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Subject: Re: [PATCH v4 3/5] stack: Optionally randomize kernel stack offset
 each syscall
To:     Kees Cook <keescook@chromium.org>,
        Thomas Gleixner <tglx@linutronix.de>
Cc:     Elena Reshetova <elena.reshetova@intel.com>, x86@kernel.org,
        Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Potapenko <glider@google.com>,
        Alexander Popov <alex.popov@linux.com>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Jann Horn <jannh@google.com>,
        kernel-hardening@lists.openwall.com,
        linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <20200622193146.2985288-1-keescook@chromium.org>
 <20200622193146.2985288-4-keescook@chromium.org>
From:   Randy Dunlap <rdunlap@infradead.org>
Message-ID: <87a7b943-ed15-8521-773e-c182a37ee61e@infradead.org>
Date:   Mon, 22 Jun 2020 12:40:49 -0700
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On 6/22/20 12:31 PM, Kees Cook wrote:
> This provides the ability for architectures to enable kernel stack base
> address offset randomization. This feature is controlled by the boot
> param "randomize_kstack_offset=on/off", with its default value set by
> CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT.
> 
> Co-developed-by: Elena Reshetova <elena.reshetova@intel.com>
> Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
> Link: https://lore.kernel.org/r/20190415060918.3766-1-elena.reshetova@intel.com
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  Makefile                         |  4 ++++
>  arch/Kconfig                     | 23 ++++++++++++++++++
>  include/linux/randomize_kstack.h | 40 ++++++++++++++++++++++++++++++++
>  init/main.c                      | 23 ++++++++++++++++++
>  4 files changed, 90 insertions(+)
>  create mode 100644 include/linux/randomize_kstack.h

Hi,
Please add documentation for the new kernel boot parameter to
Documentation/admin-guide/kernel-parameters.txt.


> diff --git a/arch/Kconfig b/arch/Kconfig
> index 1ea61290900a..1f52c9cfefca 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -883,6 +883,29 @@ config VMAP_STACK
>  	  virtual mappings with real shadow memory, and KASAN_VMALLOC must
>  	  be enabled.
>  
> +config HAVE_ARCH_RANDOMIZE_KSTACK_OFFSET
> +	def_bool n
> +	help
> +	  An arch should select this symbol if it can support kernel stack
> +	  offset randomization with calls to add_random_kstack_offset()
> +	  during syscall entry and choose_random_kstack_offset() during
> +	  syscall exit. Downgrading of -fstack-protector-strong to
> +	  -fstack-protector should also be applied to the entry code and
> +	  closely examined, as the artificial stack bump looks like an array
> +	  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"
> +	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.


thanks.
-- 
~Randy