All of lore.kernel.org
 help / color / mirror / Atom feed
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 v8 3/6] stack: Optionally randomize kernel stack offset each syscall
Date: Thu, 1 Apr 2021 09:30:35 +0100	[thread overview]
Message-ID: <20210401083034.GA8554@willie-the-truck> (raw)
In-Reply-To: <20210330205750.428816-4-keescook@chromium.org>

On Tue, Mar 30, 2021 at 01:57:47PM -0700, Kees Cook wrote:
> diff --git a/include/linux/randomize_kstack.h b/include/linux/randomize_kstack.h
> new file mode 100644
> index 000000000000..351520803006
> --- /dev/null
> +++ b/include/linux/randomize_kstack.h
> @@ -0,0 +1,55 @@
> +/* 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
> + * The asm statement is designed to convince the compiler to keep the
> + * allocation around even after "ptr" goes out of scope.
> + */
> +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.
> + */
> +#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);		\
> +		u8 *ptr = __builtin_alloca(KSTACK_OFFSET_MAX(offset));	\
> +		asm volatile("" : "=m"(*ptr) :: "memory");		\

Using the "m" constraint here is dangerous if you don't actually evaluate it
inside the asm. For example, if the compiler decides to generate an
addressing mode relative to the stack but with writeback (autodecrement), then
the stack pointer will be off by 8 bytes. Can you use "o" instead?

Will

WARNING: multiple messages have this Message-ID
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 v8 3/6] stack: Optionally randomize kernel stack offset each syscall
Date: Thu, 1 Apr 2021 09:30:35 +0100	[thread overview]
Message-ID: <20210401083034.GA8554@willie-the-truck> (raw)
In-Reply-To: <20210330205750.428816-4-keescook@chromium.org>

On Tue, Mar 30, 2021 at 01:57:47PM -0700, Kees Cook wrote:
> diff --git a/include/linux/randomize_kstack.h b/include/linux/randomize_kstack.h
> new file mode 100644
> index 000000000000..351520803006
> --- /dev/null
> +++ b/include/linux/randomize_kstack.h
> @@ -0,0 +1,55 @@
> +/* 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
> + * The asm statement is designed to convince the compiler to keep the
> + * allocation around even after "ptr" goes out of scope.
> + */
> +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.
> + */
> +#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);		\
> +		u8 *ptr = __builtin_alloca(KSTACK_OFFSET_MAX(offset));	\
> +		asm volatile("" : "=m"(*ptr) :: "memory");		\

Using the "m" constraint here is dangerous if you don't actually evaluate it
inside the asm. For example, if the compiler decides to generate an
addressing mode relative to the stack but with writeback (autodecrement), then
the stack pointer will be off by 8 bytes. Can you use "o" instead?

Will

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

  parent reply	other threads:[~2021-04-01  8:31 UTC|newest]

Thread overview: 41+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-03-30 20:57 [PATCH v8 0/6] " Kees Cook
2021-03-30 20:57 ` Kees Cook
2021-03-30 20:57 ` [PATCH v8 1/6] jump_label: Provide CONFIG-driven build state defaults Kees Cook
2021-03-30 20:57   ` Kees Cook
2021-03-30 20:57 ` [PATCH v8 2/6] init_on_alloc: Optimize static branches Kees Cook
2021-03-30 20:57   ` Kees Cook
2021-03-30 20:57 ` [PATCH v8 3/6] stack: Optionally randomize kernel stack offset each syscall Kees Cook
2021-03-30 20:57   ` Kees Cook
2021-03-31  7:53   ` Thomas Gleixner
2021-03-31  7:53     ` Thomas Gleixner
2021-03-31 21:54     ` Kees Cook
2021-03-31 21:54       ` Kees Cook
2021-03-31 22:38       ` Thomas Gleixner
2021-03-31 22:38         ` Thomas Gleixner
2021-04-01  6:31         ` Kees Cook
2021-04-01  6:31           ` Kees Cook
2021-04-01  8:30   ` Will Deacon [this message]
2021-04-01  8:30     ` Will Deacon
2021-04-01 11:15     ` David Laight
2021-04-01 11:15       ` David Laight
2021-04-01 11:15       ` David Laight
2021-04-01 22:42       ` Kees Cook
2021-04-01 22:42         ` Kees Cook
2021-04-01 22:42         ` Kees Cook
2021-03-30 20:57 ` [PATCH v8 4/6] x86/entry: Enable random_kstack_offset support Kees Cook
2021-03-30 20:57   ` Kees Cook
2021-03-31  7:50   ` Thomas Gleixner
2021-03-31  7:50     ` Thomas Gleixner
2021-03-30 20:57 ` [PATCH v8 5/6] arm64: entry: " Kees Cook
2021-03-30 20:57   ` Kees Cook
2021-03-30 20:57 ` [PATCH v8 6/6] lkdtm: Add REPORT_STACK for checking stack offsets Kees Cook
2021-03-30 20:57   ` Kees Cook
2021-04-01 19:17 ` [PATCH] Where we are for this patch? Roy Yang
2021-04-01 19:17   ` Roy Yang
2021-04-01 19:17   ` Roy Yang
2021-04-01 19:48   ` Al Viro
2021-04-01 19:48     ` Al Viro
2021-04-01 20:13     ` Theodore Ts'o
2021-04-01 20:13       ` Theodore Ts'o
2021-04-01 21:46   ` [PATCH v8 0/6] Optionally randomize kernel stack offset each syscall Kees Cook
2021-04-01 21:46     ` Kees Cook

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20210401083034.GA8554@willie-the-truck \
    --to=will@kernel.org \
    --cc=akpm@linux-foundation.org \
    --cc=alex.popov@linux.com \
    --cc=ard.biesheuvel@linaro.org \
    --cc=catalin.marinas@arm.com \
    --cc=corbet@lwn.net \
    --cc=david@redhat.com \
    --cc=elena.reshetova@intel.com \
    --cc=glider@google.com \
    --cc=jannh@google.com \
    --cc=keescook@chromium.org \
    --cc=kernel-hardening@lists.openwall.com \
    --cc=linux-arm-kernel@lists.infradead.org \
    --cc=linux-hardening@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-mm@kvack.org \
    --cc=luto@kernel.org \
    --cc=mark.rutland@arm.com \
    --cc=peterz@infradead.org \
    --cc=rdunlap@infradead.org \
    --cc=rppt@linux.ibm.com \
    --cc=tglx@linutronix.de \
    --cc=vbabka@suse.cz \
    --cc=x86@kernel.org \
    --subject='Re: [PATCH v8 3/6] stack: Optionally randomize kernel stack offset each syscall' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.