Re: [PATCH] [RFC/RFT]SCS:Add gcc plugin to support Shadow Call Stack

[Ard Biesheuvel]

On Mon, 20 Sept 2021 at 20:53, Dan Li <ashimida@xxxxxxxxxxxxxxxxx> wrote:
>
> Hi Ard,
>
> Thanks for your comment.
>
> I pasted a copy of the config code in my last email, could you please check it again?
>
> On 9/20/21 3:18 PM, Ard Biesheuvel wrote:
> > Hi Dan,
> >
> > On Sun, 19 Sept 2021 at 18:37, Dan Li <ashimida@xxxxxxxxxxxxxxxxx> wrote:
> >>
> >> The Clang-based shadow call stack protection has been integrated into the
> >> mainline, but kernel compiled by gcc cannot enable this feature for now.
> >>
> >> This Patch supports gcc-based SCS protection by adding a plugin.
> >>
> >
> > Thanks for working on this. I had a stab at this myself about 2 years
> > ago and couldn't make it work.
> >
> >> For each function that x30 will be pushed onto the stack during execution,
> >> this plugin:
> >> 1) insert "str x30, [x18], #8" at the entry of the function to save x30
> >>     to current SCS
> >> 2) insert "ldr x30, [x18, #-8]!"  before the exit of this function to
> >>     restore x30
> >>
> >
> > This logic seems sound to me, but it would be nice if someone more
> > familiar with Clang's implementation could confirm that it is really
> > this simple.
> >
> > Looking at your plugin, there is an issue with tail calls, and I don't
> > think Clang simply disables those altogether as well, right?
>
> I am not familiar with clang's code, the logic comes from clang's description and the
> disassembled binary code for now, so it may be different from the actual situation.
>

OK

> The tail call could be handled (theoretically), and I will try to solve the issue in
> the next version.
> >
> >>   ifdef CONFIG_SHADOW_CALL_STACK
> >> -CC_FLAGS_SCS   := -fsanitize=shadow-call-stack
> >> +CC_FLAGS_SCS   := $(if $(CONFIG_CC_IS_CLANG),-fsanitize=shadow-call-stack,)
> >
> > This variable should contain whatever needs to be added to the
> > compiler comamand line
>    In the new code, an 'enable' option is added here to enable the plugin
> >>   KBUILD_CFLAGS  += $(CC_FLAGS_SCS)
> >>   export CC_FLAGS_SCS
> >>   endif
> >> diff --git a/arch/Kconfig b/arch/Kconfig
> >> index 98db634..81ff127 100644
> >> --- a/arch/Kconfig
> >> +++ b/arch/Kconfig
> >> @@ -594,7 +594,7 @@ config ARCH_SUPPORTS_SHADOW_CALL_STACK
> >>
> >>   config SHADOW_CALL_STACK
> >>          bool "Clang Shadow Call Stack"
> >> -       depends on CC_IS_CLANG && ARCH_SUPPORTS_SHADOW_CALL_STACK
> >> +       depends on (CC_IS_CLANG && ARCH_SUPPORTS_SHADOW_CALL_STACK) || GCC_PLUGIN_SHADOW_CALL_STACK
> >
> > This logic needs to be defined in such a way that a builtin
> > implementation provided by GCC will take precedence once it becomes
> > available.
> >
>    In new code, if gcc supports SCS in the future, the plugin will be closed due to
>    CC_HAVE_SHADOW_CALL_STACK is true.
> >>          depends on DYNAMIC_FTRACE_WITH_REGS || !FUNCTION_GRAPH_TRACER
> >>          help
> >>            This option enables Clang's Shadow Call Stack, which uses a
> >> diff --git a/scripts/gcc-plugins/Kconfig b/scripts/gcc-plugins/Kconfig
> >> index ab9eb4c..2534195e 100644
> >> --- a/scripts/gcc-plugins/Kconfig
> >> +++ b/scripts/gcc-plugins/Kconfig
> >> @@ -19,6 +19,14 @@ menuconfig GCC_PLUGINS
> >>
> >>   if GCC_PLUGINS
> >>
> >> +config GCC_PLUGIN_SHADOW_CALL_STACK
> >> +       bool "GCC Shadow Call Stack plugin"
> >> +       select SHADOW_CALL_STACK
> >
> > You shouldn't 'select' something like this if the symbol has its own
> > dependencies which may be unsatisfied, as this causes a Kconfig
> > warning. Also, en/disabling shadow call stacks for the architecture
> > should be done from the arch's 'kernel features' menu, it shouldn't be
> > buried in the GCC plugins menu.
>     I removed 'select' in the new version.
>     SCS's enable is changed to rely on CONFIG_SHADOW_CALL_STACK in arch/kernel,
>     the GCC_PLUGIN_SHADOW_CALL_STACK config is just to add a usable platform to it.
> >> +       help
> >> +         This plugin is used to support the kernel CONFIG_SHADOW_CALL_STACK
> >> +         compiled by gcc. Its principle is basically the same as that of CLANG.
> >> +         For more information, please refer to "config SHADOW_CALL_STACK"
> >> +
> >> +__visible int plugin_is_GPL_compatible;
> >> +
> >> +static struct plugin_info arm64_scs_plugin_info = {
> >> +       .version        = "20210926vanilla",
> >
> > I will respond to this obvious invitation at bikeshedding by saying
> > that 'salted caramel' is clearly the superior flavor of ice cream.
>    I'm sorry, as a non-native English speaker, I think I might not understand
>    what you mean here. My intention is to say that this is the first/initial
>    version, do I miss something?

