In the Linux kernel, the following vulnerability has been resolved: perf/x86/intel: Improve...

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.