Re: [PATCH v5 05/14] add support for Clang's Shadow Call Stack (SCS)

[Mark Rutland]

On Tue, Nov 05, 2019 at 03:55:59PM -0800, Sami Tolvanen wrote:
> This change adds generic support for Clang's Shadow Call Stack,
> which uses a shadow stack to protect return addresses from being
> overwritten by an attacker. Details are available here:
> 
>   https://clang.llvm.org/docs/ShadowCallStack.html
> 
> Note that security guarantees in the kernel differ from the
> ones documented for user space. The kernel must store addresses
> of shadow stacks used by other tasks and interrupt handlers in
> memory, which means an attacker capable reading and writing
> arbitrary memory may be able to locate them and hijack control
> flow by modifying shadow stacks that are not currently in use.
> 
> Signed-off-by: Sami Tolvanen <samitolvanen@xxxxxxxxxx>
> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Reviewed-by: Miguel Ojeda <miguel.ojeda.sandonis@xxxxxxxxx>
> ---
>  Makefile                       |   6 ++
>  arch/Kconfig                   |  33 ++++++
>  include/linux/compiler-clang.h |   6 ++
>  include/linux/compiler_types.h |   4 +
>  include/linux/scs.h            |  57 ++++++++++
>  init/init_task.c               |   8 ++
>  kernel/Makefile                |   1 +
>  kernel/fork.c                  |   9 ++
>  kernel/sched/core.c            |   2 +
>  kernel/scs.c                   | 187 +++++++++++++++++++++++++++++++++
>  10 files changed, 313 insertions(+)
>  create mode 100644 include/linux/scs.h
>  create mode 100644 kernel/scs.c
> 
> diff --git a/Makefile b/Makefile
> index b37d0e8fc61d..7f3a4c5c7dcc 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -846,6 +846,12 @@ ifdef CONFIG_LIVEPATCH
>  KBUILD_CFLAGS += $(call cc-option, -flive-patching=inline-clone)
>  endif
>  
> +ifdef CONFIG_SHADOW_CALL_STACK
> +CC_FLAGS_SCS	:= -fsanitize=shadow-call-stack
> +KBUILD_CFLAGS	+= $(CC_FLAGS_SCS)
> +export CC_FLAGS_SCS
> +endif
> +
>  # arch Makefile may override CC so keep this after arch Makefile is included
>  NOSTDINC_FLAGS += -nostdinc -isystem $(shell $(CC) -print-file-name=include)
>  
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 5f8a5d84dbbe..5e34cbcd8d6a 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -521,6 +521,39 @@ config STACKPROTECTOR_STRONG
>  	  about 20% of all kernel functions, which increases the kernel code
>  	  size by about 2%.
>  
> +config ARCH_SUPPORTS_SHADOW_CALL_STACK
> +	bool
> +	help
> +	  An architecture should select this if it supports Clang's Shadow
> +	  Call Stack, has asm/scs.h, and implements runtime support for shadow
> +	  stack switching.
> +
> +config SHADOW_CALL_STACK_VMAP
> +	bool
> +	depends on SHADOW_CALL_STACK
> +	help
> +	  Use virtually mapped shadow call stacks. Selecting this option
> +	  provides better stack exhaustion protection, but increases per-thread
> +	  memory consumption as a full page is allocated for each shadow stack.

The bool needs some display text to make it selectable.

This should probably be below SHADOW_CALL_STACK so that when it shows up
in menuconfig it's where you'd expect it to be.

I locally hacked that in, but when building defconfig +
SHADOW_CALL_STACK + SHADOW_CALL_STACK_VMAP, the build explodes as below:

| [mark@lakrids:~/src/linux]% usellvm 9.0.0 usekorg 8.1.0 make ARCH=arm64 CROSS_COMPILE=aarch64-linux- CC=clang -j56 -s
| arch/arm64/kernel/scs.c:28:7: error: use of undeclared identifier 'VMALLOC_START'
|                                          VMALLOC_START, VMALLOC_END,
|                                          ^
| arch/arm64/kernel/scs.c:28:22: error: use of undeclared identifier 'VMALLOC_END'
|                                          VMALLOC_START, VMALLOC_END,
|                                                         ^
| arch/arm64/kernel/scs.c:29:7: error: use of undeclared identifier 'SCS_GFP'
|                                          SCS_GFP, PAGE_KERNEL,
|                                          ^
| arch/arm64/kernel/scs.c:29:16: error: use of undeclared identifier 'PAGE_KERNEL'
|                                          SCS_GFP, PAGE_KERNEL,
|                                                   ^
| 4 errors generated.
| scripts/Makefile.build:265: recipe for target 'arch/arm64/kernel/scs.o' failed
| make[2]: *** [arch/arm64/kernel/scs.o] Error 1
| scripts/Makefile.build:509: recipe for target 'arch/arm64/kernel' failed
| make[1]: *** [arch/arm64/kernel] Error 2
| Makefile:1655: recipe for target 'arch/arm64' failed
| make: *** [arch/arm64] Error 2
| make: *** Waiting for unfinished jobs....

Other than that, this largely looks good to me!

Thanks,
Mark.