In the Linux kernel, the following vulnerability has been resolved: perf/x86/intel: Improve validation and configuration of ACR masks Curr

In the Linux kernel, the following vulnerability has been resolved:

perf/x86/intel: Improve validation and configuration of ACR masks

Currently there are several issues on the user space ACR mask validation
and configuration.

• The validation for user space ACR mask (attr.config2) is incomplete,
  e.g., the ACR mask could include the index which belongs to another
  ACR events group, but it's not validated.
• An early return on an invalid ACR mask caused all subsequent ACR groups
  to be skipped.
• The stale hardware ACR mask (hw.config1) is not cleared before setting
  new hardware ACR mask.

The following changes address all of the above issues.

• Figure out the event index group of an ACR group. Any bits in the
  user-space mask not present in the index group are now dropped.
• Instead of an early return on invalid bits, drop only the invalid
  portions and continue iterating through all ACR events to ensure full
  configuration.
• Explicitly clear the stale hardware ACR mask for each event prior to
  writing the new configuration.

Besides, a non-leader event member of ACR group could be disabled in
theory. This could cause bit-shifting errors in the acr_mask of remaining
group members. But since ACR sampling requires all events to be active,
this should not be a big concern in real use case. Add a "FIXME" comment
to notice this risk.