Re: [PATCH 1/2] stack: Introduce CONFIG_RANDOMIZE_KSTACK_OFFSET

[Nathan Chancellor <nathan@kernel.org>]

On Fri, Jan 28, 2022 at 12:44:45PM +0100, Marco Elver wrote:
> The randomize_kstack_offset feature is unconditionally compiled in when
> the architecture supports it.
> 
> To add constraints on compiler versions, we require a dedicated Kconfig
> variable. Therefore, introduce RANDOMIZE_KSTACK_OFFSET.
> 
> Furthermore, this option is now also configurable by EXPERT kernels:
> while the feature is supposed to have zero performance overhead when
> disabled, due to its use of static branches, there are few cases where
> giving a distribution the option to disable the feature entirely makes
> sense. For example, in very resource constrained environments, which
> would never enable the feature to begin with, in which case the
> additional kernel code size increase would be redundant.
> 
> Signed-off-by: Marco Elver <elver@google.com>

From a Kconfig perspective:

Reviewed-by: Nathan Chancellor <nathan@kernel.org>

> ---
>  arch/Kconfig                     | 23 ++++++++++++++++++-----
>  include/linux/randomize_kstack.h |  5 +++++
>  init/main.c                      |  2 +-
>  3 files changed, 24 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 678a80713b21..2cde48d9b77c 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -1159,16 +1159,29 @@ config HAVE_ARCH_RANDOMIZE_KSTACK_OFFSET
>  	  to the compiler, so it will attempt to add canary checks regardless
>  	  of the static branch state.
>  
> -config RANDOMIZE_KSTACK_OFFSET_DEFAULT
> -	bool "Randomize kernel stack offset on syscall entry"
> +config RANDOMIZE_KSTACK_OFFSET
> +	bool "Support for randomizing kernel stack offset on syscall entry" if EXPERT
> +	default y
>  	depends on HAVE_ARCH_RANDOMIZE_KSTACK_OFFSET
>  	help
>  	  The kernel stack offset can be randomized (after pt_regs) by
>  	  roughly 5 bits of entropy, frustrating memory corruption
>  	  attacks that depend on stack address determinism or
> -	  cross-syscall address exposures. This feature is controlled
> -	  by kernel boot param "randomize_kstack_offset=on/off", and this
> -	  config chooses the default boot state.
> +	  cross-syscall address exposures.
> +
> +	  The feature is controlled via the "randomize_kstack_offset=on/off"
> +	  kernel boot param, and if turned off has zero overhead due to its use
> +	  of static branches (see JUMP_LABEL).
> +
> +	  If unsure, say Y.
> +
> +config RANDOMIZE_KSTACK_OFFSET_DEFAULT
> +	bool "Default state of kernel stack offset randomization"
> +	depends on RANDOMIZE_KSTACK_OFFSET
> +	help
> +	  Kernel stack offset randomization is controlled by kernel boot param
> +	  "randomize_kstack_offset=on/off", and this config chooses the default
> +	  boot state.
>  
>  config ARCH_OPTIONAL_KERNEL_RWX
>  	def_bool n
> diff --git a/include/linux/randomize_kstack.h b/include/linux/randomize_kstack.h
> index bebc911161b6..91f1b990a3c3 100644
> --- a/include/linux/randomize_kstack.h
> +++ b/include/linux/randomize_kstack.h
> @@ -2,6 +2,7 @@
>  #ifndef _LINUX_RANDOMIZE_KSTACK_H
>  #define _LINUX_RANDOMIZE_KSTACK_H
>  
> +#ifdef CONFIG_RANDOMIZE_KSTACK_OFFSET
>  #include <linux/kernel.h>
>  #include <linux/jump_label.h>
>  #include <linux/percpu-defs.h>
> @@ -50,5 +51,9 @@ void *__builtin_alloca(size_t size);
>  		raw_cpu_write(kstack_offset, offset);			\
>  	}								\
>  } while (0)
> +#else /* CONFIG_RANDOMIZE_KSTACK_OFFSET */
> +#define add_random_kstack_offset()		do { } while (0)
> +#define choose_random_kstack_offset(rand)	do { } while (0)
> +#endif /* CONFIG_RANDOMIZE_KSTACK_OFFSET */
>  
>  #endif
> diff --git a/init/main.c b/init/main.c
> index 65fa2e41a9c0..560f45c27ffe 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -853,7 +853,7 @@ static void __init mm_init(void)
>  	pti_init();
>  }
>  
> -#ifdef CONFIG_HAVE_ARCH_RANDOMIZE_KSTACK_OFFSET
> +#ifdef CONFIG_RANDOMIZE_KSTACK_OFFSET
>  DEFINE_STATIC_KEY_MAYBE_RO(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,
>  			   randomize_kstack_offset);
>  DEFINE_PER_CPU(u32, kstack_offset);
> -- 
> 2.35.0.rc0.227.g00780c9af4-goog
>