objtool: Add entry UNRET validation

Since entry asm is tricky, add a validation pass that ensures the
retbleed mitigation has been done before the first actual RET
instruction.

Entry points are those that either have UNWIND_HINT_ENTRY, which acts
as UNWIND_HINT_EMPTY but marks the instruction as an entry point, or
those that have UWIND_HINT_IRET_REGS at +0.

This is basically a variant of validate_branch() that is
intra-function and it will simply follow all branches from marked
entry points and ensures that all paths lead to ANNOTATE_UNRET_END.

If a path hits RET or an indirection the path is a fail and will be
reported.

There are 3 ANNOTATE_UNRET_END instances:

 - UNTRAIN_RET itself
 - exception from-kernel; this path doesn't need UNTRAIN_RET
 - all early exceptions; these also don't need UNTRAIN_RET
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>

arch/x86/entry/entry_64.S

@@ -85,7 +85,7 @@
  */
 
 SYM_CODE_START(entry_SYSCALL_64)
-	UNWIND_HINT_EMPTY
+	UNWIND_HINT_ENTRY
 	ENDBR
 
 	swapgs
@@ -1095,6 +1095,7 @@ SYM_CODE_START_LOCAL(error_entry)
 .Lerror_entry_done_lfence:
 	FENCE_SWAPGS_KERNEL_ENTRY
 	leaq	8(%rsp), %rax			/* return pt_regs pointer */
+	ANNOTATE_UNRET_END
 	RET
 
 .Lbstep_iret:

arch/x86/entry/entry_64_compat.S

@@ -49,7 +49,7 @@
  * 0(%ebp) arg6
  */
 SYM_CODE_START(entry_SYSENTER_compat)
-	UNWIND_HINT_EMPTY
+	UNWIND_HINT_ENTRY
 	ENDBR
 	/* Interrupts are off on entry. */
 	swapgs
@@ -179,7 +179,7 @@ SYM_CODE_END(entry_SYSENTER_compat)
  * 0(%esp) arg6
  */
 SYM_CODE_START(entry_SYSCALL_compat)
-	UNWIND_HINT_EMPTY
+	UNWIND_HINT_ENTRY
 	ENDBR
 	/* Interrupts are off on entry. */
 	swapgs
@@ -305,7 +305,7 @@ SYM_CODE_END(entry_SYSCALL_compat)
  * ebp  arg6
  */
 SYM_CODE_START(entry_INT80_compat)
-	UNWIND_HINT_EMPTY
+	UNWIND_HINT_ENTRY
 	ENDBR
 	/*
 	 * Interrupts are off on entry.

arch/x86/include/asm/nospec-branch.h

@@ -81,6 +81,17 @@
  */
 #define ANNOTATE_UNRET_SAFE ANNOTATE_RETPOLINE_SAFE
 
+/*
+ * Abuse ANNOTATE_RETPOLINE_SAFE on a NOP to indicate UNRET_END, should
+ * eventually turn into it's own annotation.
+ */
+.macro ANNOTATE_UNRET_END
+#ifdef CONFIG_DEBUG_ENTRY
+	ANNOTATE_RETPOLINE_SAFE
+	nop
+#endif
+.endm
+
 /*
  * JMP_NOSPEC and CALL_NOSPEC macros can be used instead of a simple
  * indirect jmp/call which may be susceptible to the Spectre variant 2
@@ -131,6 +142,7 @@
  */
 .macro UNTRAIN_RET
 #ifdef CONFIG_RETPOLINE
+	ANNOTATE_UNRET_END
 	ALTERNATIVE_2 "",						\
 	              "call zen_untrain_ret", X86_FEATURE_UNRET,	\
 		      "call entry_ibpb", X86_FEATURE_ENTRY_IBPB

arch/x86/include/asm/unwind_hints.h

@@ -11,6 +11,10 @@
 	UNWIND_HINT sp_reg=ORC_REG_UNDEFINED type=UNWIND_HINT_TYPE_CALL end=1
 .endm
 
+.macro UNWIND_HINT_ENTRY
+	UNWIND_HINT sp_reg=ORC_REG_UNDEFINED type=UNWIND_HINT_TYPE_ENTRY end=1
+.endm
+
 .macro UNWIND_HINT_REGS base=%rsp offset=0 indirect=0 extra=1 partial=0
 	.if \base == %rsp
 		.if \indirect

arch/x86/kernel/head_64.S

@@ -389,6 +389,8 @@ SYM_CODE_START_NOALIGN(vc_boot_ghcb)
 	UNWIND_HINT_IRET_REGS offset=8
 	ENDBR
 
+	ANNOTATE_UNRET_END
+
 	/* Build pt_regs */
 	PUSH_AND_CLEAR_REGS
 
@@ -448,6 +450,7 @@ SYM_CODE_END(early_idt_handler_array)
 
