linux-kernel - [RFC][PATCH 3/6] x86: Add ENDBR to IRET-to-Self

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Date:   Mon, 22 Nov 2021 18:03:04 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     x86@...nel.org, joao@...rdrivepizza.com, hjl.tools@...il.com,
        jpoimboe@...hat.com, andrew.cooper3@...rix.com
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org,
        ndesaulniers@...gle.com, keescook@...omium.org,
        samitolvanen@...gle.com
Subject: [RFC][PATCH 3/6] x86: Add ENDBR to IRET-to-Self

The IRET-to-Self chunks trigger forward code references without ENDBR,
fix that.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 arch/x86/entry/entry_64.S        |    2 ++
 arch/x86/include/asm/sync_core.h |    2 ++
 2 files changed, 4 insertions(+)

--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -39,6 +39,7 @@
 #include <asm/trapnr.h>
 #include <asm/nospec-branch.h>
 #include <asm/fsgsbase.h>
+#include <asm/ibt.h>
 #include <linux/err.h>
 
 #include "calling.h"
@@ -1309,6 +1310,7 @@ SYM_CODE_START(asm_exc_nmi)
 	iretq			/* continues at repeat_nmi below */
 	UNWIND_HINT_IRET_REGS
 1:
+	ENDBR
 #endif
 
 repeat_nmi:
--- a/arch/x86/include/asm/sync_core.h
+++ b/arch/x86/include/asm/sync_core.h
@@ -6,6 +6,7 @@
 #include <asm/processor.h>
 #include <asm/cpufeature.h>
 #include <asm/special_insns.h>
+#include <asm/ibt.h>
 
 #ifdef CONFIG_X86_32
 static inline void iret_to_self(void)
@@ -34,6 +35,7 @@ static inline void iret_to_self(void)
 		"pushq $1f\n\t"
 		"iretq\n\t"
 		"1:"
+		ASM_ENDBR
 		: "=&r" (tmp), ASM_CALL_CONSTRAINT : : "cc", "memory");
 }
 #endif /* CONFIG_X86_32 */