Re: randomize_kstack: To init or not to init?

Kees Cook
2021-12-09T20:48:47Z

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. :(

--
Kees Cook