Messages in this thread | | | Date | Wed, 20 Jan 2021 10:43:54 +0530 | From | Sai Prakash Ranjan <> | Subject | Re: [PATCH] coresight: etm4x: Add config to exclude kernel mode tracing |
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Hi Al,
On 2021-01-19 17:26, Al Grant wrote: >> From: Suzuki K Poulose <suzuki.poulose@arm.com> >> On 1/19/21 9:51 AM, Sai Prakash Ranjan wrote: >> > Hi Al, >> > >> > On 2021-01-19 14:06, Al Grant wrote: >> >> Hi Sai, >> >> >> >>> From: saiprakash.ranjan=codeaurora.org@mg.codeaurora.org >> >>> Hi Mathieu, >> >>> >> >>> On 2021-01-19 01:53, Mathieu Poirier wrote: >> >>> > On Fri, Jan 15, 2021 at 11:16:24AM +0530, Sai Prakash Ranjan wrote: >> >>> >> Hello Mathieu, Suzuki >> >>> >> >> >>> >> On 2020-10-15 21:32, Mathieu Poirier 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. >> >>> >> > >> >>> >> >> >>> >> Apart from the VM usecase discussed, I am told there are other >> >>> >> security concerns here regarding need to exclude kernel mode >> >>> >> tracing even for the privileged users/root. One such case being >> >>> >> the ability to analyze cryptographic code execution since ETMs >> >>> >> can record all branch instructions including timestamps in the >> >>> >> kernel and there may be other cases as well which I may not be >> >>> >> aware of and hence have added Denis and Mattias. Please let us >> >>> >> know if you have any questions further regarding this not being a security >> concern. >> >>> > >> >>> > Even if we were to apply this patch there are many ways to >> >>> > compromise a system or get the kernel to reveal important >> >>> > information using the perf subsystem. I would perfer to tackle >> >>> > the problem at that level rather than concentrating on coresight. >> >>> > >> >>> >> >>> Sorry but I did not understand your point. We are talking about the >> >>> capabilities of coresight etm tracing which has the instruction level tracing >> and a lot more. >> >>> Perf subsystem is just the framework used for it. >> >>> In other words, its not the perf subsystem which does instruction >> >>> level tracing, its the coresight etm. Why the perf subsystem should >> >>> be modified to lockdown kernel mode? If we were to let perf handle >> >>> all the trace filtering for different exception levels, then why do >> >>> we need the register settings in coresight etm driver to filter out >> >>> NS EL* tracing? And more importantly, how do you suppose we handle sysfs >> mode of coresight tracing with perf subsystem? >> >> >> >> You both have good points. Mathieu is right that this is not a >> >> CoreSight issue specifically, it is a matter of kernel security >> >> policy, and other hardware tracing mechanisms ought to be within its >> >> scope. There should be a general "anti kernel exfiltration" config >> >> that applies to all mechanisms within its scope, and we'd definitely expect >> that to include Intel PT as well as ETM. >> >> >> > >> > I agree with this part where there should be a generic config for all >> > hardware tracing families(atleast for Intel PT and ARM Coresight), >> > Suzuki suggested that as well. I am under the impression that Mathieu >> > didn't like adding such a config and wanted perf subsystem to handle >> > it since initial discussion was around whether root compromise meant >> > everything is lost already and such a kconfig would not help, but >> > Mattias already gave some good examples where that is not true. >> > >> >> A kernel config that forced exclude_kernel on all perf events would >> >> deal with ETM and PT in one place, but miss the sysfs interface to ETM. >> >> >> >> On the other hand, doing it in the ETM drivers would cover the perf >> >> and sysfs interfaces to ETM, but would miss Intel PT. >> >> >> >> So I think what is needed is a general config option that is both >> >> implemented in perf (excluding all kernel tracing events) and by any >> >> drivers that provide an alternative interface to hardware tracing events. >> >> >> > >> > I am good with this approach, once Mathieu confirms, I can add a >> > kernel wide kconfig as Suzuki suggested earlier and make ETM{3,4}x as >> > the initial users. Someone more familiar with Intel PTs can then make >> > use of this kconfig. >> >> Instead of adding the support for individual drivers, you could handle >> this in the >> generic perf layer. e.g, Fail perf_event create with an attribute >> which allows >> kernel tracing ? >> >> if (!attr.exclude_kernel) >> return -EINVAL; >> >> Or even exclude the kernel silently always. >> >> This could also be limited to PMUs with PERF_PMU_CAP_ITRACE, if you >> want to >> limit this to PMUs that instruction level tracing. > > The sysfs interface to ETM also needs to deny access to kernel trace, > so it's > safest to enforce it in the drivers in addition to any enforcement done > in perf. >
Yes, it will be done in drivers for sysfs interface as well based on the same kconfig.
> Also, forcing exclude_kernel on all perf events may be too strong. > Including > the kernel in counted events e.g. cache misses can help understand the > effect > of system calls on performance, and isn't a big side channel compared > to > userspace event counts. It doesn't reveal detailed timings in the way > trace does. > > So there's an argument for locking out kernel trace specifically (ETM > or PT > on the kernel); or even, for locking out timed trace with timestamps > and > cycle counts, and allowing untimed trace. So, that could be done in > perf, with > a more specific test on the type of event, before it forced > exclude_kernel. >
Yes exclude_kernel for all events might not be possible, so it would be better if it is initially applied for PMUs with PERF_PMU_CAP_ITRACE as Suzuki suggested.
Thanks, Sai
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