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

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Message-ID: <20250716063320.1337818-1-xin@zytor.com>
Date: Tue, 15 Jul 2025 23:33:20 -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 v3 1/1] x86/fred: Remove ENDBR64 from FRED entry points

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
-- 
2.50.1