oci/spec: deny /sys/devices/virtual/powercap

The ability to read these files may offer a power-based sidechannel
attack against any workloads running on the same kernel.

This was originally [CVE-2020-8694][1], which was fixed in
[949dd0104c496fa7c14991a23c03c62e44637e71][2] by restricting read access
to root. However, since many containers run as root, this is not
sufficient for our use case.

While untrusted code should ideally never be run, we can add some
defense in depth here by masking out the device class by default.

[Other mechanisms][3] to access this hardware exist, but they should not
be accessible to a container due to other safeguards in the
kernel/container stack (e.g. capabilities, perf paranoia).

[1]: https://nvd.nist.gov/vuln/detail/CVE-2020-8694
[2]: torvalds/linux@949dd01
[3]: https://web.eece.maine.edu/~vweaver/projects/rapl/

Signed-off-by: Bjorn Neergaard <bjorn.neergaard@docker.com>
(cherry picked from commit 106a9b7)
Signed-off-by: Bjorn Neergaard <bjorn.neergaard@docker.com>

oci/spec.go

@@ -175,6 +175,7 @@ func populateDefaultUnixSpec(ctx context.Context, s *Spec, id string) error {
 				"/proc/timer_stats",
 				"/proc/sched_debug",
 				"/sys/firmware",
+				"/sys/devices/virtual/powercap",
 				"/proc/scsi",
 			},
 			ReadonlyPaths: []string{