 100644 --- a/init/Kconfig +++ b/init/Kconfig @@ -1832,6 +1832,18 @@ config DEBUG_PERF_USE_VMALLOC
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+config EXCLUDE_KERNEL_HW_ITRACE + bool "Exclude kernel mode hardware assisted instruction tracing" + depends on PERF_EVENTS + help + Exclude kernel mode instruction tracing by hardware tracing + family such as ARM Coresight ETM, Intel PT and so on. + + This option allows to disable kernel mode instruction tracing + offered by hardware assisted tracing for all users(including root) + especially for production systems where only userspace tracing might + be preferred for security reasons. + config VM_EVENT_COUNTERS default y bool "Enable VM event counters for /proc/vmstat" if EXPERT diff --git a/kernel/events/core.c b/kernel/events/core.c index aece2fe19693..044a774cef6d 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -11866,6 +11866,12 @@ SYSCALL_DEFINE5(perf_event_open, goto err_task; }
+ if (IS_ENABLED(CONFIG_EXCLUDE_KERNEL_HW_ITRACE) && + (event->pmu->capabilities & PERF_PMU_CAP_ITRACE) && !attr.exclude_kernel) { + err = -EACCES; + goto err_alloc; + } + if (is_sampling_event(event)) { if (event->pmu->capabilities & PERF_PMU_CAP_NO_INTERRUPT) { err = -EOPNOTSUPP;