linux-kernel - Re: [PATCH v2 3/5] stack: Optionally randomize kernel stack offset each syscall

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Message-ID: <20200330112536.GD1309@C02TD0UTHF1T.local>
Date:   Mon, 30 Mar 2020 12:25:36 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Elena Reshetova <elena.reshetova@...el.com>, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Alexander Potapenko <glider@...gle.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Jann Horn <jannh@...gle.com>,
        "Perla, Enrico" <enrico.perla@...el.com>,
        kernel-hardening@...ts.openwall.com,
        linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/5] stack: Optionally randomize kernel stack offset
 each syscall

On Tue, Mar 24, 2020 at 01:32:29PM -0700, Kees Cook wrote:
> +/*
> + * Do not use this anywhere else in the kernel. This is used here because
> + * it provides an arch-agnostic way to grow the stack with correct
> + * alignment. Also, since this use is being explicitly masked to a max of
> + * 10 bits, stack-clash style attacks are unlikely. For more details see
> + * "VLAs" in Documentation/process/deprecated.rst
> + */
> +void *__builtin_alloca(size_t size);
> +
> +#define add_random_kstack_offset() do {					\
> +	if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,	\
> +				&randomize_kstack_offset)) {		\
> +		u32 offset = this_cpu_read(kstack_offset);		\
> +		char *ptr = __builtin_alloca(offset & 0x3FF);		\
> +		asm volatile("" : "=m"(*ptr));				\

Is this asm() a homebrew OPTIMIZER_HIDE_VAR(*ptr)? If the asm
constraints generate metter code, could we add those as alternative
constraints in OPTIMIZER_HIDE_VAR() ?

Mark.