[PATCH v4 20/30] objtool: Add noinstr validation for static branches/calls

[Valentin Schneider <vschneid@redhat.com>]

From: Josh Poimboeuf <jpoimboe@kernel.org>

Warn about static branches/calls in noinstr regions, unless the
corresponding key is RO-after-init or has been manually whitelisted with
DEFINE_STATIC_KEY_*_NOINSTR(().

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
[Added NULL check for insn_call_dest() return value]
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
---
 include/linux/jump_label.h              | 17 +++--
 include/linux/objtool.h                 |  7 ++
 include/linux/static_call.h             |  3 +
 tools/objtool/Documentation/objtool.txt | 34 +++++++++
 tools/objtool/check.c                   | 92 ++++++++++++++++++++++---
 tools/objtool/include/objtool/check.h   |  1 +
 tools/objtool/include/objtool/elf.h     |  1 +
 tools/objtool/include/objtool/special.h |  1 +
 tools/objtool/special.c                 | 18 ++++-
 9 files changed, 156 insertions(+), 18 deletions(-)

diff --git a/include/linux/jump_label.h b/include/linux/jump_label.h
index 88bb6e32fdcbc..dc8a82a62c109 100644
--- a/include/linux/jump_label.h
+++ b/include/linux/jump_label.h
@@ -75,6 +75,7 @@
 
 #include <linux/types.h>
 #include <linux/compiler.h>
+#include <linux/objtool.h>
 
 extern bool static_key_initialized;
 
@@ -373,8 +374,9 @@ struct static_key_false {
 #define DEFINE_STATIC_KEY_TRUE(name)	\
 	struct static_key_true name = STATIC_KEY_TRUE_INIT
 
-#define DEFINE_STATIC_KEY_TRUE_RO(name)	\
-	struct static_key_true name __ro_after_init = STATIC_KEY_TRUE_INIT
+#define DEFINE_STATIC_KEY_TRUE_RO(name)						\
+	struct static_key_true name __ro_after_init = STATIC_KEY_TRUE_INIT;	\
+	ANNOTATE_NOINSTR_ALLOWED(name)
 
 #define DECLARE_STATIC_KEY_TRUE(name)	\
 	extern struct static_key_true name
@@ -382,8 +384,9 @@ struct static_key_false {
 #define DEFINE_STATIC_KEY_FALSE(name)	\
 	struct static_key_false name = STATIC_KEY_FALSE_INIT
 
-#define DEFINE_STATIC_KEY_FALSE_RO(name)	\
-	struct static_key_false name __ro_after_init = STATIC_KEY_FALSE_INIT
+#define DEFINE_STATIC_KEY_FALSE_RO(name)					\
+	struct static_key_false name __ro_after_init = STATIC_KEY_FALSE_INIT;	\
+	ANNOTATE_NOINSTR_ALLOWED(name)
 
 /*
  * The _NOINSTR variants are used to tell objtool the static key is allowed to
@@ -397,10 +400,12 @@ struct static_key_false {
  * definition with the rationale.
  */
 #define DEFINE_STATIC_KEY_TRUE_NOINSTR(name)					\
-	DEFINE_STATIC_KEY_TRUE(name)
+	DEFINE_STATIC_KEY_TRUE(name);						\
+	ANNOTATE_NOINSTR_ALLOWED(name)
 
 #define DEFINE_STATIC_KEY_FALSE_NOINSTR(name)					\
-	DEFINE_STATIC_KEY_FALSE(name)
+	DEFINE_STATIC_KEY_FALSE(name);						\
+	ANNOTATE_NOINSTR_ALLOWED(name)
 
 #define DECLARE_STATIC_KEY_FALSE(name)	\
 	extern struct static_key_false name
diff --git a/include/linux/objtool.h b/include/linux/objtool.h
index b3b8d3dab52d5..1a7389f273063 100644
--- a/include/linux/objtool.h
+++ b/include/linux/objtool.h
@@ -34,6 +34,12 @@
 	static void __used __section(".discard.func_stack_frame_non_standard") \
 		*__func_stack_frame_non_standard_##func = func
 
