Re: [PATCH v3 1/1] x86/fred: Clear WFE in missing-ENDBRANCH #CPs

[Dave Hansen <dave.hansen@intel.com>]

On 11/13/24 01:13, Xin Li (Intel) wrote:
> An indirect branch instruction sets the CPU indirect branch tracker
> (IBT) into WAIT_FOR_ENDBRANCH (WFE) state, and WFE stays asserted
> across the instruction boundary.  When decoder finds an instruction
> and WFE is set, and the instruction is not the appropriate ENDBR, it
> raises a #CP fault.
> 
> For the kernel IBT no ENDBR selftest where #CPs are deliberately
> triggerred, 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 ibt_clear_fred_wfe() and the
> !ibt_fatal code path when execution is allowed to continue.
> 
> Clobbering WFE in any other circumstance is a security-relevant bug.

With the minor fixes Ingo mentioned:

Acked-by: Dave Hansen <dave.hansen@linux.intel.com>