[PATCH 2/5] gcc-plugins/stackleak: Use asm instrumentation to avoid useless register saving

Alexander Popov
2020-06-04T13:50:59Z

The kernel code instrumentation in stackleak gcc plugin works in two stages.
At first, stack tracking is added to GIMPLE representation of every function
(except some special cases). And later, when stack frame size info is
available, stack tracking is removed from the RTL representation of the
functions with small stack frame. There is an unwanted side-effect for these
functions: some of them do useless work with caller-saved registers.

As an example of such case, proc_sys_write without instrumentation:
55 push %rbp
41 b8 01 00 00 00 mov $0x1,%r8d
48 89 e5 mov %rsp,%rbp
e8 11 ff ff ff callq ffffffff81284610 <proc_sys_call_handler>
5d pop %rbp
c3 retq
0f 1f 44 00 00 nopl 0x0(%rax,%rax,1)
66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1)
00 00 00

proc_sys_write with instrumentation:
55 push %rbp
48 89 e5 mov %rsp,%rbp
41 56 push %r14
41 55 push %r13
41 54 push %r12
53 push %rbx
49 89 f4 mov %rsi,%r12
48 89 fb mov %rdi,%rbx
49 89 d5 mov %rdx,%r13
49 89 ce mov %rcx,%r14
4c 89 f1 mov %r14,%rcx
4c 89 ea mov %r13,%rdx
4c 89 e6 mov %r12,%rsi
48 89 df mov %rbx,%rdi
41 b8 01 00 00 00 mov $0x1,%r8d
e8 f2 fe ff ff callq ffffffff81298e80 <proc_sys_call_handler>
5b pop %rbx
41 5c pop %r12
41 5d pop %r13
41 5e pop %r14
5d pop %rbp
c3 retq
66 0f 1f 84 00 00 00 nopw 0x0(%rax,%rax,1)
00 00

Let's improve the instrumentation to avoid this:

1. Make stackleak_track_stack() save all register that it works with.
Use no_caller_saved_registers attribute for that function. This attribute
is available for x86_64 and i386 starting from gcc-7.

2. Insert calling stackleak_track_stack() in asm:
asm volatile("call stackleak_track_stack" :: "r" (current_stack_pointer))
Here we use ASM_CALL_CONSTRAINT trick from arch/x86/include/asm/asm.h.
The input constraint is taken into account during gcc shrink-wrapping
optimization. It is needed to be sure that stackleak_track_stack() call is
inserted after the prologue of the containing function, when the stack
frame is prepared.

This work is a deep reengineering of the idea described on grsecurity blog
https://grsecurity.net/resolving_an_unfortunate_stackleak_interaction

Signed-off-by: Alexander Popov <alex.popov@linux.com>
---
include/linux/compiler_attributes.h | 13 ++
kernel/stackleak.c | 16 +-
scripts/Makefile.gcc-plugins | 2 +
scripts/gcc-plugins/stackleak_plugin.c | 206 +++++++++++++++++++++----
4 files changed, 196 insertions(+), 41 deletions(-)

diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
index cdf016596659..522d57ae8532 100644
--- a/include/linux/compiler_attributes.h
+++ b/include/linux/compiler_attributes.h
@@ -41,6 +41,7 @@
# define __GCC4_has_attribute___nonstring__ 0
# define __GCC4_has_attribute___no_sanitize_address__ (__GNUC_MINOR__ >= 8)
# define __GCC4_has_attribute___fallthrough__ 0
+# define __GCC4_has_attribute___no_caller_saved_registers__ 0
#endif

/*
@@ -175,6 +176,18 @@
*/
#define __mode(x) __attribute__((__mode__(x)))

+/*
+ * Optional: only supported since gcc >= 7
+ *
+ * gcc: https://gcc.gnu.org/onlinedocs/gcc/x86-Function-Attributes.html#index-no_005fcaller_005fsaved_005fregisters-function-attribute_002c-x86
+ * clang: https://clang.llvm.org/docs/AttributeReference.html#no-caller-saved-registers
+ */
+#if __has_attribute(__no_caller_saved_registers__)
+# define __no_caller_saved_registers __attribute__((__no_caller_saved_registers__))
+#else
+# define __no_caller_saved_registers
+#endif
+
/*
* Optional: not supported by clang
*
diff --git a/kernel/stackleak.c b/kernel/stackleak.c
index b193a59fc05b..a8fc9ae1d03d 100644
--- a/kernel/stackleak.c
+++ b/kernel/stackleak.c
@@ -104,19 +104,9 @@ asmlinkage void notrace stackleak_erase(void)
}
NOKPROBE_SYMBOL(stackleak_erase);

