linux-kernel - [PATCH v4 4/4] objtool, xen: Fix INSN_SYSCALL / INSN

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Message-ID: <19453dfe9a0431b7f016e9dc16d031cad3812a50.1744095216.git.jpoimboe@kernel.org>
Date: Tue,  8 Apr 2025 00:02:16 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: x86@...nel.org
Cc: linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Juergen Gross <jgross@...e.com>
Subject: [PATCH v4 4/4] objtool, xen: Fix INSN_SYSCALL / INSN_SYSRET semantics

Objtool uses an arbitrary rule for INSN_SYSCALL and INSN_SYSRET that
almost works by accident: if it's in a function, control flow continues
after the instruction, otherwise it terminates.

That behavior should instead be based on the semantics of the underlying
instruction.  Change INSN_SYSCALL to always preserve control flow and
INSN_SYSRET to always terminate it.

The changed semantic for INSN_SYSCALL requires a tweak to the
!CONFIG_IA32_EMULATION version of xen_entry_SYSCALL_compat().  In Xen,
SYSCALL is a hypercall which usually returns.  But in this case it's a
hypercall to IRET which doesn't return.  Add UD2 to tell objtool to
terminate control flow, and to prevent undefined behavior at runtime.

Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
---
 arch/x86/xen/xen-asm.S |  4 +---
 tools/objtool/check.c  | 23 ++++++++++++++---------
 2 files changed, 15 insertions(+), 12 deletions(-)

diff --git a/arch/x86/xen/xen-asm.S b/arch/x86/xen/xen-asm.S
index 109af12f7647..461bb1526502 100644
--- a/arch/x86/xen/xen-asm.S
+++ b/arch/x86/xen/xen-asm.S
@@ -226,9 +226,7 @@ SYM_CODE_END(xen_early_idt_handler_array)
 	push %rax
 	mov  $__HYPERVISOR_iret, %eax
 	syscall		/* Do the IRET. */
-#ifdef CONFIG_MITIGATION_SLS
-	int3
-#endif
+	ud2		/* The SYSCALL should never return. */
 .endm
 
 SYM_CODE_START(xen_iret)
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 2dd89b0f4d5e..69f94bc47499 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -3685,14 +3685,19 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
 			break;
 
 		case INSN_SYSCALL:
-		case INSN_SYSRET:
-			if (func) {
-				if (!next_insn || !next_insn->hint) {
-					WARN_INSN(insn, "unsupported instruction in callable function");
-					return 1;
-				}
-				break;
+			if (func && (!next_insn || !next_insn->hint)) {
+				WARN_INSN(insn, "unsupported instruction in callable function");
+				return 1;
 			}
+
+			break;
+
+		case INSN_SYSRET:
+			if (func && (!next_insn || !next_insn->hint)) {
+				WARN_INSN(insn, "unsupported instruction in callable function");
+				return 1;
+			}
+
 			return 0;
 
 		case INSN_STAC:
@@ -3888,9 +3893,9 @@ static int validate_unret(struct objtool_file *file, struct instruction *insn)
 			return 1;
 
 		case INSN_SYSCALL:
+			break;
+
 		case INSN_SYSRET:
-			if (insn_func(insn))
-				break;
 			return 0;
 
 		case INSN_NOP:
-- 
2.49.0