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>,
Alexander Popov <alex.popov@linux.com>,
Ard Biesheuvel <ard.biesheuvel@linaro.org>,
Jann Horn <jannh@google.com>, Vlastimil Babka <vbabka@suse.cz>,
David Hildenbrand <david@redhat.com>,
Mike Rapoport <rppt@linux.ibm.com>,
Andrew Morton <akpm@linux-foundation.org>,
Jonathan Corbet <corbet@lwn.net>,
Randy Dunlap <rdunlap@infradead.org>,
kernel-hardening@lists.openwall.com,
linux-hardening@vger.kernel.org,
linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH v10 3/6] stack: Optionally randomize kernel stack offset each syscall
Date: Wed, 7 Apr 2021 22:37:02 +0100 [thread overview]
Message-ID: <20210407213702.GB16569@willie-the-truck> (raw)
In-Reply-To: <20210401232347.2791257-4-keescook@chromium.org>
On Thu, Apr 01, 2021 at 04:23:44PM -0700, Kees Cook wrote:
> This provides the ability for architectures to enable kernel stack base
> address offset randomization. This feature is controlled by the boot
> param "randomize_kstack_offset=on/off", with its default value set by
> CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT.
>
> This feature is based on the original idea from the last public release
> of PaX's RANDKSTACK feature: https://pax.grsecurity.net/docs/randkstack.txt
> All the credit for the original idea goes to the PaX team. Note that
> the design and implementation of this upstream randomize_kstack_offset
> feature differs greatly from the RANDKSTACK feature (see below).
[...]
> diff --git a/include/linux/randomize_kstack.h b/include/linux/randomize_kstack.h
> new file mode 100644
> index 000000000000..fd80fab663a9
> --- /dev/null
> +++ b/include/linux/randomize_kstack.h
> @@ -0,0 +1,54 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +#ifndef _LINUX_RANDOMIZE_KSTACK_H
> +#define _LINUX_RANDOMIZE_KSTACK_H
> +
> +#include <linux/kernel.h>
> +#include <linux/jump_label.h>
> +#include <linux/percpu-defs.h>
> +
> +DECLARE_STATIC_KEY_MAYBE(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,
> + randomize_kstack_offset);
> +DECLARE_PER_CPU(u32, kstack_offset);
> +
> +/*
> + * 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);
> +/*
> + * Use, at most, 10 bits of entropy. We explicitly cap this to keep the
> + * "VLA" from being unbounded (see above). 10 bits leaves enough room for
> + * per-arch offset masks to reduce entropy (by removing higher bits, since
> + * high entropy may overly constrain usable stack space), and for
> + * compiler/arch-specific stack alignment to remove the lower bits.
> + */
> +#define KSTACK_OFFSET_MAX(x) ((x) & 0x3FF)
> +
> +/*
> + * These macros must be used during syscall entry when interrupts and
> + * preempt are disabled, and after user registers have been stored to
> + * the stack.
> + */
This comment is out of date, as this is called from preemptible context on
arm64. Does that matter in terms of offset randomness?
Will
next prev parent reply other threads:[~2021-04-07 21:37 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-01 23:23 [PATCH v10 0/6] Optionally randomize kernel stack offset each syscall Kees Cook
2021-04-01 23:23 ` [PATCH v10 1/6] jump_label: Provide CONFIG-driven build state defaults Kees Cook
2021-04-01 23:23 ` [PATCH v10 2/6] init_on_alloc: Optimize static branches Kees Cook
2021-04-01 23:23 ` [PATCH v10 3/6] stack: Optionally randomize kernel stack offset each syscall Kees Cook
2021-04-07 21:37 ` Will Deacon [this message]
2021-04-01 23:23 ` [PATCH v10 4/6] x86/entry: Enable random_kstack_offset support Kees Cook
2021-04-01 23:23 ` [PATCH v10 5/6] arm64: entry: " Kees Cook
2021-04-07 21:35 ` Will Deacon
2021-04-01 23:23 ` [PATCH v10 6/6] lkdtm: Add REPORT_STACK for checking stack offsets Kees Cook
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