 SYM_CODE_START_LOCAL(early_idt_handler_common)
 	UNWIND_HINT_IRET_REGS offset=16
+	ANNOTATE_UNRET_END
 	/*
 	 * The stack is the hardware frame, an error code or zero, and the
 	 * vector number.
@@ -497,6 +500,8 @@ SYM_CODE_START_NOALIGN(vc_no_ghcb)
 	UNWIND_HINT_IRET_REGS offset=8
 	ENDBR
 
+	ANNOTATE_UNRET_END
+
 	/* Build pt_regs */
 	PUSH_AND_CLEAR_REGS
 

arch/x86/xen/xen-asm.S

@@ -121,7 +121,7 @@ SYM_FUNC_END(xen_read_cr2_direct);
 
 .macro xen_pv_trap name
 SYM_CODE_START(xen_\name)
-	UNWIND_HINT_EMPTY
+	UNWIND_HINT_ENTRY
 	ENDBR
 	pop %rcx
 	pop %r11
@@ -235,7 +235,7 @@ SYM_CODE_END(xenpv_restore_regs_and_return_to_usermode)
 
 /* Normal 64-bit system call target */
 SYM_CODE_START(xen_entry_SYSCALL_64)
-	UNWIND_HINT_EMPTY
+	UNWIND_HINT_ENTRY
 	ENDBR
 	popq %rcx
 	popq %r11
@@ -255,7 +255,7 @@ SYM_CODE_END(xen_entry_SYSCALL_64)
 
 /* 32-bit compat syscall target */
 SYM_CODE_START(xen_entry_SYSCALL_compat)
-	UNWIND_HINT_EMPTY
+	UNWIND_HINT_ENTRY
 	ENDBR
 	popq %rcx
 	popq %r11
@@ -273,7 +273,7 @@ SYM_CODE_END(xen_entry_SYSCALL_compat)
 
 /* 32-bit compat sysenter target */
 SYM_CODE_START(xen_entry_SYSENTER_compat)
-	UNWIND_HINT_EMPTY
+	UNWIND_HINT_ENTRY
 	ENDBR
 	/*
 	 * NB: Xen is polite and clears TF from EFLAGS for us.  This means
@@ -297,7 +297,7 @@ SYM_CODE_END(xen_entry_SYSENTER_compat)
 
 SYM_CODE_START(xen_entry_SYSCALL_compat)
 SYM_CODE_START(xen_entry_SYSENTER_compat)
-	UNWIND_HINT_EMPTY
+	UNWIND_HINT_ENTRY
 	ENDBR
 	lea 16(%rsp), %rsp	/* strip %rcx, %r11 */
 	mov $-ENOSYS, %rax

include/linux/objtool.h

@@ -32,11 +32,14 @@ struct unwind_hint {
  *
  * UNWIND_HINT_FUNC: Generate the unwind metadata of a callable function.
  * Useful for code which doesn't have an ELF function annotation.
+ *
+ * UNWIND_HINT_ENTRY: machine entry without stack, SYSCALL/SYSENTER etc.
  */
 #define UNWIND_HINT_TYPE_CALL		0
 #define UNWIND_HINT_TYPE_REGS		1
 #define UNWIND_HINT_TYPE_REGS_PARTIAL	2
 #define UNWIND_HINT_TYPE_FUNC		3
+#define UNWIND_HINT_TYPE_ENTRY		4
 
 #ifdef CONFIG_OBJTOOL
 

scripts/Makefile.vmlinux_o

@@ -44,7 +44,7 @@ objtool-enabled := $(or $(delay-objtool),$(CONFIG_NOINSTR_VALIDATION))
 
 objtool_args := \
 	$(if $(delay-objtool),$(objtool_args)) \
-	$(if $(CONFIG_NOINSTR_VALIDATION), --noinstr) \
+	$(if $(CONFIG_NOINSTR_VALIDATION), --noinstr $(if $(CONFIG_RETPOLINE), --unret)) \
 	$(if $(CONFIG_GCOV_KERNEL), --no-unreachable) \
 	--link
 

tools/include/linux/objtool.h

@@ -32,11 +32,14 @@ struct unwind_hint {
  *
  * UNWIND_HINT_FUNC: Generate the unwind metadata of a callable function.
  * Useful for code which doesn't have an ELF function annotation.
+ *
+ * UNWIND_HINT_ENTRY: machine entry without stack, SYSCALL/SYSENTER etc.
  */
 #define UNWIND_HINT_TYPE_CALL		0
 #define UNWIND_HINT_TYPE_REGS		1
 #define UNWIND_HINT_TYPE_REGS_PARTIAL	2
 #define UNWIND_HINT_TYPE_FUNC		3
+#define UNWIND_HINT_TYPE_ENTRY		4
 
