CVE-2024-56761: x86/fred: Clear WFE in missing-ENDBRANCH #CPs

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

x86/fred: Clear WFE in missing-ENDBRANCH #CPs

An indirect branch instruction sets the CPU indirect branch tracker (IBT) into WAITFORENDBRANCH (WFE) state and WFE stays asserted across the instruction boundary. When the decoder finds an inappropriate instruction while WFE is set ENDBR, the CPU raises a #CP fault.

For the "kernel IBT no ENDBR" selftest where #CPs are deliberately triggered, the WFE state of the interrupted context needs to be cleared to let execution continue. Otherwise when the CPU resumes from the instruction that just caused the previous #CP, another missing-ENDBRANCH #CP is raised and the CPU enters a dead loop.

This is not a problem with IDT because it doesn't preserve WFE and IRET doesn't set WFE. But FRED provides space on the entry stack (in an expanded CS area) to save and restore the WFE state, thus the WFE state is no longer clobbered, so software must clear it.

Clear WFE to avoid dead looping in ibtclearfredwfe() and the !ibtfatal code path when execution is allowed to continue.

Clobbering WFE in any other circumstance is a security-relevant bug.

[ dhansen: changelog rewording ]