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Date: Mon, 22 Jun 2020 14:30:17 -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 10:07:37PM +0200, Jann Horn wrote:
> On Mon, Jun 22, 2020 at 9:31 PM Kees Cook <keescook@...omium.org> 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.
> [...]
> > +#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(offset & 0x3FF); \
> > + asm volatile("" : "=m"(*ptr)); \
> > + } \
> > +} while (0)
>
> clang generates better code here if the mask is stack-aligned -
> otherwise it needs to round the stack pointer / the offset:
Interesting. I was hoping to avoid needing to know the architecture
stack alignment (leaving it up to the compiler).
>
> $ cat alloca_align.c
> #include <alloca.h>
> void callee(void);
>
> void alloca_blah(unsigned long rand) {
> asm volatile(""::"r"(alloca(rand & MASK)));
> callee();
> }
> $ clang -O3 -c -o alloca_align.o alloca_align.c -DMASK=0x3ff
> $ objdump -d alloca_align.o
> [...]
> 0: 55 push %rbp
> 1: 48 89 e5 mov %rsp,%rbp
> 4: 81 e7 ff 03 00 00 and $0x3ff,%edi
> a: 83 c7 0f add $0xf,%edi
> d: 83 e7 f0 and $0xfffffff0,%edi
> 10: 48 89 e0 mov %rsp,%rax
> 13: 48 29 f8 sub %rdi,%rax
> 16: 48 89 c4 mov %rax,%rsp
> 19: e8 00 00 00 00 callq 1e <alloca_blah+0x1e>
> 1e: 48 89 ec mov %rbp,%rsp
> 21: 5d pop %rbp
> 22: c3 retq
> $ clang -O3 -c -o alloca_align.o alloca_align.c -DMASK=0x3f0
> $ objdump -d alloca_align.o
> [...]
> 0: 55 push %rbp
> 1: 48 89 e5 mov %rsp,%rbp
> 4: 48 89 e0 mov %rsp,%rax
> 7: 81 e7 f0 03 00 00 and $0x3f0,%edi
> d: 48 29 f8 sub %rdi,%rax
> 10: 48 89 c4 mov %rax,%rsp
> 13: e8 00 00 00 00 callq 18 <alloca_blah+0x18>
> 18: 48 89 ec mov %rbp,%rsp
> 1b: 5d pop %rbp
> 1c: c3 retq
> $
>
> (From a glance at the assembly, gcc seems to always assume that the
> length may be misaligned.)
Right -- this is why I didn't bother with it, since it didn't seem to
notice what I'd already done to the alloca() argument. (But from what I
could measure on cycle counts, the additional ALU didn't seem to really
make much difference ... it _would_ be nice to avoid it, of course.)
> Maybe this should be something along the lines of
> __builtin_alloca(offset & (0x3ff & ARCH_STACK_ALIGN_MASK)) (with
> appropriate definitions of the stack alignment mask depending on the
> architecture's choice of stack alignment for kernel code).
Is that explicitly selected anywhere in the kernel? I thought the
alignment was left up to the compiler (as in I've seen bugs fixed where
the kernel had to deal with the alignment choices the compiler was
making...)
--
Kees Cook
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