randomize_kstack: To init or not to init?

Marco Elver
2021-12-09T09:58:10Z

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.

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.

C. Introduce a new __builtin_alloca_uninitialized().

I think #C would be the most robust solution, but means this would
remain as-is for a while.

Preferences?

Thanks,
-- Marco