linux-kernel - [PATCH] objtool: Detect assembly code falling through to INT3 padding

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Date:   Thu, 17 May 2018 08:49:34 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Alexey Dobriyan <adobriyan@...il.com>,
        Peter Anvin <h.peter.anvin@...el.com>,
        kernel test robot <xiaolong.ye@...el.com>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Peter Anvin <hpa@...or.com>, tipbuild@...or.com,
        LKP <lkp@...org>
Subject: [PATCH] objtool: Detect assembly code falling through to INT3 padding

With the following commit:

  51bad67ffbce ("x86/asm: Pad assembly functions with INT3 instructions")

... asm function alignments are padded with INT3, so it's no longer safe
to fall through to an aligned function.  Make sure we catch any such
cases with objtool.

Note this only adds checking for 64-bit, since objtool doesn't support
x86-32.

Suggested-by: Thomas Gleixner <tglx@...utronix.de>
Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
---
 arch/x86/kernel/head_64.S       |  2 --
 tools/objtool/arch.h            |  3 ++-
 tools/objtool/arch/x86/decode.c |  2 +-
 tools/objtool/check.c           | 11 ++++++++++-
 4 files changed, 13 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 8344dd2f310a..3ed8cec6e765 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -285,11 +285,9 @@ ENTRY(early_idt_handler_array)
 	.endif
 	pushq $i		# 72(%rsp) Vector number
 	jmp early_idt_handler_common
-	UNWIND_HINT_IRET_REGS
 	i = i + 1
 	.fill early_idt_handler_array + i*EARLY_IDT_HANDLER_SIZE - ., 1, 0xcc
 	.endr
-	UNWIND_HINT_IRET_REGS offset=16
 END(early_idt_handler_array)
 
 early_idt_handler_common:
diff --git a/tools/objtool/arch.h b/tools/objtool/arch.h
index b0d7dc3d71b5..6eb058a8ac00 100644
--- a/tools/objtool/arch.h
+++ b/tools/objtool/arch.h
@@ -33,7 +33,8 @@
 #define INSN_STACK		8
 #define INSN_BUG		9
 #define INSN_NOP		10
-#define INSN_OTHER		11
+#define INSN_PADDING		11
+#define INSN_OTHER		12
 #define INSN_LAST		INSN_OTHER
 
 enum op_dest_type {
diff --git a/tools/objtool/arch/x86/decode.c b/tools/objtool/arch/x86/decode.c
index 7e86a743f851..82b41bb93c02 100644
--- a/tools/objtool/arch/x86/decode.c
+++ b/tools/objtool/arch/x86/decode.c
@@ -421,7 +421,7 @@ int arch_decode_instruction(struct elf *elf, struct section *sec,
 
 	case 0xcc:
 		/* int3: used for asm function padding by the __ALIGN macro */
-		*type = INSN_NOP;
+		*type = INSN_PADDING;
 		break;
 
 	case 0xe3:
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index f4bbce838433..4d387448519f 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -1936,6 +1936,14 @@ static int validate_branch(struct objtool_file *file, struct instruction *first,
 			}
 			return 0;
 
+		case INSN_PADDING:
+			if (!file->c_file) {
+				WARN_FUNC("code falls through to INT3 padding",
+					  insn->sec, insn->offset);
+				return 1;
+			}
+			break;
+
 		case INSN_STACK:
 			if (update_insn_state(insn, &state))
 				return 1;
@@ -2032,7 +2040,8 @@ static bool ignore_unreachable_insn(struct instruction *insn)
 {
 	int i;
 
-	if (insn->ignore || insn->type == INSN_NOP)
+	if (insn->ignore || insn->type == INSN_NOP ||
+	    insn->type == INSN_PADDING)
 		return true;
 
 	/*
-- 
2.17.0