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

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Date:   Mon, 22 Nov 2021 19:09:47 +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, ndesaulniers@...gle.com,
        keescook@...omium.org, samitolvanen@...gle.com
Subject: Re: [RFC][PATCH 3/6] x86: Add ENDBR to IRET-to-Self

On Mon, Nov 22, 2021 at 06:03:04PM +0100, Peter Zijlstra wrote:
> The IRET-to-Self chunks trigger forward code references without ENDBR,
> fix that.

Andy corrected me, IRET doesn't take ENBR, the alternative is the below.

---
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1316,7 +1316,6 @@ SYM_CODE_START(asm_exc_nmi)
 	iretq			/* continues at repeat_nmi below */
 	UNWIND_HINT_IRET_REGS entry=0
 1:
-	ENDBR
 #endif
 
 repeat_nmi:
--- a/arch/x86/include/asm/sync_core.h
+++ b/arch/x86/include/asm/sync_core.h
@@ -6,7 +6,6 @@
 #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)
@@ -35,7 +34,6 @@ 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 */
--- a/tools/objtool/arch/x86/decode.c
+++ b/tools/objtool/arch/x86/decode.c
@@ -598,6 +598,7 @@ int arch_decode_instruction(struct objto
 				op->dest.type = OP_DEST_REG;
 				op->dest.reg = CFI_SP;
 			}
+			*type = INSN_IRET;
 			break;
 		}
 
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -3587,7 +3587,7 @@ static int validate_ibt_reloc(struct obj
 static void validate_ibt_insn(struct objtool_file *file, struct instruction *insn)
 {
 	struct reloc *reloc = insn_reloc(file, insn);
-	struct instruction *target;
+	struct instruction *target, *n;
 	unsigned long offset;
 
 	if (!reloc)
@@ -3599,8 +3599,16 @@ static void validate_ibt_insn(struct obj
 	offset = reloc->sym->offset + reloc->addend;
 
 	target = find_insn(file, reloc->sym->sec, offset);
-	if (target && insn->func == target->func && target->this_ip)
-		return;
+	if (target && insn->func == target->func) {
+		if (target->this_ip)
+			return;
+
+		for (n = insn; n->offset <= target->offset;
+		     n = next_insn_same_func(file, n)) {
+			if (n->type == INSN_IRET)
+				return;
+		}
+	}
 
 	WARN_FUNC("relocation to !ENDBR: %s+0x%lx",
 		  insn->sec, insn->offset,
--- a/tools/objtool/include/objtool/arch.h
+++ b/tools/objtool/include/objtool/arch.h
@@ -27,6 +27,7 @@ enum insn_type {
 	INSN_STD,
 	INSN_CLD,
 	INSN_ENDBR,
+	INSN_IRET,
 	INSN_OTHER,
 };