+#define __ANNOTATE_NOINSTR_ALLOWED(key) \
+	static void __used __section(".discard.noinstr_allowed") \
+		*__annotate_noinstr_allowed_##key = &key
+
+#define ANNOTATE_NOINSTR_ALLOWED(key) __ANNOTATE_NOINSTR_ALLOWED(key)
+
 /*
  * STACK_FRAME_NON_STANDARD_FP() is a frame-pointer-specific function ignore
  * for the case where a function is intentionally missing frame pointer setup,
@@ -157,6 +163,7 @@
 #define STACK_FRAME_NON_STANDARD_FP(func)
 #define ANNOTATE_NOENDBR
 #define ASM_REACHABLE
+#define ANNOTATE_NOINSTR_ALLOWED(key)
 #else
 #define ANNOTATE_INTRA_FUNCTION_CALL
 .macro UNWIND_HINT type:req sp_reg=0 sp_offset=0 signal=0
diff --git a/include/linux/static_call.h b/include/linux/static_call.h
index ea6ca57e2a829..0d4b16d348501 100644
--- a/include/linux/static_call.h
+++ b/include/linux/static_call.h
@@ -133,6 +133,7 @@
 
 #include <linux/types.h>
 #include <linux/cpu.h>
+#include <linux/objtool.h>
 #include <linux/static_call_types.h>
 
 #ifdef CONFIG_HAVE_STATIC_CALL
@@ -198,6 +199,7 @@ extern long __static_call_return0(void);
 		.func = _func,						\
 		.type = 1,						\
 	};								\
+	ANNOTATE_NOINSTR_ALLOWED(STATIC_CALL_TRAMP(name));		\
 	ARCH_DEFINE_STATIC_CALL_TRAMP(name, _func)
 
 #define DEFINE_STATIC_CALL_NULL(name, _func)				\
@@ -214,6 +216,7 @@ extern long __static_call_return0(void);
 		.func = NULL,						\
 		.type = 1,						\
 	};								\
+	ANNOTATE_NOINSTR_ALLOWED(STATIC_CALL_TRAMP(name));		\
 	ARCH_DEFINE_STATIC_CALL_NULL_TRAMP(name)
 
 #define DEFINE_STATIC_CALL_RET0(name, _func)				\
diff --git a/tools/objtool/Documentation/objtool.txt b/tools/objtool/Documentation/objtool.txt
index 7c3ee959b63c7..922d3b41541d0 100644
--- a/tools/objtool/Documentation/objtool.txt
+++ b/tools/objtool/Documentation/objtool.txt
@@ -447,6 +447,40 @@ the objtool maintainers.
    names and does not use module_init() / module_exit() macros to create
    them.
 
+13. file.o: warning: func()+0x2a: key: non-RO static key usage in noinstr code
+    file.o: warning: func()+0x2a: key: non-RO static call usage in noinstr code
+
+  This means that noinstr function func() uses a static key or
+  static call named 'key' which can be modified at runtime.  This is
+  discouraged because it prevents code patching IPIs from being
+  deferred.
+
+  You have the following options:
+
+  1) Check whether the static key/call in question is only modified
+     during init.  If so, define it as read-only-after-init with
+     DEFINE_STATIC_KEY_*_RO() or DEFINE_STATIC_CALL_RO().
+
+  2) Avoid the runtime patching.  For static keys this can be done by
+     using static_key_enabled() or by getting rid of the static key
+     altogether if performance is not a concern.
+
+     For static calls, something like the following could be done:
+
+       target = static_call_query(foo);
+       if (target == func1)
+	       func1();
+	else if (target == func2)
+		func2();
+	...
+
+  3) Silence the warning by defining the static key/call with
+     DEFINE_STATIC_*_NOINSTR().  This decision should not
+     be taken lightly as it may result in code patching IPIs getting
+     sent to isolated NOHZ_FULL CPUs running in pure userspace.  A
+     comment should be added above the definition explaining the
+     rationale for the decision.
+
 