-void __used notrace stackleak_track_stack(void)
+void __used __no_caller_saved_registers notrace stackleak_track_stack(void)
{
- /*
- * N.B. stackleak_erase() fills the kernel stack with the poison value,
- * which has the register width. That code assumes that the value
- * of 'lowest_stack' is aligned on the register width boundary.
- *
- * That is true for x86 and x86_64 because of the kernel stack
- * alignment on these platforms (for details, see 'cc_stack_align' in
- * arch/x86/Makefile). Take care of that when you port STACKLEAK to
- * new platforms.
- */
- unsigned long sp = (unsigned long)&sp;
+ unsigned long sp = current_stack_pointer;

/*
* Having CONFIG_STACKLEAK_TRACK_MIN_SIZE larger than
@@ -125,6 +115,8 @@ void __used notrace stackleak_track_stack(void)
*/
BUILD_BUG_ON(CONFIG_STACKLEAK_TRACK_MIN_SIZE > STACKLEAK_SEARCH_DEPTH);

+ /* 'lowest_stack' should be aligned on the register width boundary */
+ sp = ALIGN(sp, sizeof(unsigned long));
if (sp < current->lowest_stack &&
sp >= (unsigned long)task_stack_page(current) +
sizeof(unsigned long)) {
diff --git a/scripts/Makefile.gcc-plugins b/scripts/Makefile.gcc-plugins
index 5f7df50cfe7a..952e46876329 100644
--- a/scripts/Makefile.gcc-plugins
+++ b/scripts/Makefile.gcc-plugins
@@ -33,6 +33,8 @@ gcc-plugin-cflags-$(CONFIG_GCC_PLUGIN_STACKLEAK) \
+= -DSTACKLEAK_PLUGIN
gcc-plugin-cflags-$(CONFIG_GCC_PLUGIN_STACKLEAK) \
+= -fplugin-arg-stackleak_plugin-track-min-size=$(CONFIG_STACKLEAK_TRACK_MIN_SIZE)
+gcc-plugin-cflags-$(CONFIG_GCC_PLUGIN_STACKLEAK) \
+ += -fplugin-arg-stackleak_plugin-arch=$(SRCARCH)
ifdef CONFIG_GCC_PLUGIN_STACKLEAK
DISABLE_STACKLEAK_PLUGIN += -fplugin-arg-stackleak_plugin-disable
endif
diff --git a/scripts/gcc-plugins/stackleak_plugin.c b/scripts/gcc-plugins/stackleak_plugin.c
index 1ecfe50d0bf5..0769c5b9156d 100644
--- a/scripts/gcc-plugins/stackleak_plugin.c
+++ b/scripts/gcc-plugins/stackleak_plugin.c
@@ -19,7 +19,7 @@
*
* Debugging:
* - use fprintf() to stderr, debug_generic_expr(), debug_gimple_stmt(),
- * print_rtl() and print_simple_rtl();
+ * print_rtl_single() and debug_rtx();
* - add "-fdump-tree-all -fdump-rtl-all" to the plugin CFLAGS in
* Makefile.gcc-plugins to see the verbose dumps of the gcc passes;
* - use gcc -E to understand the preprocessing shenanigans;
@@ -31,6 +31,7 @@
__visible int plugin_is_GPL_compatible;

static int track_frame_size = -1;
+static bool build_for_x86 = false;
static const char track_function[] = "stackleak_track_stack";

/*
@@ -42,27 +43,28 @@ static GTY(()) tree track_function_decl;
static struct plugin_info stackleak_plugin_info = {
.version = "201707101337",
.help = "track-min-size=nn\ttrack stack for functions with a stack frame size >= nn bytes\n"
+ "arch=target_arch\tspecify target build arch\n"
"disable\t\tdo not activate the plugin\n"
};

-static void stackleak_add_track_stack(gimple_stmt_iterator *gsi)
+static void add_stack_tracking_gcall(gimple_stmt_iterator *gsi)
{
gimple stmt;
- gcall *stackleak_track_stack;
+ gcall *gimple_call;
cgraph_node_ptr node;
basic_block bb;

