Re: [PATCH v4 3/5] stack: Optionally randomize kernel stack offset each syscall

[Jann Horn]

On Mon, Jun 22, 2020 at 11:30 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> 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@xxxxxxxxxxxx> 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:
[...]
> > 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...)

No, at least on x86-64 and x86 Linux overrides the normal ABI. From
arch/x86/Makefile:

# 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

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.