linux-kernel - Re: [PATCH v3 1/1] x86/fred: Remove ENDBR64 from FRED entry points

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Message-ID: <d1f20289-a6d6-44d6-988f-08fb87faa067@zytor.com>
Date: Mon, 11 Aug 2025 15:48:47 -0700
From: Xin Li <xin@...or.com>
To: linux-kernel@...r.kernel.org
Cc: luto@...nel.org, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        jmill@....edu, peterz@...radead.org, andrew.cooper3@...rix.com,
        stable@...r.kernel.org
Subject: Re: [PATCH v3 1/1] x86/fred: Remove ENDBR64 from FRED entry points

On 7/15/2025 11:33 PM, Xin Li (Intel) wrote:
> The FRED specification has been changed in v9.0 to state that there
> is no need for FRED event handlers to begin with ENDBR64, because
> in the presence of supervisor indirect branch tracking, FRED event
> delivery does not enter the WAIT_FOR_ENDBRANCH state.
> 
> As a result, remove ENDBR64 from FRED entry points.
> 
> Then add ANNOTATE_NOENDBR to indicate that FRED entry points will
> never be used for indirect calls to suppress an objtool warning.
> 
> This change implies that any indirect CALL/JMP to FRED entry points
> causes #CP in the presence of supervisor indirect branch tracking.
> 
> Credit goes to Jennifer Miller <jmill@....edu> and other contributors
> from Arizona State University whose research shows that placing ENDBR
> at entry points has negative value thus led to this change.
> 
> Fixes: 14619d912b65 ("x86/fred: FRED entry/exit and dispatch code")
> Link: https://lore.kernel.org/linux-hardening/Z60NwR4w%2F28Z7XUa@ubun/
> Reviewed-by: H. Peter Anvin (Intel) <hpa@...or.com>
> Reviewed-by: Andrew Cooper <andrew.cooper3@...rix.com>
> Signed-off-by: Xin Li (Intel) <xin@...or.com>
> Cc: Jennifer Miller <jmill@....edu>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Andrew Cooper <andrew.cooper3@...rix.com>
> Cc: H. Peter Anvin <hpa@...or.com>
> Cc: stable@...r.kernel.org # v6.9+
> ---
> 
> Change in v3:
> *) Revise the FRED spec change description to clearly indicate that it
>     deviates from previous versions and is based on new research showing
>     that placing ENDBR at entry points has negative value (Andrew Cooper).
> 
> Change in v2:
> *) CC stable and add a fixes tag (PeterZ).
> ---
>   arch/x86/entry/entry_64_fred.S | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/entry/entry_64_fred.S b/arch/x86/entry/entry_64_fred.S
> index 29c5c32c16c3..907bd233c6c1 100644
> --- a/arch/x86/entry/entry_64_fred.S
> +++ b/arch/x86/entry/entry_64_fred.S
> @@ -16,7 +16,7 @@
>   
>   .macro FRED_ENTER
>   	UNWIND_HINT_END_OF_STACK
> -	ENDBR
> +	ANNOTATE_NOENDBR
>   	PUSH_AND_CLEAR_REGS
>   	movq	%rsp, %rdi	/* %rdi -> pt_regs */
>   .endm


Ping :)