[PATCH RFC 0/2] Selective mitigation for trusted userspace

[Pawan Gupta <pawan.kumar.gupta@linux.intel.com>]

Hi,

This is an experimental series exploring the feasibility of selectively
applying CPU vulnerability mitigations on a per-process basis. The
motivation behind this work is to address the performance degradation
experienced by trusted user-space applications due to system-wide CPU
mitigations.

Currently, the mitigations are applied universally across the system,
without discrimination between trusted and untrusted user-space processes.
This results in a performance penalty for all applications, regardless of
their trustworthiness. The proposed solution aims to provide a mechanism
for system administrators to explicitly mark certain applications as
trusted, allowing them to bypass these mitigations and regain lost
performance.

The series introduces a new cgroup attribute and a separate kernel
entry/exit path that can be used to selectively disable CPU mitigations for
processes that are deemed trustworthy by the system administrator. This
approach provides a tool to the administrator who understands the security
implications and is aware of trustworthiness of the applications that they
care.

The rationale for choosing the cgroup interface over other potential
interfaces, such as LSMs, is cgroup's inherent support for core scheduling.
Core scheduling allows the grouping of tasks such that they are scheduled
to run on the same cores. By leveraging core scheduling, we can minimize
the performance overhead caused by the MSR writes during context switching
between trusted and untrusted processes. With the end goal being trusted
and untrusted processes run on separate cores, enhancing the security.

Patch 1 adds the unmitigated entry/exit path.
Patch 2 provides a cgroup knob to bypass CPU mitigations.

This series is lightly tested. Feedback and discussion are welcome.

TODO:
- Add CONFIG_MITIGATION_PER_PROCESS
- Add support for skipping other mitigations like RSB filling.
- Update suspend/resume paths to handle the new entry/exit path.
- Should child processes inherit the parent's unmitigated status?
- Add documentation.

Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
---
Pawan Gupta (2):
      x86/entry_64: Add a separate unmitigated entry/exit path
      cpu/bugs: cgroup: Add a cgroup knob to bypass CPU mitigations

 arch/x86/entry/entry_64.S        | 66 +++++++++++++++++++++++++++++++++-------
 arch/x86/include/asm/proto.h     | 15 ++++++---
 arch/x86/include/asm/ptrace.h    | 15 ++++++---
 arch/x86/include/asm/switch_to.h | 10 ++++++
 arch/x86/kernel/cpu/bugs.c       | 21 +++++++++++++
 arch/x86/kernel/cpu/common.c     |  2 +-
 include/linux/cgroup-defs.h      |  3 ++
 kernel/cgroup/cgroup.c           | 42 +++++++++++++++++++++++++
 kernel/sched/core.c              |  2 +-
 9 files changed, 155 insertions(+), 21 deletions(-)
---
base-commit: 98f7e32f20d28ec452afb208f9cffc08448a2652
change-id: 20240919-selective-mitigation-6d02c4bbb72b

-- 
Thanks,
Pawan