 If the error doesn't seem to make sense, it could be a bug in objtool.
 Feel free to ask the objtool maintainer for help.
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 6aa9259fc9940..24219538c1587 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -1068,6 +1068,45 @@ static int create_direct_call_sections(struct objtool_file *file)
 	return 0;
 }
 
+static int read_noinstr_allowed(struct objtool_file *file)
+{
+	struct section *rsec;
+	struct symbol *sym;
+	struct reloc *reloc;
+
+	rsec = find_section_by_name(file->elf, ".rela.discard.noinstr_allowed");
+	if (!rsec)
+		return 0;
+
+	for_each_reloc(rsec, reloc) {
+		switch (reloc->sym->type) {
+		case STT_OBJECT:
+		case STT_FUNC:
+			sym = reloc->sym;
+			break;
+
+		case STT_SECTION:
+			sym = find_symbol_by_offset(reloc->sym->sec,
+						    reloc_addend(reloc));
+			if (!sym) {
+				WARN_FUNC("can't find static key/call symbol",
+					  reloc->sym->sec, reloc_addend(reloc));
+				return -1;
+			}
+			break;
+
+		default:
+			WARN("unexpected relocation symbol type in %s: %d",
+			     rsec->name, reloc->sym->type);
+			return -1;
+		}
+
+		sym->noinstr_allowed = 1;
+	}
+
+	return 0;
+}
+
 /*
  * Warnings shouldn't be reported for ignored functions.
  */
@@ -1955,6 +1994,8 @@ static int handle_jump_alt(struct objtool_file *file,
 		return -1;
 	}
 
+	orig_insn->key = special_alt->key;
+
 	if (opts.hack_jump_label && special_alt->key_addend & 2) {
 		struct reloc *reloc = insn_reloc(file, orig_insn);
 
@@ -2731,6 +2772,10 @@ static int decode_sections(struct objtool_file *file)
 	if (ret)
 		return ret;
 
+	ret = read_noinstr_allowed(file);
+	if (ret)
+		return ret;
+
 	return 0;
 }
 
@@ -3494,9 +3539,9 @@ static bool pv_call_dest(struct objtool_file *file, struct instruction *insn)
 	return file->pv_ops[idx].clean;
 }
 
-static inline bool noinstr_call_dest(struct objtool_file *file,
-				     struct instruction *insn,
-				     struct symbol *func)
+static inline bool noinstr_call_allowed(struct objtool_file *file,
+					struct instruction *insn,
+					struct symbol *func)
 {
 	/*
 	 * We can't deal with indirect function calls at present;
@@ -3516,10 +3561,10 @@ static inline bool noinstr_call_dest(struct objtool_file *file,
 		return true;
 
 	/*
-	 * If the symbol is a static_call trampoline, we can't tell.
+	 * Only DEFINE_STATIC_CALL_*_RO allowed.
 	 */
 	if (func->static_call_tramp)
-		return true;
+		return func->noinstr_allowed;
 
 	/*
 	 * The __ubsan_handle_*() calls are like WARN(), they only happen when
@@ -3532,14 +3577,29 @@ static inline bool noinstr_call_dest(struct objtool_file *file,
 	return false;
 }
 
+static char *static_call_name(struct symbol *func)
+{
+	return func->name + strlen("__SCT__");
+}
+
 static int validate_call(struct objtool_file *file,
 			 struct instruction *insn,
 			 struct insn_state *state)
 {
-	if (state->noinstr && state->instr <= 0 &&
-	    !noinstr_call_dest(file, insn, insn_call_dest(insn))) {
-		WARN_INSN(insn, "call to %s() leaves .noinstr.text section", call_dest_name(file, insn));
-		return 1;
+	if (state->noinstr && state->instr <= 0) {
+		struct symbol *dest = insn_call_dest(insn);
+
+		if (dest && dest->static_call_tramp) {
+			if (!dest->noinstr_allowed) {
+				WARN_INSN(insn, "%s: non-RO static call usage in noinstr",
+					  static_call_name(dest));
+			}
+
+		} else if (dest && !noinstr_call_allowed(file, insn, dest)) {
+			WARN_INSN(insn, "call to %s() leaves .noinstr.text section",
+				  call_dest_name(file, insn));
+			return 1;
+		}
 	}
 
 	if (state->uaccess && !func_uaccess_safe(insn_call_dest(insn))) {
@@ -3604,6 +3664,17 @@ static int validate_return(struct symbol *func, struct instruction *insn, struct
 	return 0;
 }
 