It was a joke - don't worry about it.

> >> +       .help           = "disable\tdo not activate plugin\n"
> >> +                         "verbose\tprint all debug infos\n",
> >> +};
> >> +static unsigned int arm64_scs_execute(void)
> >> +{
> >> +       rtx_insn *insn;
> >> +       enum scs_state state = SCS_SEARCHING_FIRST_INSN;
> >> +
> >> +       for (insn = get_insns(); insn; insn = NEXT_INSN(insn)) {
> >> +               rtx mark = NULL;
> >> +
> >> +               switch (GET_CODE(insn)) {
> >> +               case NOTE:
> >> +               case BARRIER:
> >> +               case CODE_LABEL:
> >> +               case INSN:
> >> +               case DEBUG_INSN:
> >> +               case JUMP_INSN:
> >> +               case JUMP_TABLE_DATA:
> >> +                       break;
> >> +               case CALL_INSN:
> >> +                       if (SIBLING_CALL_P(insn)) {
> >> +                               error(G_("Sibling call found in func:%s, file:%s\n"),
> >> +                                               get_name(current_function_decl),
> >> +                                               main_input_filename);
> >> +                               gcc_unreachable();
> >> +                       }
> >
> > Sibling calls are an important optimization, not only for performance
> > but also for stack utilization, so this needs to be fixed. Can you
> > elaborate on the issue you are working around here?
> >
>    Since the ARM64 has disabled sibling calls (-fno-optimize-sibling-calls) by default,
>    there is almost no sibling call appear in the kernel I encountered.

What do you mean this is disabled by default? Is that a compiler
setting or a Linux setting?




>    So I did not provide support for it, and I will fix this issue in the next version.
> >> +                       break;
> >> +               default:
> >> +                       error(G_("Invalid rtx_insn seqs found with type:%s in func:%s, file:%s\n"),
> >> +                                       GET_RTX_NAME(GET_CODE(insn)),
> >> +                                       get_name(current_function_decl), main_input_filename);
> >> +                       gcc_unreachable();
> >> +                       break;
> >> +               }
> >> +               /* A function return insn was found */
> >> +               if (ANY_RETURN_P(PATTERN(insn))) {
> >> +                       /* There should be an epilogue before 'RETURN' inst */
> >> +                       if (GET_CODE(PATTERN(insn)) == RETURN) {
> >> +                               gcc_assert(state == SCS_FOUND_ONE_EPILOGUE_NOTE);
> >> +                               state = SCS_SEARCHING_FUNC_RETURN;
> >> +                       }
> >> +
> >> +                       /* There is no epilogue before 'SIMPLE_RETURN' insn */
> >> +                       if (GET_CODE(PATTERN(insn)) == SIMPLE_RETURN)
> >> +                               gcc_assert(state == SCS_SEARCHING_FUNC_RETURN);
> >
> > These assert()s will crash the compiler if the RTL doesn't have quite
> > the right structure, correct? Could we issue a warning instead, saying
> > function 'x' could not be handled, and back out gracefully (i.e.,
> > don't insert the push either)?
> >
>     Sure, I think I need to dynamically mark all instrumented positions here,
>     and then confirm that the instruction sequence is correct before inserting in batches.

Yes, that sounds more suitable.

> >> +
> >> +                       /* Insert scs pop instruction(s) before return insn */
> >> +                       mark = gen_scs_pop(RESERVED_LOCATION_COUNT);
> >> +                       emit_insn_before(mark, insn);
> >> +               }
> >> +       }
> >> +       return 0;
> >> +}
> >> +
> >> +static tree handle_noscs_attribute(tree *node, tree name, tree args __unused, int flags,
> >> +               bool *no_add_attrs)
> >> +{
> >> +       *no_add_attrs = true;
> >> +
> >> +       gcc_assert(DECL_P(*node));
> >> +       switch (TREE_CODE(*node)) {
> >> +       default:
> >> +               error(G_("%qE attribute can be applies to function decl only (%qE)"), name, *node);
> >> +               gcc_unreachable();
> >> +
> >> +       case FUNCTION_DECL:     /* the attribute is only used for function declarations */
> >> +               break;
> >> +       }
> >> +
> >> +       *no_add_attrs = false;
> >
> > I'm not familiar with this idiom: what is the purpose of setting this
> > to true initially and then to false again when the expected flow
> > through the function is to do nothing at all?
> >
>     This is my mistake, at the beginning default case only return 0 directly after a warning;
>     At that time, if *no_add_attrs is true, the corresponding attribute will not be added to 'node',
>     and it means __noscs attribute can only be added for FUNCTION_DECL.
>     For now, *no_add_attrs = true; is useless, it should be deleted.
>
>     But if, as you said, try to back out gracefully, is it better to report warning in the default case?

error() just terminates the compile with an error, right? I think that is fine.


