From: Kees Cook <keescook@chromium.org>
To: Mark Rutland <mark.rutland@arm.com>
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>,
Will Deacon <will@kernel.org>,
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 3/5] stack: Optionally randomize kernel stack offset each syscall
Date: Mon, 30 Mar 2020 11:27:19 -0700 [thread overview]
Message-ID: <202003301122.354B722@keescook> (raw)
In-Reply-To: <20200330112536.GD1309@C02TD0UTHF1T.local>
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
next prev parent reply other threads:[~2020-03-31 14:25 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-24 20:32 [PATCH v2 0/5] Optionally randomize kernel stack offset each syscall Kees Cook
2020-03-24 20:32 ` [PATCH v2 1/5] jump_label: Provide CONFIG-driven build state defaults Kees Cook
2020-03-24 22:06 ` Peter Zijlstra
2020-03-24 20:32 ` [PATCH v2 2/5] init_on_alloc: Unpessimize default-on builds Kees Cook
2020-03-26 15:48 ` Alexander Potapenko
2020-03-24 20:32 ` [PATCH v2 3/5] stack: Optionally randomize kernel stack offset each syscall Kees Cook
2020-03-30 11:25 ` Mark Rutland
2020-03-30 18:18 ` Kees Cook
2020-03-30 18:27 ` Kees Cook [this message]
2020-03-24 20:32 ` [PATCH v2 4/5] x86/entry: Enable random_kstack_offset support Kees Cook
2020-03-28 22:26 ` Kees Cook
2020-03-24 20:32 ` [PATCH v2 5/5] arm64: entry: " Kees Cook
2020-03-25 13:21 ` Mark Rutland
2020-03-25 20:22 ` Kees Cook
2020-03-26 11:15 ` Mark Rutland
2020-03-26 16:31 ` Kees Cook
2020-03-30 11:26 ` Mark Rutland
2020-04-20 20:54 ` Will Deacon
2020-04-20 22:34 ` Kees Cook
2020-04-21 7:02 ` Will Deacon
2020-03-24 21:28 ` [PATCH v2 0/5] Optionally randomize kernel stack offset each syscall Jann Horn
2020-03-24 23:07 ` Kees Cook
2020-03-25 12:15 ` Reshetova, Elena
2020-03-25 20:27 ` Kees Cook
2020-03-25 23:20 ` Jann Horn
2020-03-26 17:18 ` Kees Cook
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