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x86/bugs, kvm: Introduce boot-time control of L1TF mitigations

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Introduce the 'l1tf=' kernel command line option to allow for boot-time
switching of mitigation that is used on processors affected by L1TF.

The possible values are:

  full
	Provides all available mitigations for the L1TF vulnerability. Disables
	SMT and enables all mitigations in the hypervisors. SMT control via
	/sys/devices/system/cpu/smt/control is still possible after boot.
	Hypervisors will issue a warning when the first VM is started in
	a potentially insecure configuration, i.e. SMT enabled or L1D flush
	disabled.

  full,force
	Same as 'full', but disables SMT control. Implies the 'nosmt=force'
	command line option. sysfs control of SMT and the hypervisor flush
	control is disabled.

  flush
	Leaves SMT enabled and enables the conditional hypervisor mitigation.
	Hypervisors will issue a warning when the first VM is started in a
	potentially insecure configuration, i.e. SMT enabled or L1D flush
	disabled.

  flush,nosmt
	Disables SMT and enables the conditional hypervisor mitigation. SMT
	control via /sys/devices/system/cpu/smt/control is still possible
	after boot. If SMT is reenabled or flushing disabled at runtime
	hypervisors will issue a warning.

  flush,nowarn
	Same as 'flush', but hypervisors will not warn when
	a VM is started in a potentially insecure configuration.

  off
	Disables hypervisor mitigations and doesn't emit any warnings.

Default is 'flush'.

Let KVM adhere to these semantics, which means:

  - 'lt1f=full,force'	: Performe L1D flushes. No runtime control
    			  possible.

  - 'l1tf=full'
  - 'l1tf-flush'
  - 'l1tf=flush,nosmt'	: Perform L1D flushes and warn on VM start if
			  SMT has been runtime enabled or L1D flushing
			  has been run-time enabled
			  
  - 'l1tf=flush,nowarn'	: Perform L1D flushes and no warnings are emitted.
  
  - 'l1tf=off'		: L1D flushes are not performed and no warnings
			  are emitted.

KVM can always override the L1D flushing behavior using its 'vmentry_l1d_flush'
module parameter except when lt1f=full,force is set.

This makes KVM's private 'nosmt' option redundant, and as it is a bit
non-systematic anyway (this is something to control globally, not on
hypervisor level), remove that option.

Add the missing Documentation entry for the l1tf vulnerability sysfs file
while at it.

Signed-off-by: Jiri Kosina <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Tested-by: Jiri Kosina <[email protected]>
Reviewed-by: Greg Kroah-Hartman <[email protected]>
Reviewed-by: Josh Poimboeuf <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]
This commit is contained in:
Jiri Kosina 2018-07-13 16:23:25 +02:00 • committed by Thomas Gleixner
commit d90a7a0ec8
5 changed files with 165 additions and 19 deletions
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4
Documentation/ABI/testing/sysfs-devices-system-cpu
View file

@ -476,6 +476,7 @@ What: /sys/devices/system/cpu/vulnerabilities
/sys/devices/system/cpu/vulnerabilities/spectre_v1
/sys/devices/system/cpu/vulnerabilities/spectre_v2
/sys/devices/system/cpu/vulnerabilities/spec_store_bypass
/sys/devices/system/cpu/vulnerabilities/l1tf
Date: January 2018
Contact: Linux kernel mailing list <[email protected]>
Description: Information about CPU vulnerabilities
@ -488,6 +489,9 @@ Description: Information about CPU vulnerabilities
"Vulnerable" CPU is affected and no mitigation in effect
"Mitigation: $M" CPU is affected and mitigation $M is in effect
Details about the l1tf file can be found in
Documentation/admin-guide/l1tf.rst
What: /sys/devices/system/cpu/smt
/sys/devices/system/cpu/smt/active
/sys/devices/system/cpu/smt/control