[RFC][PATCH] objtool: STAC/CLAC validation

From: Peter Zijlstra
Date: Fri Feb 22 2019 - 17:26:48 EST

On Fri, Feb 22, 2019 at 07:10:34PM +0100, Thomas Gleixner wrote:

> But correct :)

> I agree, that a function which is doing the actual copy should be callable,
> but random other functions? NO!

So find the below patch -- which spotted fail in ptrace.c

It has an AC_SAFE(func) annotation which allows marking specific
functions as safe to call. The patch includes 2 instances which were
required to make arch/x86 'build':

arch/x86/ia32/ia32_signal.o: warning: objtool: ia32_restore_sigcontext()+0x3d: call to native_load_gs_index() with AC set
arch/x86/kernel/ptrace.o: warning: objtool: genregs_get()+0x8e: call to getreg() with AC set

It also screams (provided one has CONFIG_FUNCTION_TRACE=y) about the
lack of notrace annotations on functions marked AC_SAFE():

arch/x86/kernel/ptrace.o: warning: objtool: getreg()+0x0: call to __fentry__() with AC set

It builds arch/x86 relatively clean; it only complains about some
redundant CLACs in entry_64.S because it doesn't understand interrupts
and I've not bothered creating an annotation for them yet.

arch/x86/entry/entry_64.o: warning: objtool: .altinstr_replacement+0x4d: redundant CLAC
arch/x86/entry/entry_64.o: warning: objtool: .altinstr_replacement+0x5a: redundant CLAC
...
arch/x86/entry/entry_64.o: warning: objtool: .altinstr_replacement+0xb1: redundant CLAC

Also, I realized we don't need special annotations for preempt_count;
preempt_disable() emits a CALL instruction which should readily trigger
the warnings added here.

The VDSO thing is a bit of a hack, but I couldn't quickly find anything
better.

Comments?

---
arch/x86/include/asm/special_insns.h | 2 ++
arch/x86/kernel/ptrace.c | 3 +-
include/linux/frame.h | 23 ++++++++++++++
tools/objtool/arch.h | 4 ++-
tools/objtool/arch/x86/decode.c | 14 ++++++++-
tools/objtool/check.c | 59 ++++++++++++++++++++++++++++++++++++
tools/objtool/check.h | 3 +-
tools/objtool/elf.h | 1 +
8 files changed, 105 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
index 43c029cdc3fe..cd31e4433f4c 100644
--- a/arch/x86/include/asm/special_insns.h
+++ b/arch/x86/include/asm/special_insns.h
@@ -5,6 +5,7 @@

#ifdef __KERNEL__

+#include <linux/frame.h>
#include <asm/nops.h>

/*
@@ -135,6 +136,7 @@ static inline void native_wbinvd(void)
}

extern asmlinkage void native_load_gs_index(unsigned);
+AC_SAFE(native_load_gs_index);

static inline unsigned long __read_cr4(void)
{
diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
index 4b8ee05dd6ad..e278b4055a8b 100644
--- a/arch/x86/kernel/ptrace.c
+++ b/arch/x86/kernel/ptrace.c
@@ -420,7 +420,7 @@ static int putreg(struct task_struct *child,
return 0;
}

-static unsigned long getreg(struct task_struct *task, unsigned long offset)
+static notrace unsigned long getreg(struct task_struct *task, unsigned long offset)
{
switch (offset) {
case offsetof(struct user_regs_struct, cs):
@@ -444,6 +444,7 @@ static unsigned long getreg(struct task_struct *task, unsigned long offset)

return *pt_regs_access(task_pt_regs(task), offset);
}
+AC_SAFE(getreg);

static int genregs_get(struct task_struct *target,
const struct user_regset *regset,
diff --git a/include/linux/frame.h b/include/linux/frame.h
index 02d3ca2d9598..5d354cf42a56 100644
--- a/include/linux/frame.h
+++ b/include/linux/frame.h
@@ -21,4 +21,27 @@

#endif /* CONFIG_STACK_VALIDATION */

+#if defined(CONFIG_STACK_VALIDATION) && !defined(BUILD_VDSO) && !defined(BUILD_VDSO32)
+/*
+ * This macro marks functions as AC-safe, that is, it is safe to call from an
+ * EFLAGS.AC enabled region (typically user_access_begin() /
+ * user_access_end()).
+ *
+ * These functions in turn will only call AC-safe functions themselves (which
+ * precludes tracing, including __fentry__ and scheduling, including
+ * preempt_enable).
+ *
+ * AC-safe functions will obviously also not change EFLAGS.AC themselves.
+ *
+ * Since STAC/CLAC are OPL-0 only, this is all irrelevant for VDSO builds
+ * (and the generated symbol reference will in fact cause link failures).
+ */
+#define AC_SAFE(func) \
+ static void __used __section(.discard.ac_safe) \
+ *__func_ac_safe_##func = func
+
+#else
+#define AC_SAFE(func)
+#endif
+
#endif /* _LINUX_FRAME_H */
diff --git a/tools/objtool/arch.h b/tools/objtool/arch.h
index b0d7dc3d71b5..48327099466d 100644
--- a/tools/objtool/arch.h
+++ b/tools/objtool/arch.h
@@ -33,7 +33,9 @@
#define INSN_STACK 8
#define INSN_BUG 9
#define INSN_NOP 10
-#define INSN_OTHER 11
+#define INSN_STAC 11
+#define INSN_CLAC 12
+#define INSN_OTHER 13
#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 540a209b78ab..d1e99d1460a5 100644
--- a/tools/objtool/arch/x86/decode.c
+++ b/tools/objtool/arch/x86/decode.c
@@ -369,7 +369,19 @@ int arch_decode_instruction(struct elf *elf, struct section *sec,

case 0x0f:

