Re: [RFC][PATCH] x86,nospec: Simplify {JMP,CALL}_NOSPEC

[Peter Zijlstra]

On Tue, Jul 19, 2022 at 11:23:07PM +0200, Peter Zijlstra wrote:
> Subject: x86,nospec: Simplify {JMP,CALL}_NOSPEC
> 
> Have {JMP,CALL}_NOSPEC generate the same code GCC does for indirect
> calls and rely on the objtool retpoline patching infrastructure.
> 
> There's no reason these should be alternatives while the vast bulk of
> compiler generated retpolines are not.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> ---
>  arch/x86/include/asm/nospec-branch.h | 24 ++++++++++++++++++------
>  1 file changed, 18 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
> index 10a3bfc1eb23..7bb319d2932c 100644
> --- a/arch/x86/include/asm/nospec-branch.h
> +++ b/arch/x86/include/asm/nospec-branch.h
> @@ -93,6 +93,19 @@
>  #endif
>  .endm
>  
> +/*
> + * Equivalent to -mindirect-branch-cs-prefix; emit the 5 byte jmp/call
> + * to the retpoline thunk with a CS prefix when the register requires
> + * a RAX prefix byte to encode. Also see apply_alternatives().

Obviously I meant: apply_retpolines() ...

> + */
> +.macro __CS_PREFIX reg:req
> +	.irp rs,r8,r9,r10,r11,r12,r13,r14,r15
> +	.ifc \reg,\rs
> +	.byte 0x2e
> +	.endif
> +	.endr
> +.endm
> +
>  /*
>   * JMP_NOSPEC and CALL_NOSPEC macros can be used instead of a simple
>   * indirect jmp/call which may be susceptible to the Spectre variant 2
> @@ -100,19 +113,18 @@
>   */
>  .macro JMP_NOSPEC reg:req
>  #ifdef CONFIG_RETPOLINE
> -	ALTERNATIVE_2 __stringify(ANNOTATE_RETPOLINE_SAFE; jmp *%\reg), \
> -		      __stringify(jmp __x86_indirect_thunk_\reg), X86_FEATURE_RETPOLINE, \
> -		      __stringify(lfence; ANNOTATE_RETPOLINE_SAFE; jmp *%\reg), X86_FEATURE_RETPOLINE_LFENCE
> +	__CS_PREFIX \reg
> +	jmp	__x86_indirect_thunk_\reg
>  #else
>  	jmp	*%\reg
> +	int3
>  #endif
>  .endm
>  
>  .macro CALL_NOSPEC reg:req
>  #ifdef CONFIG_RETPOLINE
> -	ALTERNATIVE_2 __stringify(ANNOTATE_RETPOLINE_SAFE; call *%\reg), \
> -		      __stringify(call __x86_indirect_thunk_\reg), X86_FEATURE_RETPOLINE, \
> -		      __stringify(lfence; ANNOTATE_RETPOLINE_SAFE; call *%\reg), X86_FEATURE_RETPOLINE_LFENCE
> +	__CS_PREFIX \reg
> +	call	__x86_indirect_thunk_\reg
>  #else
>  	call	*%\reg
>  #endif