 #ifdef CONFIG_OBJTOOL
 

tools/objtool/builtin-check.c

@@ -68,6 +68,7 @@ const struct option check_options[] = {
 	OPT_BOOLEAN('n', "noinstr", &opts.noinstr, "validate noinstr rules"),
 	OPT_BOOLEAN('o', "orc", &opts.orc, "generate ORC metadata"),
 	OPT_BOOLEAN('r', "retpoline", &opts.retpoline, "validate and annotate retpoline usage"),
+	OPT_BOOLEAN(0,   "unret", &opts.unret, "validate entry unret placement"),
 	OPT_BOOLEAN('l', "sls", &opts.sls, "validate straight-line-speculation mitigations"),
 	OPT_BOOLEAN('s', "stackval", &opts.stackval, "validate frame pointer rules"),
 	OPT_BOOLEAN('t', "static-call", &opts.static_call, "annotate static calls"),
@@ -163,6 +164,11 @@ static bool link_opts_valid(struct objtool_file *file)
 		return false;
 	}
 
+	if (opts.unret) {
+		ERROR("--unret requires --link");
+		return false;
+	}
+
 	return true;
 }
 

tools/objtool/check.c

@@ -2032,16 +2032,24 @@ static int read_unwind_hints(struct objtool_file *file)
 
 		insn->hint = true;
 
-		if (opts.ibt && hint->type == UNWIND_HINT_TYPE_REGS_PARTIAL) {
+		if (hint->type == UNWIND_HINT_TYPE_REGS_PARTIAL) {
 			struct symbol *sym = find_symbol_by_offset(insn->sec, insn->offset);
 
-			if (sym && sym->bind == STB_GLOBAL &&
-			    insn->type != INSN_ENDBR && !insn->noendbr) {
-				WARN_FUNC("UNWIND_HINT_IRET_REGS without ENDBR",
-					  insn->sec, insn->offset);
+			if (sym && sym->bind == STB_GLOBAL) {
+				if (opts.ibt && insn->type != INSN_ENDBR && !insn->noendbr) {
+					WARN_FUNC("UNWIND_HINT_IRET_REGS without ENDBR",
+						  insn->sec, insn->offset);
+				}
+
+				insn->entry = 1;
 			}
 		}
 
+		if (hint->type == UNWIND_HINT_TYPE_ENTRY) {
+			hint->type = UNWIND_HINT_TYPE_CALL;
+			insn->entry = 1;
+		}
+
 		if (hint->type == UNWIND_HINT_TYPE_FUNC) {
 			insn->cfi = &func_cfi;
 			continue;
@@ -2116,8 +2124,9 @@ static int read_retpoline_hints(struct objtool_file *file)
 
 		if (insn->type != INSN_JUMP_DYNAMIC &&
 		    insn->type != INSN_CALL_DYNAMIC &&
-		    insn->type != INSN_RETURN) {
-			WARN_FUNC("retpoline_safe hint not an indirect jump/call/ret",
+		    insn->type != INSN_RETURN &&
+		    insn->type != INSN_NOP) {
+			WARN_FUNC("retpoline_safe hint not an indirect jump/call/ret/nop",
 				  insn->sec, insn->offset);
 			return -1;
 		}
@@ -3305,8 +3314,8 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
 			return 1;
 		}
 
-		visited = 1 << state.uaccess;
-		if (insn->visited) {
+		visited = VISITED_BRANCH << state.uaccess;
+		if (insn->visited & VISITED_BRANCH_MASK) {
 			if (!insn->hint && !insn_cfi_match(insn, &state.cfi))
 				return 1;
 
@@ -3520,6 +3529,145 @@ static int validate_unwind_hints(struct objtool_file *file, struct section *sec)
 	return warnings;
 }
 
