[PATCH] coresight: etm4x: Add config to exclude kernel mode tracing

[Mattias Nissler]

On Fri, Jan 15, 2021 at 6:46 AM Sai Prakash Ranjan
<saiprakash.ranjan at codeaurora.org> 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.

Well, the idea that root privileges != full control over the kernel
isn't new and at the very least since lockdown became part of mainline
[1] no longer an esoteric edge case. Regarding the use case Sai hints
at (namely protection of secrets in the kernel), Matthew Garret
actually has some more thoughts about confidentiality mode for
lockdown for secret protection [2]. And thus, unless someone can make
a compelling case that instruction-level tracing will not leak secrets
held by the kernel, I think an option for the kernel to prevent itself
from being traced (even by root) is valuable.

Finally, to sketch a practical use case scenario: 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.

[1] https://lwn.net/Articles/796866/
[2] https://mjg59.dreamwidth.org/55105.html

>
> After this discussion, I would like to post a v2 based on Suzuki's
> feedback earlier. @Suzuki, I have a common config for ETM3 and ETM4
> but couldn't get much idea on how to implement it for Intel PTs, if
> you have any suggestions there, please do share or we can have this
> only for Coresight ETMs.
>
> Thanks,
> Sai
>
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