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Date:   Mon, 20 Apr 2020 21:54:58 +0100
From:   Will Deacon <will@kernel.org>
To:     Kees Cook <keescook@chromium.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        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>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Potapenko <glider@google.com>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Jann Horn <jannh@google.com>,
        "Perla, Enrico" <enrico.perla@intel.com>,
        kernel-hardening@lists.openwall.com,
        linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 5/5] arm64: entry: Enable random_kstack_offset support
Message-ID: <20200420205458.GC29998@willie-the-truck>
References: <20200324203231.64324-1-keescook@chromium.org>
 <20200324203231.64324-6-keescook@chromium.org>
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On Tue, Mar 24, 2020 at 01:32:31PM -0700, Kees Cook wrote:
> Allow for a randomized stack offset on a per-syscall basis, with roughly
> 5 bits of entropy.
> 
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  arch/arm64/Kconfig          |  1 +
>  arch/arm64/kernel/syscall.c | 10 ++++++++++
>  2 files changed, 11 insertions(+)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 0b30e884e088..4d5aa4959f72 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -127,6 +127,7 @@ config ARM64
>  	select HAVE_ARCH_MMAP_RND_BITS
>  	select HAVE_ARCH_MMAP_RND_COMPAT_BITS if COMPAT
>  	select HAVE_ARCH_PREL32_RELOCATIONS
> +	select HAVE_ARCH_RANDOMIZE_KSTACK_OFFSET
>  	select HAVE_ARCH_SECCOMP_FILTER
>  	select HAVE_ARCH_STACKLEAK
>  	select HAVE_ARCH_THREAD_STRUCT_WHITELIST
> diff --git a/arch/arm64/kernel/syscall.c b/arch/arm64/kernel/syscall.c
> index a12c0c88d345..238dbd753b44 100644
> --- a/arch/arm64/kernel/syscall.c
> +++ b/arch/arm64/kernel/syscall.c
> @@ -5,6 +5,7 @@
>  #include <linux/errno.h>
>  #include <linux/nospec.h>
>  #include <linux/ptrace.h>
> +#include <linux/randomize_kstack.h>
>  #include <linux/syscalls.h>
>  
>  #include <asm/daifflags.h>
> @@ -42,6 +43,8 @@ static void invoke_syscall(struct pt_regs *regs, unsigned int scno,
>  {
>  	long ret;
>  
> +	add_random_kstack_offset();
> +
>  	if (scno < sc_nr) {
>  		syscall_fn_t syscall_fn;
>  		syscall_fn = syscall_table[array_index_nospec(scno, sc_nr)];
> @@ -51,6 +54,13 @@ static void invoke_syscall(struct pt_regs *regs, unsigned int scno,
>  	}
>  
>  	regs->regs[0] = ret;
> +
> +	/*
> +	 * Since the compiler chooses a 4 bit alignment for the stack,
> +	 * let's save one additional bit (9 total), which gets us up
> +	 * near 5 bits of entropy.
> +	 */
> +	choose_random_kstack_offset(get_random_int() & 0x1FF);

Hmm, this comment doesn't make any sense to me. I mean, I get that 0x1ff
is 9 bits, and that is 4+5 but so what?

Will