- /* Insert call to void stackleak_track_stack(void) */
+ /* Insert calling stackleak_track_stack() */
stmt = gimple_build_call(track_function_decl, 0);
- stackleak_track_stack = as_a_gcall(stmt);
- gsi_insert_before(gsi, stackleak_track_stack, GSI_SAME_STMT);
+ gimple_call = as_a_gcall(stmt);
+ gsi_insert_before(gsi, gimple_call, GSI_SAME_STMT);

/* Update the cgraph */
- bb = gimple_bb(stackleak_track_stack);
+ bb = gimple_bb(gimple_call);
node = cgraph_get_create_node(track_function_decl);
gcc_assert(node);
cgraph_create_edge(cgraph_get_node(current_function_decl), node,
- stackleak_track_stack, bb->count,
+ gimple_call, bb->count,
compute_call_stmt_bb_frequency(current_function_decl, bb));
}

@@ -79,6 +81,76 @@ static bool is_alloca(gimple stmt)
return false;
}

+static tree get_current_stack_pointer_decl(void)
+{
+ varpool_node_ptr node;
+
+ FOR_EACH_VARIABLE(node) {
+ tree var = NODE_DECL(node);
+ tree name = DECL_NAME(var);
+
+ if (DECL_NAME_LENGTH(var) != sizeof("current_stack_pointer") - 1)
+ continue;
+
+ if (strcmp(IDENTIFIER_POINTER(name), "current_stack_pointer"))
+ continue;
+
+ return var;
+ }
+
+ return NULL_TREE;
+}
+
+static void add_stack_tracking_gasm(gimple_stmt_iterator *gsi)
+{
+ gasm *asm_call = NULL;
+ tree sp_decl, input;
+ vec<tree, va_gc> *inputs = NULL;
+
+ /* 'no_caller_saved_registers' is currently supported only for x86 */
+ gcc_assert(build_for_x86);
+
+ /*
+ * Insert calling stackleak_track_stack() in asm:
+ * asm volatile("call stackleak_track_stack"
+ * :: "r" (current_stack_pointer))
+ * Use ASM_CALL_CONSTRAINT trick from arch/x86/include/asm/asm.h.
+ * This constraint is taken into account during gcc shrink-wrapping
+ * optimization. It is needed to be sure that stackleak_track_stack()
+ * call is inserted after the prologue of the containing function,
+ * when the stack frame is prepared.
+ */
+ sp_decl = get_current_stack_pointer_decl();
+ if (sp_decl == NULL_TREE) {
+ add_stack_tracking_gcall(gsi);
+ return;
+ }
+ input = build_tree_list(NULL_TREE, build_const_char_string(2, "r"));
+ input = chainon(NULL_TREE, build_tree_list(input, sp_decl));
+ vec_safe_push(inputs, input);
+ asm_call = gimple_build_asm_vec("call stackleak_track_stack",
+ inputs, NULL, NULL, NULL);
+ gimple_asm_set_volatile(asm_call, true);
+ gsi_insert_before(gsi, asm_call, GSI_SAME_STMT);
+ update_stmt(asm_call);
+}
+
+static void add_stack_tracking(gimple_stmt_iterator *gsi)
+{
+ /*
+ * The 'no_caller_saved_registers' attribute is used for
+ * stackleak_track_stack(). If the compiler supports this attribute for
+ * the target arch, we can add calling stackleak_track_stack() in asm.
+ * That improves performance: we avoid useless operations with the
+ * caller-saved registers in the functions from which we will remove
+ * stackleak_track_stack() call during the stackleak_cleanup pass.
+ */
+ if (lookup_attribute_spec(get_identifier("no_caller_saved_registers")))
+ add_stack_tracking_gasm(gsi);
+ else
+ add_stack_tracking_gcall(gsi);
+}
+
/*
* Work with the GIMPLE representation of the code. Insert the
* stackleak_track_stack() call into the beginning of the function.
@@ -151,7 +223,7 @@ static unsigned int stackleak_instrument_execute(void)
bb = single_succ(ENTRY_BLOCK_PTR_FOR_FN(cfun));
}
gsi = gsi_after_labels(bb);
- stackleak_add_track_stack(&gsi);
+ add_stack_tracking(&gsi);

return 0;
}
@@ -165,29 +237,10 @@ static bool large_stack_frame(void)
#endif
}

-/*
- * Work with the RTL representation of the code.
- * Remove the unneeded stackleak_track_stack() calls from the functions
- * that don't have a large enough stack frame size.
- */
-static unsigned int stackleak_cleanup_execute(void)
+static void remove_stack_tracking_gcall(void)
{
rtx_insn *insn, *next;

