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Message-ID: <202006221451.2E80C90FF7@keescook>
Date: Mon, 22 Jun 2020 15:04:40 -0700
From: Kees Cook <keescook@...omium.org>
To: Jann Horn <jannh@...gle.com>
Cc: Thomas Gleixner <tglx@...utronix.de>,
Elena Reshetova <elena.reshetova@...el.com>,
the arch/x86 maintainers <x86@...nel.org>,
Andy Lutomirski <luto@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
Catalin Marinas <catalin.marinas@....com>,
Will Deacon <will@...nel.org>, Mark Rutland <mark.rutland@....com>,
Alexander Potapenko <glider@...gle.com>,
Alexander Popov <alex.popov@...ux.com>,
Ard Biesheuvel <ard.biesheuvel@...aro.org>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>,
Linux ARM <linux-arm-kernel@...ts.infradead.org>,
Linux-MM <linux-mm@...ck.org>,
kernel list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 3/5] stack: Optionally randomize kernel stack offset
each syscall
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
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