Re: [PATCH 2/2] stack: Constrain stack offset randomization with Clang builds

[Marco Elver <elver@google.com>]

On Fri, 28 Jan 2022 at 19:55, Nathan Chancellor <nathan@kernel.org> wrote:
[...]
>
> Reviewed-by: Nathan Chancellor <nathan@kernel.org>
>
> One comment below.

Thanks!

Though with Kees's requested changes I'll probably let you re-review it.

> > ---
> >  arch/Kconfig                     |  1 +
> >  include/linux/randomize_kstack.h | 14 ++++++++++++--
> >  2 files changed, 13 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/Kconfig b/arch/Kconfig
> > index 2cde48d9b77c..c5b50bfe31c1 100644
> > --- a/arch/Kconfig
> > +++ b/arch/Kconfig
> > @@ -1163,6 +1163,7 @@ config RANDOMIZE_KSTACK_OFFSET
> >       bool "Support for randomizing kernel stack offset on syscall entry" if EXPERT
> >       default y
> >       depends on HAVE_ARCH_RANDOMIZE_KSTACK_OFFSET
> > +     depends on INIT_STACK_NONE || !CC_IS_CLANG || CLANG_VERSION >= 140000
> >       help
> >         The kernel stack offset can be randomized (after pt_regs) by
> >         roughly 5 bits of entropy, frustrating memory corruption
> > diff --git a/include/linux/randomize_kstack.h b/include/linux/randomize_kstack.h
> > index 91f1b990a3c3..5c711d73ed10 100644
> > --- a/include/linux/randomize_kstack.h
> > +++ b/include/linux/randomize_kstack.h
> > @@ -17,8 +17,18 @@ DECLARE_PER_CPU(u32, kstack_offset);
> >   * alignment. Also, since this use is being explicitly masked to a max of
> >   * 10 bits, stack-clash style attacks are unlikely. For more details see
> >   * "VLAs" in Documentation/process/deprecated.rst
> > + *
> > + * The normal alloca() can be initialized with INIT_STACK_ALL. Initializing the
> > + * unused area on each syscall entry is expensive, and generating an implicit
> > + * call to memset() may also be problematic (such as in noinstr functions).
> > + * Therefore, if the compiler provides it, use the "uninitialized" variant.
> >   */
> > -void *__builtin_alloca(size_t size);
>
> Is it okay to remove the declaration? Why was it even added in the first
> place (Kees)?

Declaring __builtins is redundant for as long as I remember.