- /*
- * gcc before version 7 called allocate_dynamic_stack_space() from
- * expand_stack_vars() for runtime alignment of constant-sized stack
- * variables. That caused cfun->calls_alloca to be set for functions
- * that don't use alloca().
- * For more info see gcc commit 7072df0aae0c59ae437e.
- * Let's leave such functions instrumented.
- */
- if (cfun->calls_alloca)
- return 0;
-
- if (large_stack_frame())
- return 0;
-
/*
* Find stackleak_track_stack() calls. Loop through the chain of insns,
* which is an RTL representation of the code for a function.
@@ -248,6 +301,92 @@ static unsigned int stackleak_cleanup_execute(void)
}
#endif
}
+}
+
+static bool remove_stack_tracking_gasm(void)
+{
+ bool removed = false;
+ rtx_insn *insn, *next;
+
+ /* 'no_caller_saved_registers' is currently supported only for x86 */
+ gcc_assert(build_for_x86);
+
+ /*
+ * Find stackleak_track_stack() asm calls. Loop through the chain of
+ * insns, which is an RTL representation of the code for a function.
+ *
+ * The example of a matching insn:
+ * (insn 11 5 12 2 (parallel [ (asm_operands/v
+ * ("call stackleak_track_stack") ("") 0
+ * [ (reg/v:DI 7 sp [ current_stack_pointer ]) ]
+ * [ (asm_input:DI ("r")) ] [])
+ * (clobber (reg:CC 17 flags)) ]) -1 (nil))
+ */
+ for (insn = get_insns(); insn; insn = next) {
+ rtx body;
+
+ next = NEXT_INSN(insn);
+
+ /* Check the expression code of the insn */
+ if (!NONJUMP_INSN_P(insn))
+ continue;
+
+ /*
+ * Check the expression code of the insn body, which is an RTL
+ * Expression (RTX) describing the side effect performed by
+ * that insn.
+ */
+ body = PATTERN(insn);
+
+ if (GET_CODE(body) != PARALLEL)
+ continue;
+
+ body = XVECEXP(body, 0, 0);
+
+ if (GET_CODE(body) != ASM_OPERANDS)
+ continue;
+
+ if (strcmp(ASM_OPERANDS_TEMPLATE(body),
+ "call stackleak_track_stack")) {
+ continue;
+ }
+
+ delete_insn_and_edges(insn);
+ gcc_assert(!removed);
+ removed = true;
+ }
+
+ return removed;
+}
+
+/*
+ * Work with the RTL representation of the code.
+ * Remove the unneeded stackleak_track_stack() calls from the functions
+ * that don't have a large enough stack frame size.
+ */
+static unsigned int stackleak_cleanup_execute(void)
+{
+ bool removed = false;
+
+ /*
+ * gcc before version 7 called allocate_dynamic_stack_space() from
+ * expand_stack_vars() for runtime alignment of constant-sized stack
+ * variables. That caused cfun->calls_alloca to be set for functions
+ * that don't use alloca().
+ * For more info see gcc commit 7072df0aae0c59ae437e.
+ * Let's leave such functions instrumented.
+ */
+ if (cfun->calls_alloca)
+ return 0;
+
+ if (large_stack_frame())
+ return 0;
+
+ if (lookup_attribute_spec(get_identifier("no_caller_saved_registers")))
+ removed = remove_stack_tracking_gasm();
+
+ if (!removed)
+ remove_stack_tracking_gcall();

return 0;
}
@@ -383,6 +522,15 @@ __visible int plugin_init(struct plugin_name_args *plugin_info,
plugin_name, argv[i].key, argv[i].value);
return 1;
}
+ } else if (!strcmp(argv[i].key, "arch")) {
+ if (!argv[i].value) {
+ error(G_("no value supplied for option '-fplugin-arg-%s-%s'"),
+ plugin_name, argv[i].key);
+ return 1;
+ }
+
+ if (!strcmp(argv[i].value, "x86"))
+ build_for_x86 = true;
} else {
error(G_("unknown option '-fplugin-arg-%s-%s'"),
plugin_name, argv[i].key);
--
2.25.2