From: Kees Cook <keescook@chromium.org>
To: Jann Horn <jannh@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
Elena Reshetova <elena.reshetova@intel.com>,
the arch/x86 maintainers <x86@kernel.org>,
Andy Lutomirski <luto@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Catalin Marinas <catalin.marinas@arm.com>,
Will Deacon <will@kernel.org>,
Mark Rutland <mark.rutland@arm.com>,
Alexander Potapenko <glider@google.com>,
Alexander Popov <alex.popov@linux.com>,
Ard Biesheuvel <ard.biesheuvel@linaro.org>,
Kernel Hardening <kernel-hardening@lists.openwall.com>,
Linux ARM <linux-arm-kernel@lists.infradead.org>,
Linux-MM <linux-mm@kvack.org>,
kernel list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v4 3/5] stack: Optionally randomize kernel stack offset each syscall
Date: Mon, 22 Jun 2020 15:04:40 -0700 [thread overview]
Message-ID: <202006221451.2E80C90FF7@keescook> (raw)
In-Reply-To: <CAG48ez1b_wMkQGj+z=dWSVctikzzw72V3SPexEPm3Aw8LrXGWQ@mail.gmail.com>
On Mon, Jun 22, 2020 at 11:42:29PM +0200, Jann Horn wrote:
> No, at least on x86-64 and x86 Linux overrides the normal ABI. From
> arch/x86/Makefile:
Ah! Thanks for the pointer.
>
> # For gcc stack alignment is specified with -mpreferred-stack-boundary,
> # clang has the option -mstack-alignment for that purpose.
> ifneq ($(call cc-option, -mpreferred-stack-boundary=4),)
> cc_stack_align4 := -mpreferred-stack-boundary=2
> cc_stack_align8 := -mpreferred-stack-boundary=3
> else ifneq ($(call cc-option, -mstack-alignment=16),)
> cc_stack_align4 := -mstack-alignment=4
> cc_stack_align8 := -mstack-alignment=8
> endif
> [...]
> ifeq ($(CONFIG_X86_32),y)
> [...]
> # Align the stack to the register width instead of using the default
> # alignment of 16 bytes. This reduces stack usage and the number of
> # alignment instructions.
> KBUILD_CFLAGS += $(call cc-option,$(cc_stack_align4))
> [...]
> else
> [...]
> # By default gcc and clang use a stack alignment of 16 bytes for x86.
> # However the standard kernel entry on x86-64 leaves the stack on an
> # 8-byte boundary. If the compiler isn't informed about the actual
> # alignment it will generate extra alignment instructions for the
> # default alignment which keep the stack *mis*aligned.
> # Furthermore an alignment to the register width reduces stack usage
> # and the number of alignment instructions.
> KBUILD_CFLAGS += $(call cc-option,$(cc_stack_align8))
> [...]
> endif
And it seems that only x86 does this. No other architecture specifies
-mpreferred-stack-boundary...
> Normal x86-64 ABI has 16-byte stack alignment; Linux kernel x86-64 ABI
> has 8-byte stack alignment.
> Similarly, the normal Linux 32-bit x86 ABI is 16-byte aligned;
> meanwhile Linux kernel x86 ABI has 4-byte stack alignment.
>
> This is because userspace code wants the stack to be sufficiently
> aligned for fancy SSE instructions and such; the kernel, on the other
> hand, never uses those in normal code, and cares about stack usage and
> such very much.
This makes it nicer for Clang:
diff --git a/include/linux/randomize_kstack.h b/include/linux/randomize_kstack.h
index 1df0dc52cadc..f7e1f68fb50c 100644
--- a/include/linux/randomize_kstack.h
+++ b/include/linux/randomize_kstack.h
@@ -10,6 +10,14 @@ DECLARE_STATIC_KEY_MAYBE(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,
randomize_kstack_offset);
DECLARE_PER_CPU(u32, kstack_offset);
+#ifdef CONFIG_X86_64
+#define ARCH_STACK_ALIGN_MASK ~((1 << 8) - 1)
+#elif defined(CONFIG_X86_32)
+#define ARCH_STACK_ALIGN_MASK ~((1 << 4) - 1)
+#else
+#define ARCH_STACK_ALIGN_MASK ~(0)
+#endif
+
/*
* 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
@@ -23,7 +31,8 @@ void *__builtin_alloca(size_t size);
if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \
&randomize_kstack_offset)) { \
u32 offset = this_cpu_read(kstack_offset); \
- u8 *ptr = __builtin_alloca(offset & 0x3FF); \
+ u8 *ptr = __builtin_alloca(offset & 0x3FF & \
+ ARCH_STACK_ALIGN_MASK); \
asm volatile("" : "=m"(*ptr)); \
} \
} while (0)
But I don't like open-coding the x86-ony stack alignment... it should be
in Kconfig or something, I think?
--
Kees Cook
next prev parent reply other threads:[~2020-06-22 22:04 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-22 19:31 [PATCH v4 0/5] Optionally randomize kernel stack offset each syscall Kees Cook
2020-06-22 19:31 ` [PATCH v4 1/5] jump_label: Provide CONFIG-driven build state defaults Kees Cook
2020-06-22 19:31 ` [PATCH v4 2/5] init_on_alloc: Unpessimize default-on builds Kees Cook
2020-06-22 19:31 ` [PATCH v4 3/5] stack: Optionally randomize kernel stack offset each syscall Kees Cook
2020-06-22 19:40 ` Randy Dunlap
2020-06-22 21:26 ` Kees Cook
2020-06-22 20:07 ` Jann Horn
2020-06-22 21:30 ` Kees Cook
2020-06-22 21:42 ` Jann Horn
2020-06-22 22:04 ` Kees Cook [this message]
2020-06-22 22:56 ` Arvind Sankar
2020-06-22 23:07 ` Kees Cook
2020-06-23 0:05 ` Arvind Sankar
2020-06-23 0:56 ` Kees Cook
2020-06-23 13:42 ` David Laight
2020-06-23 12:38 ` Alexander Popov
2020-06-22 19:31 ` [PATCH v4 4/5] x86/entry: Enable random_kstack_offset support Kees Cook
2020-06-22 19:31 ` [PATCH v4 5/5] arm64: entry: " Kees Cook
2020-06-23 9:40 ` Mark Rutland
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=202006221451.2E80C90FF7@keescook \
--to=keescook@chromium.org \
--cc=alex.popov@linux.com \
--cc=ard.biesheuvel@linaro.org \
--cc=catalin.marinas@arm.com \
--cc=elena.reshetova@intel.com \
--cc=glider@google.com \
--cc=jannh@google.com \
--cc=kernel-hardening@lists.openwall.com \
--cc=linux-arm-kernel@lists.infradead.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=tglx@linutronix.de \
--cc=will@kernel.org \
--cc=x86@kernel.org \
/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
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).