+/*
+ * Validate rethunk entry constraint: must untrain RET before the first RET.
+ *
+ * Follow every branch (intra-function) and ensure ANNOTATE_UNRET_END comes
+ * before an actual RET instruction.
+ */
+static int validate_entry(struct objtool_file *file, struct instruction *insn)
+{
+	struct instruction *next, *dest;
+	int ret, warnings = 0;
+
+	for (;;) {
+		next = next_insn_to_validate(file, insn);
+
+		if (insn->visited & VISITED_ENTRY)
+			return 0;
+
+		insn->visited |= VISITED_ENTRY;
+
+		if (!insn->ignore_alts && !list_empty(&insn->alts)) {
+			struct alternative *alt;
+			bool skip_orig = false;
+
+			list_for_each_entry(alt, &insn->alts, list) {
+				if (alt->skip_orig)
+					skip_orig = true;
+
+				ret = validate_entry(file, alt->insn);
+				if (ret) {
+				        if (opts.backtrace)
+						BT_FUNC("(alt)", insn);
+					return ret;
+				}
+			}
+
+			if (skip_orig)
+				return 0;
+		}
+
+		switch (insn->type) {
+
+		case INSN_CALL_DYNAMIC:
+		case INSN_JUMP_DYNAMIC:
+		case INSN_JUMP_DYNAMIC_CONDITIONAL:
+			WARN_FUNC("early indirect call", insn->sec, insn->offset);
+			return 1;
+
+		case INSN_JUMP_UNCONDITIONAL:
+		case INSN_JUMP_CONDITIONAL:
+			if (!is_sibling_call(insn)) {
+				if (!insn->jump_dest) {
+					WARN_FUNC("unresolved jump target after linking?!?",
+						  insn->sec, insn->offset);
+					return -1;
+				}
+				ret = validate_entry(file, insn->jump_dest);
+				if (ret) {
+					if (opts.backtrace) {
+						BT_FUNC("(branch%s)", insn,
+							insn->type == INSN_JUMP_CONDITIONAL ? "-cond" : "");
+					}
+					return ret;
+				}
+
+				if (insn->type == INSN_JUMP_UNCONDITIONAL)
+					return 0;
+
+				break;
+			}
+
+			/* fallthrough */
+		case INSN_CALL:
+			dest = find_insn(file, insn->call_dest->sec,
+					 insn->call_dest->offset);
+			if (!dest) {
+				WARN("Unresolved function after linking!?: %s",
+				     insn->call_dest->name);
+				return -1;
+			}
+
+			ret = validate_entry(file, dest);
+			if (ret) {
+				if (opts.backtrace)
+					BT_FUNC("(call)", insn);
+				return ret;
+			}
+			/*
+			 * If a call returns without error, it must have seen UNTRAIN_RET.
+			 * Therefore any non-error return is a success.
+			 */
+			return 0;
+
+		case INSN_RETURN:
+			WARN_FUNC("RET before UNTRAIN", insn->sec, insn->offset);
+			return 1;
+
+		case INSN_NOP:
+			if (insn->retpoline_safe)
+				return 0;
+			break;
+
+		default:
+			break;
+		}
+
+		if (!next) {
+			WARN_FUNC("teh end!", insn->sec, insn->offset);
+			return -1;
+		}
+		insn = next;
+	}
+
+	return warnings;
+}
+
+/*
+ * Validate that all branches starting at 'insn->entry' encounter UNRET_END
+ * before RET.
+ */
+static int validate_unret(struct objtool_file *file)
+{
+	struct instruction *insn;
+	int ret, warnings = 0;
+
+	for_each_insn(file, insn) {
+		if (!insn->entry)
+			continue;
+
+		ret = validate_entry(file, insn);
+		if (ret < 0) {
+			WARN_FUNC("Failed UNRET validation", insn->sec, insn->offset);
+			return ret;
+		}
+		warnings += ret;
+	}
+
+	return warnings;
+}
+
 static int validate_retpoline(struct objtool_file *file)
 {
 	struct instruction *insn;
@@ -4039,6 +4187,17 @@ int check(struct objtool_file *file)
 		warnings += ret;
 	}
 
+	if (opts.unret) {
+		/*
+		 * Must be after validate_branch() and friends, it plays
+		 * further games with insn->visited.
+		 */
+		ret = validate_unret(file);
+		if (ret < 0)
+			return ret;
+		warnings += ret;
+	}
+
 	if (opts.ibt) {
 		ret = validate_ibt(file);
 		if (ret < 0)

tools/objtool/include/objtool/builtin.h

@@ -19,6 +19,7 @@ struct opts {
 	bool noinstr;
 	bool orc;
 	bool retpoline;
+	bool unret;
 	bool sls;
 	bool stackval;
 	bool static_call;

tools/objtool/include/objtool/check.h

@@ -51,8 +51,10 @@ struct instruction {
 	   ignore_alts	: 1,
 	   hint		: 1,
 	   retpoline_safe : 1,
-	   noendbr	: 1;
-		/* 2 bit hole */
+	   noendbr	: 1,
+	   entry	: 1;
+		/* 1 bit hole */
+
 	s8 instr;
 	u8 visited;
 	/* u8 hole */
@@ -69,6 +71,11 @@ struct instruction {
 	struct cfi_state *cfi;
 };
 
+#define VISITED_BRANCH		0x01
+#define VISITED_BRANCH_UACCESS	0x02
+#define VISITED_BRANCH_MASK	0x03
+#define VISITED_ENTRY		0x04
+
 static inline bool is_static_jump(struct instruction *insn)
 {
 	return insn->type == INSN_JUMP_CONDITIONAL ||