Re: [PATCH v2 2/2] stack: Constrain and fix stack offset randomization with Clang builds

[Kees Cook <keescook@chromium.org>]

On Mon, Jan 31, 2022 at 10:05:21AM +0100, Marco Elver wrote:
> All supported versions of Clang perform auto-init of __builtin_alloca()
> when stack auto-init is on (CONFIG_INIT_STACK_ALL_{ZERO,PATTERN}).
> 
> add_random_kstack_offset() uses __builtin_alloca() to add a stack
> offset. This means, when CONFIG_INIT_STACK_ALL_{ZERO,PATTERN} is
> enabled, add_random_kstack_offset() will auto-init that unused portion
> of the stack used to add an offset.
> 
> There are several problems with this:
> 
> 	1. These offsets can be as large as 1023 bytes. Performing
> 	   memset() on them isn't exactly cheap, and this is done on
> 	   every syscall entry.
> 
> 	2. Architectures adding add_random_kstack_offset() to syscall
> 	   entry implemented in C require them to be 'noinstr' (e.g. see
> 	   x86 and s390). The potential problem here is that a call to
> 	   memset may occur, which is not noinstr.
> 
> A x86_64 defconfig kernel with Clang 11 and CONFIG_VMLINUX_VALIDATION shows:
> 
>  | vmlinux.o: warning: objtool: do_syscall_64()+0x9d: call to memset() leaves .noinstr.text section
>  | vmlinux.o: warning: objtool: do_int80_syscall_32()+0xab: call to memset() leaves .noinstr.text section
>  | vmlinux.o: warning: objtool: __do_fast_syscall_32()+0xe2: call to memset() leaves .noinstr.text section
>  | vmlinux.o: warning: objtool: fixup_bad_iret()+0x2f: call to memset() leaves .noinstr.text section
> 
> Clang 14 (unreleased) will introduce a way to skip alloca initialization
> via __builtin_alloca_uninitialized() (https://reviews.llvm.org/D115440).
> 
> Constrain RANDOMIZE_KSTACK_OFFSET to only be enabled if no stack
> auto-init is enabled, the compiler is GCC, or Clang is version 14+. Use
> __builtin_alloca_uninitialized() if the compiler provides it, as is done
> by Clang 14.
> 
> Link: https://lkml.kernel.org/r/YbHTKUjEejZCLyhX@elver.google.com
> Fixes: 39218ff4c625 ("stack: Optionally randomize kernel stack offset each syscall")
> Signed-off-by: Marco Elver <elver@google.com>

Thanks for the tweaks; this looks good to me now.

Acked-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook