Re: randomize_kstack: To init or not to init?

[Marco Elver <elver@google.com>]

On Thu, 9 Dec 2021 at 21:48, Kees Cook <keescook@chromium.org> wrote:
>
> On Thu, Dec 09, 2021 at 10:58:01AM +0100, Marco Elver wrote:
> > Clang supports CONFIG_INIT_STACK_ALL_ZERO, which appears to be the
> > default since dcb7c0b9461c2, which is why this came on my radar. And
> > Clang also performs auto-init of allocas when auto-init is on
> > (https://reviews.llvm.org/D60548), with no way to skip. As far as I'm
> > aware, GCC 12's upcoming -ftrivial-auto-var-init= doesn't yet auto-init
> > allocas.
> >
> > 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 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
> >
> > Switching to INIT_STACK_ALL_NONE resolves the warnings as expected.
> >
> > To figure out what the right solution is, the first thing to figure out
> > is, do we actually want that offset portion of the stack to be
> > auto-init'd?
> >
> > There are several options:
> >
> >       A. Make memset (and probably all other mem-transfer functions)
> >          noinstr compatible, if that is even possible. This only solves
> >          problem #2.
>
> I'd agree: "A" isn't going to work well here.
>
> >
> >       B. A workaround could be using a VLA with
> >          __attribute__((uninitialized)), but requires some restructuring
> >          to make sure the VLA remains in scope and other trickery to
> >          convince the compiler to not give up that stack space.
>
> I was hoping the existing trickery would work for a VLA, but it seems
> not. It'd be nice if it could work with a VLA, which could just gain the
> attribute and we'd be done.
>
> >       C. Introduce a new __builtin_alloca_uninitialized().
>
> Hrm, this means conditional logic between compilers, too. :(

And as Segher just pointed out, I think Clang has a "bug" because
explicit alloca() calls aren't "automatic storage". I think Clang
needs a new -mllvm param.

Because I think making #B work is quite ugly and also brittle. :-/