Re: [PATCH 5.15 v3 13/16] x86/its: Align RETs in BHB clear sequence to avoid thunking

[Sasha Levin <sashal@kernel.org>]

[ Sasha's backport helper bot ]

Hi,

✅ All tests passed successfully. No issues detected.
No action required from the submitter.

The upstream commit SHA1 provided is correct: f0cd7091cc5a032c8870b4285305d9172569d126

Status in newer kernel trees:
6.14.y | Present (different SHA1: bd83f74dcbb9)
6.12.y | Present (different SHA1: 56aabf26bac5)
6.6.y | Present (different SHA1: fb3103513d53)
6.1.y | Present (different SHA1: 31e4ffa249de)

Note: The patch differs from the upstream commit:
---
1:  f0cd7091cc5a0 ! 1:  fde65bafa97bf x86/its: Align RETs in BHB clear sequence to avoid thunking
    @@ Metadata
      ## Commit message ##
         x86/its: Align RETs in BHB clear sequence to avoid thunking
     
    +    commit f0cd7091cc5a032c8870b4285305d9172569d126 upstream.
    +
         The software mitigation for BHI is to execute BHB clear sequence at syscall
         entry, and possibly after a cBPF program. ITS mitigation thunks RETs in the
         lower half of the cacheline. This causes the RETs in the BHB clear sequence
    @@ arch/x86/entry/entry_64.S: SYM_CODE_END(rewind_stack_and_make_dead)
     + * Target Selection, rather than taking the slowpath via its_return_thunk.
       */
      SYM_FUNC_START(clear_bhb_loop)
    - 	ANNOTATE_NOENDBR
    + 	push	%rbp
     @@ arch/x86/entry/entry_64.S: SYM_FUNC_START(clear_bhb_loop)
      	call	1f
      	jmp	5f
---

Results of testing on various branches:

| Branch                    | Patch Apply | Build Test |
|---------------------------|-------------|------------|
| stable/linux-6.1.y        |  Success    |  Success   |