Re: [PATCH v2 4/5] x86/entry: Enable random_kstack_offset support

[Kees Cook <keescook@chromium.org>]

On Tue, Mar 24, 2020 at 01:32:30PM -0700, Kees Cook wrote:
> Allow for a randomized stack offset on a per-syscall basis, with roughly
> 5 bits of entropy.
> 
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  arch/x86/Kconfig        |  1 +
>  arch/x86/entry/common.c | 12 +++++++++++-
>  2 files changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index beea77046f9b..b9d449581eb6 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -150,6 +150,7 @@ config X86
>  	select HAVE_ARCH_TRANSPARENT_HUGEPAGE
>  	select HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD if X86_64
>  	select HAVE_ARCH_VMAP_STACK		if X86_64
> +	select HAVE_ARCH_RANDOMIZE_KSTACK_OFFSET
>  	select HAVE_ARCH_WITHIN_STACK_FRAMES
>  	select HAVE_ASM_MODVERSIONS
>  	select HAVE_CMPXCHG_DOUBLE
> diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
> index 9747876980b5..086d7af570af 100644
> --- a/arch/x86/entry/common.c
> +++ b/arch/x86/entry/common.c
> @@ -26,6 +26,7 @@
>  #include <linux/livepatch.h>
>  #include <linux/syscalls.h>
>  #include <linux/uaccess.h>
> +#include <linux/randomize_kstack.h>
>  
>  #include <asm/desc.h>
>  #include <asm/traps.h>
> @@ -189,6 +190,13 @@ __visible inline void prepare_exit_to_usermode(struct pt_regs *regs)
>  	lockdep_assert_irqs_disabled();
>  	lockdep_sys_exit();
>  
> +	/*
> +	 * x86_64 stack alignment means 3 bits are ignored, so keep
> +	 * the top 5 bits. x86_32 needs only 2 bits of alignment, so
> +	 * the top 6 bits will be used.
> +	 */
> +	choose_random_kstack_offset(rdtsc() & 0xFF);
> +
>  	cached_flags = READ_ONCE(ti->flags);
>  
>  	if (unlikely(cached_flags & EXIT_TO_USERMODE_LOOP_FLAGS))
> @@ -283,6 +291,7 @@ __visible void do_syscall_64(unsigned long nr, struct pt_regs *regs)
>  {
>  	struct thread_info *ti;
>  
> +	add_random_kstack_offset();
>  	enter_from_user_mode();
>  	local_irq_enable();
>  	ti = current_thread_info();

So, I got an email from 0day that this caused a performance regression[1]
with things _turned off_. On closer examination:

Before (objdump -dS vmlinux):

__visible void do_syscall_64(unsigned long nr, struct pt_regs *regs)
{
ffffffff81003920:       41 54                   push   %r12
ffffffff81003922:       55                      push   %rbp
ffffffff81003923:       48 89 f5                mov    %rsi,%rbp
ffffffff81003926:       53                      push   %rbx
ffffffff81003927:       48 89 fb                mov    %rdi,%rbx
        struct thread_info *ti;

        enter_from_user_mode();
        local_irq_enable();
...


After:

__visible void do_syscall_64(unsigned long nr, struct pt_regs *regs)
{ 
ffffffff81003960:       55                      push   %rbp
ffffffff81003961:       48 89 e5                mov    %rsp,%rbp
ffffffff81003964:       41 55                   push   %r13
ffffffff81003966:       41 54                   push   %r12
ffffffff81003968:       49 89 f4                mov    %rsi,%r12
ffffffff8100396b:       53                      push   %rbx
ffffffff8100396c:       48 89 fb                mov    %rdi,%rbx
ffffffff8100396f:       48 83 ec 08             sub    $0x8,%rsp
ffffffff81003973:       65 48 8b 04 25 28 00    mov    %gs:0x28,%rax
ffffffff8100397a:       00 00 
ffffffff8100397c:       48 89 45 e0             mov    %rax,-0x20(%rbp)
ffffffff81003980:       31 c0                   xor    %eax,%eax
        asm_volatile_goto("1:"
ffffffff81003982:       0f 1f 44 00 00          nopl   0x0(%rax,%rax,1)
        struct thread_info *ti;

        add_random_kstack_offset();
        enter_from_user_mode();
        local_irq_enable();

The "nopl" there is the static_branch code that I'd expect. However, the
preample is quite different. *drum roll* Anyone else recognize %gs:0x28?

That's the stack canary. :P It seems that GCC views this as an array:

                char *ptr = __builtin_alloca(offset & 0x3FF);
                asm volatile("" : "=m"(*ptr));

because it's locally allocated on the stack. *face palm*

I'll go figure out a way to fix this...

-Kees

[1] https://lore.kernel.org/lkml/202003281505.0F481D3@keescook/

-- 
Kees Cook