+static int validate_static_key(struct instruction *insn, struct insn_state *state)
+{
+	if (state->noinstr && state->instr <= 0 && !insn->key->noinstr_allowed) {
+		WARN_INSN(insn, "%s: non-RO static key usage in noinstr",
+			  insn->key->name);
+		return 1;
+	}
+
+	return 0;
+}
+
 static struct instruction *next_insn_to_validate(struct objtool_file *file,
 						 struct instruction *insn)
 {
@@ -3765,6 +3836,9 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
 		if (handle_insn_ops(insn, next_insn, &state))
 			return 1;
 
+		if (insn->key)
+			validate_static_key(insn, &state);
+
 		switch (insn->type) {
 
 		case INSN_RETURN:
diff --git a/tools/objtool/include/objtool/check.h b/tools/objtool/include/objtool/check.h
index daa46f1f0965a..c0da7246eac7b 100644
--- a/tools/objtool/include/objtool/check.h
+++ b/tools/objtool/include/objtool/check.h
@@ -77,6 +77,7 @@ struct instruction {
 	struct symbol *sym;
 	struct stack_op *stack_ops;
 	struct cfi_state *cfi;
+	struct symbol *key;
 };
 
 static inline struct symbol *insn_func(struct instruction *insn)
diff --git a/tools/objtool/include/objtool/elf.h b/tools/objtool/include/objtool/elf.h
index d7e815c2fd156..0cb79931262bb 100644
--- a/tools/objtool/include/objtool/elf.h
+++ b/tools/objtool/include/objtool/elf.h
@@ -69,6 +69,7 @@ struct symbol {
 	u8 embedded_insn     : 1;
 	u8 local_label       : 1;
 	u8 frame_pointer     : 1;
+	u8 noinstr_allowed   : 1;
 	struct list_head pv_target;
 	struct reloc *relocs;
 };
diff --git a/tools/objtool/include/objtool/special.h b/tools/objtool/include/objtool/special.h
index 86d4af9c5aa9d..ce4759358ec48 100644
--- a/tools/objtool/include/objtool/special.h
+++ b/tools/objtool/include/objtool/special.h
@@ -20,6 +20,7 @@ struct special_alt {
 	bool skip_alt;
 	bool jump_or_nop;
 	u8 key_addend;
+	struct symbol *key;
 
 	struct section *orig_sec;
 	unsigned long orig_off;
diff --git a/tools/objtool/special.c b/tools/objtool/special.c
index 097a69db82a0e..982d5cb55e1bb 100644
--- a/tools/objtool/special.c
+++ b/tools/objtool/special.c
@@ -119,14 +119,26 @@ static int get_alt_entry(struct elf *elf, const struct special_entry *entry,
 
 	if (entry->key) {
 		struct reloc *key_reloc;
+		struct symbol *key;
+		s64 key_addend;
 
 		key_reloc = find_reloc_by_dest(elf, sec, offset + entry->key);
 		if (!key_reloc) {
-			WARN_FUNC("can't find key reloc",
-				  sec, offset + entry->key);
+			WARN_FUNC("can't find key reloc", sec, offset + entry->key);
 			return -1;
 		}
-		alt->key_addend = reloc_addend(key_reloc);
+
+		key = key_reloc->sym;
+		key_addend = reloc_addend(key_reloc);
+
+		if (key->type == STT_SECTION)
+			key = find_symbol_by_offset(key->sec, key_addend & ~3);
+
+		/* embedded keys not supported */
+		if (key) {
+			alt->key = key;
+			alt->key_addend = key_addend;
+		}
 	}
 
 	return 0;
-- 
2.43.0