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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.
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