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

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Message-ID: <2e198fe6-c08a-4489-bb0e-aff6d2d5670b@citrix.com>
Date: Tue, 15 Jul 2025 16:49:37 +0100
From: Andrew Cooper <andrew.cooper3@...rix.com>
To: "Xin Li (Intel)" <xin@...or.com>, 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
Subject: Re: [PATCH v1 1/1] x86/fred: Remove ENDBR64 from FRED entry points

On 15/07/2025 7:44 am, 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.

I would suggest phrasing this as "The FRED specification has been
changed in v9 to..."

Simply "v9 states" can be read as "we mistook what v8 said and did it
wrong".

After all, the change here is specifically as a result of new research
showing ENDBR on the entrypoints to be of negative value.

>
> 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>

Preferably with an adjusted commit message, Reviewed-by Andrew Cooper
<andrew.cooper3@...rix.com>

Any idea when an updated SIMICS will be available?

~Andrew