linux-kernel - [PATCH v2 1/1] x86/fred: Remove ENDBR64 from FRED entry points

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Message-ID: <20250715155242.1035896-1-xin@zytor.com>
Date: Tue, 15 Jul 2025 08:52:42 -0700
From: "Xin Li (Intel)" <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: [PATCH v2 1/1] x86/fred: Remove ENDBR64 from FRED entry points

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.

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>
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 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
-- 
2.50.1