- if (op2 >= 0x80 && op2 <= 0x8f) {
+ if (op2 == 0x01) {
+
+ if (modrm == 0xca) {
+
+ *type = INSN_CLAC;
+
+ } else if (modrm == 0xcb) {
+
+ *type = INSN_STAC;
+
+ }
+
+ } else if (op2 >= 0x80 && op2 <= 0x8f) {

*type = INSN_JUMP_CONDITIONAL;

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 0414a0d52262..01852602ca31 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -127,6 +127,24 @@ static bool ignore_func(struct objtool_file *file, struct symbol *func)
return false;
}

+static bool ac_safe_func(struct objtool_file *file, struct symbol *func)
+{
+ struct rela *rela;
+
+ /* check for AC_SAFE */
+ if (file->ac_safe && file->ac_safe->rela)
+ list_for_each_entry(rela, &file->ac_safe->rela->rela_list, list) {
+ if (rela->sym->type == STT_SECTION &&
+ rela->sym->sec == func->sec &&
+ rela->addend == func->offset)
+ return true;
+ if (/* rela->sym->type == STT_FUNC && */ rela->sym == func)
+ return true;
+ }
+
+ return false;
+}
+
/*
* This checks to see if the given function is a "noreturn" function.
*
@@ -439,6 +457,8 @@ static void add_ignores(struct objtool_file *file)

for_each_sec(file, sec) {
list_for_each_entry(func, &sec->symbol_list, list) {
+ func->ac_safe = ac_safe_func(file, func);
+
if (func->type != STT_FUNC)
continue;

@@ -1902,6 +1922,11 @@ static int validate_branch(struct objtool_file *file, struct instruction *first,
switch (insn->type) {

case INSN_RETURN:
+ if (state.ac) {
+ WARN_FUNC("return with AC set", sec, insn->offset);
+ return 1;
+ }
+
if (func && has_modified_stack_frame(&state)) {
WARN_FUNC("return with modified stack frame",
sec, insn->offset);
@@ -1917,6 +1942,12 @@ static int validate_branch(struct objtool_file *file, struct instruction *first,
return 0;

case INSN_CALL:
+ if ((state.ac_safe || state.ac) && !insn->call_dest->ac_safe) {
+ WARN_FUNC("call to %s() with AC set", sec, insn->offset,
+ insn->call_dest->name);
+ return 1;
+ }
+
if (is_fentry_call(insn))
break;

@@ -1928,6 +1959,11 @@ static int validate_branch(struct objtool_file *file, struct instruction *first,

/* fallthrough */
case INSN_CALL_DYNAMIC:
+ if ((state.ac_safe || state.ac) && !insn->call_dest->ac_safe) {
+ WARN_FUNC("call to %s() with AC set", sec, insn->offset,
+ insn->call_dest->name);
+ return 1;
+ }
if (!no_fp && func && !has_valid_stack_frame(&state)) {
WARN_FUNC("call without frame pointer save/setup",
sec, insn->offset);
@@ -1980,6 +2016,26 @@ static int validate_branch(struct objtool_file *file, struct instruction *first,

break;

+ case INSN_STAC:
+ if (state.ac_safe || state.ac) {
+ WARN_FUNC("recursive STAC", sec, insn->offset);
+ return 1;
+ }
+ state.ac = true;
+ break;
+
+ case INSN_CLAC:
+ if (!state.ac) {
+ WARN_FUNC("redundant CLAC", sec, insn->offset);
+ return 1;
+ }
+ if (state.ac_safe) {
+ WARN_FUNC("AC-safe clears AC", sec, insn->offset);
+ return 1;
+ }
+ state.ac = false;
+ break;
+
default:
break;
}
@@ -2141,6 +2197,8 @@ static int validate_functions(struct objtool_file *file)
if (!insn || insn->ignore)
continue;

+ state.ac_safe = func->ac_safe;
+
ret = validate_branch(file, insn, state);
warnings += ret;
}
@@ -2198,6 +2256,7 @@ int check(const char *_objname, bool orc)
INIT_LIST_HEAD(&file.insn_list);
hash_init(file.insn_hash);
file.whitelist = find_section_by_name(file.elf, ".discard.func_stack_frame_non_standard");
+ file.ac_safe = find_section_by_name(file.elf, ".discard.ac_safe");
file.c_file = find_section_by_name(file.elf, ".comment");
file.ignore_unreachables = no_unreachable;
file.hints = false;
diff --git a/tools/objtool/check.h b/tools/objtool/check.h
index e6e8a655b556..c31ec3ca78f3 100644
--- a/tools/objtool/check.h
+++ b/tools/objtool/check.h
@@ -31,7 +31,7 @@ struct insn_state {
int stack_size;
unsigned char type;
bool bp_scratch;
- bool drap, end;
+ bool drap, end, ac, ac_safe;
int drap_reg, drap_offset;
struct cfi_reg vals[CFI_NUM_REGS];
};
@@ -61,6 +61,7 @@ struct objtool_file {
struct list_head insn_list;
DECLARE_HASHTABLE(insn_hash, 16);
struct section *whitelist;
+ struct section *ac_safe;
bool ignore_unreachables, c_file, hints, rodata;
};

diff --git a/tools/objtool/elf.h b/tools/objtool/elf.h
index bc97ed86b9cd..064c3df31e40 100644
--- a/tools/objtool/elf.h
+++ b/tools/objtool/elf.h
@@ -62,6 +62,7 @@ struct symbol {
unsigned long offset;
unsigned int len;
struct symbol *pfunc, *cfunc;
+ bool ac_safe;
};

struct rela {