linux-kernel - Re: [PATCH v1 1/1] x86/fred: Remove ENDBR64 from FRED entry points

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Message-ID: <20250715084034.GO1613200@noisy.programming.kicks-ass.net>
Date: Tue, 15 Jul 2025 10:40:34 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Xin Li <xin@...or.com>
Cc: linux-kernel@...r.kernel.org, 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,
	andrew.cooper3@...rix.com
Subject: Re: [PATCH v1 1/1] x86/fred: Remove ENDBR64 from FRED entry points

On Mon, Jul 14, 2025 at 11:50:57PM -0700, Xin Li wrote:
> On 7/14/2025 11:44 PM, Xin Li (Intel) wrote:
> > The FRED specification v9.0 states 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 work led to this change.
> > 
> > Link: https://lore.kernel.org/linux-hardening/Z60NwR4w%2F28Z7XUa@ubun/
> > Reviewed-by: H. Peter Anvin (Intel) <hpa@...or.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>
> 
> Sorry, forgot to put CC stable.

Fixes: 14619d912b65 ("x86/fred: FRED entry/exit and dispatch code")

Right?

> 
> > ---
> >   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
>