sched_ext: Maintain an online cid mask in the scheduler arenagrafted

Schedulers on the default cid mapping treat [0, nr_online_cids) as the
online set and restart on hotplug. Schedulers that install their own mapping
with scx_bpf_cid_override() have no way to learn which cids are online: the
count no longer identifies members and the CPU-form cpumask is unusable from
cid programs. This is an obvious hole in the cid API.

Add scx_bpf_online_cmask(), a kernel-maintained cmask in the scheduler's
arena, allocated alongside the per-CPU scratch masks and populated after the
cid mapping is finalized and before ops.init(), for child schedulers too.
The pointer stays valid through ops.exit() with no reference to take. It is
the arena offset as a void pointer, the same form struct_ops arena arguments
arrive in. The verifier types the void return as a scalar for the program's
arena cast.

The mask follows the SCX hotplug notifications: seeded from cpu_active_mask
and updated before ops.cid_online/offline() runs, so it lags cpu_online_mask
only inside a hotplug transition. Updates walk the scheduler list under the
lock that also serializes unlinking. Reads are live, not atomic snapshots.
Root initialization excludes hotplug.

v2: Reworded the getter kerneldoc (Andrea Righi).

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>

1 files changed, 109 insertions, 0 deletions

diff --git a/Documentation/ABI/testing/evm b/Documentation/ABI/testing/evm
new file mode 100644
index 000000000..db3007bab
--- /dev/null
+++ b/Documentation/ABI/testing/evm
@@ -0,0 +1,109 @@
+What:		/sys/kernel/security/evm
+What:		/sys/kernel/security/*/evm
+Date:		March 2011
+Contact:	Mimi Zohar <zohar@us.ibm.com>
+Description:
+		EVM protects a file's security extended attributes(xattrs)
+		against integrity attacks. The initial method maintains an
+		HMAC-sha1 value across the extended attributes, storing the
+		value as the extended attribute 'security.evm'.
+
+		EVM supports two classes of security.evm. The first is
+		an HMAC-sha1 generated locally with a
+		trusted/encrypted key stored in the Kernel Key
+		Retention System. The second is a digital signature
+		generated either locally or remotely using an
+		asymmetric key. These keys are loaded onto root's
+		keyring using keyctl, and EVM is then enabled by
+		echoing a value to <securityfs>/evm made up of the
+		following bits:
+
+		===	  ==================================================
+		Bit	  Effect
+		===	  ==================================================
+		0	  Enable HMAC validation and creation
+		1	  Enable digital signature validation
+		2	  Permit modification of EVM-protected metadata at
+			  runtime. Not supported if HMAC validation and
+			  creation is enabled (deprecated).
+		3	  Require asymmetric signatures to be version 3
+		31	  Disable further runtime modification of EVM policy
+		===	  ==================================================
+
+		For example::
+
+		  echo 1 ><securityfs>/evm
+
+		will enable HMAC validation and creation
+
+		::
+
+		  echo 0x80000003 ><securityfs>/evm
+
+		will enable HMAC and digital signature validation and
+		HMAC creation and disable all further modification of policy.
+
+		::
+
+		  echo 0x80000006 ><securityfs>/evm
+
+		will enable digital signature validation, permit
+		modification of EVM-protected metadata and
+		disable all further modification of policy. This option is now
+		deprecated in favor of::
+
+		  echo 0x80000002 ><securityfs>/evm
+
+		as the outstanding issues that prevent the usage of EVM portable
+		signatures have been solved.
+
+		Echoing a value is additive, the new value is added to the
+		existing initialization flags.
+
+		For example, after::
+
+		  echo 2 ><securityfs>/evm
+
+		another echo can be performed::
+
+		  echo 1 ><securityfs>/evm
+
+		and the resulting value will be 3.
+
+		Note that once an HMAC key has been loaded, it will no longer
+		be possible to enable metadata modification. Signaling that an
+		HMAC key has been loaded will clear the corresponding flag.
+		For example, if the current value is 6 (2 and 4 set)::
+
+		  echo 1 ><securityfs>/evm
+
+		will set the new value to 3 (4 cleared).
+
+		Loading an HMAC key is the only way to disable metadata
+		modification.
+
+		Until key loading has been signaled EVM can not create
+		or validate the 'security.evm' xattr, but returns
+		INTEGRITY_UNKNOWN.  Loading keys and signaling EVM
+		should be done as early as possible.  Normally this is
+		done in the initramfs, which has already been measured
+		as part of the trusted boot.  For more information on
+		creating and loading existing trusted/encrypted keys,
+		refer to:
+		Documentation/security/keys/trusted-encrypted.rst. Both
+		dracut (via 97masterkey and 98integrity) and systemd (via
+		core/ima-setup) have support for loading keys at boot
+		time.
+
+What:		/sys/kernel/security/*/evm/evm_xattrs
+Date:		April 2018
+Contact:	Matthew Garrett <mjg59@google.com>
+Description:
+		Shows the set of extended attributes used to calculate or
+		validate the EVM signature, and allows additional attributes
+		to be added at runtime. Any signatures generated after
+		additional attributes are added (and on files possessing those
+		additional attributes) will only be valid if the same
+		additional attributes are configured on system boot. Writing
+		a single period (.) will lock the xattr list from any further
+		modification.