> >> +       return NULL_TREE;
> >> +}
> >> +
> >> +static void (*old_override_options_after_change)(void);
> >> +
> >> +static void scs_override_options_after_change(void)
> >> +{
> >> +       if (old_override_options_after_change)
> >> +               old_override_options_after_change();
> >> +
> >> +       flag_optimize_sibling_calls = 0;
> >> +}
> >> +
> >> +static void callback_before_start_unit(void *gcc_data __unused, void *user_data __unused)
> >> +{
> >> +       /* Turn off sibling call to avoid inserting duplicate scs pop codes */
> >
> > Sibling calls will restore x30 before the calk, right? So where do the
> > duplicate pops come from?
>     a sibling call could be like:
>     stp     x29, x30, [sp, #-xx]!
>     .......
>     ldp     x29, x30, [sp], #xx
>     ---> p1
>     b       callee
>     ldp     x29, x30, [sp], #xx
>     ---> p2
>     ret
>
>     What i mean here is if we need to insert, the scs pop code should be insert in both p1/p2,

Yes, so you have to identify the 'b' insn as a function return so it
is treated the same.

> >
> >> +       old_override_options_after_change = targetm.override_options_after_change;
> >> +       targetm.override_options_after_change = scs_override_options_after_change;
> >> +
> >> +       flag_optimize_sibling_calls = 0;
> >
> > Do we need this twice?
>    I think so, there are functions similar to push/pop in gcc (cl_optimization_restore/save)
>    * callback_before_start_unit is used to set zero during initialization
>    * scs_override_options_after_change is used to reset to 0 after a 'push' occurs

OK

> >> +}
> >> +
> >> +#define PASS_NAME arm64_scs
> >> +#define TODO_FLAGS_FINISH (TODO_dump_func | TODO_verify_rtl_sharing)
> >> +#include "gcc-generate-rtl-pass.h"
> >> +
> >> +__visible int plugin_init(struct plugin_name_args *plugin_info, struct plugin_gcc_version *version)
> >> +{
> >> +       int i;
> >> +       const char * const plugin_name = plugin_info->base_name;
> >> +       const int argc = plugin_info->argc;
> >> +       const struct plugin_argument * const argv = plugin_info->argv;
> >> +       bool enable = true;
> >> +
> >> +       PASS_INFO(arm64_scs, "shorten", 1, PASS_POS_INSERT_BEFORE);
> >> +
> >> +       if (!plugin_default_version_check(version, &gcc_version)) {
> >> +               error(G_("Incompatible gcc/plugin versions"));
> >> +               return 1;
> >> +       }
> >> +
> >> +       if (strncmp(lang_hooks.name, "GNU C", 5) && !strncmp(lang_hooks.name, "GNU C+", 6)) {
> >> +               inform(UNKNOWN_LOCATION, G_("%s supports C only, not %s"), plugin_name,
> >> +                               lang_hooks.name);
> >> +               enable = false;
> >> +       }
> >> +
> >
> > Do we need this check?
>    This code is copied from structleak_plugin.c, I misunderstood the meaning here, and I will delete it later

OK. Kees should correct me if I'm wrong, but we use GCC in the kernel
only to compile C files, so this check should be redundant.


> >
> >> +       for (i = 0; i < argc; ++i) {
> >> +               if (!strcmp(argv[i].key, "disable")) {
> >> +                       enable = false;
> >> +                       continue;
> >> +               }
> >> +               if (!strcmp(argv[i].key, "verbose")) {
> >> +                       verbose = true;
> >> +                       continue;
> >> +               }
> >> +               error(G_("unknown option '-fplugin-arg-%s-%s'"), plugin_name, argv[i].key);
> >> +       }
> >> +
> >> +       register_callback(plugin_name, PLUGIN_INFO, NULL, &arm64_scs_plugin_info);
> >> +
> >> +       register_callback(plugin_name, PLUGIN_ATTRIBUTES, scs_register_attributes, NULL);
> >> +
> >> +       if (!enable) {
> >> +               v_info("Plugin disabled for file:%s\n", main_input_filename);
> >> +               return 0;
> >> +       }
> >> +
> >> +       register_callback(plugin_name, PLUGIN_START_UNIT, callback_before_start_unit, NULL);
> >> +
> >> +       register_callback(plugin_name, PLUGIN_PASS_MANAGER_SETUP, NULL, &arm64_scs_pass_info);
> >> +
> >> +       return 0;
> >> +}
> >> --
> >> 2.7.4
> >>