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From:   Jann Horn <jannh@google.com>
Date:   Mon, 22 Jun 2020 22:07:37 +0200
Message-ID: <CAG48ez0pRtMZs3Hc3R2+XGHRwt9nZAGZu6vDpPBMbE+Askr_+Q@mail.gmail.com>
Subject: Re: [PATCH v4 3/5] stack: Optionally randomize kernel stack offset
 each syscall
To:     Kees Cook <keescook@chromium.org>
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>
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On Mon, Jun 22, 2020 at 9:31 PM Kees Cook <keescook@chromium.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:

$ 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.)

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).