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

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Message-ID: <202003301122.354B722@keescook>
Date:   Mon, 30 Mar 2020 11:27:19 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Mark Rutland <mark.rutland@....com>
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 Mon, Mar 30, 2020 at 12:25:36PM +0100, Mark Rutland wrote:
> 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() ?

Er, no, sorry, not the same. I disassembled the wrong binary. :)

With     asm volatile("" : "=m"(*ptr))

ffffffff810038bc:       48 8d 44 24 0f          lea    0xf(%rsp),%rax
ffffffff810038c1:       48 83 e0 f0             and    $0xfffffffffffffff0,%rax


With   __asm__ ("" : "=r" (var) : "0" (var))

ffffffff810038bc:       48 8d 54 24 0f          lea    0xf(%rsp),%rdx
ffffffff810038c1:       48 83 e2 f0             and    $0xfffffffffffffff0,%rdx
ffffffff810038c5:       0f b6 02                movzbl (%rdx),%eax
ffffffff810038c8:       88 02                   mov    %al,(%rdx)


It looks like OPTIMIZER_HIDE_VAR() is basically just:

	var = var;

In the former case, we avoid the write and retain the allocation. So I
think don't think OPTIMIZER_HIDE_VAR() should be used here, nor should
OPTIMIZER_HIDE_VAR() be changed to remove the "0" (var) bit.

-- 
Kees Cook