Re: [PATCH v26 23/30] x86/cet/shstk: Handle thread shadow stack

Borislav Petkov
2021-05-10T14:17:52Z

On Tue, Apr 27, 2021 at 01:43:08PM -0700, Yu-cheng Yu wrote:
> @@ -181,6 +184,12 @@ int copy_thread(unsigned long clone_flags, unsigned long sp, unsigned long arg,
> if (clone_flags & CLONE_SETTLS)
> ret = set_new_tls(p, tls);
>
> +#ifdef CONFIG_X86_64

IS_ENABLED

> + /* Allocate a new shadow stack for pthread */
> + if (!ret)
> + ret = shstk_setup_thread(p, clone_flags, stack_size);
> +#endif
> +

And why is this addition here...

> if (!ret && unlikely(test_tsk_thread_flag(current, TIF_IO_BITMAP)))
> io_bitmap_share(p);

... instead of here?

<---

>
> diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
> index c815c7507830..d387df84b7f1 100644
> --- a/arch/x86/kernel/shstk.c
> +++ b/arch/x86/kernel/shstk.c
> @@ -70,6 +70,55 @@ int shstk_setup(void)
> return 0;
> }

> +int shstk_setup_thread(struct task_struct *tsk, unsigned long clone_flags,

Judging by what this function does, its name wants to be

shstk_alloc_thread_stack()

or so?

> + unsigned long stack_size)
> +{
> + unsigned long addr, size;
> + struct cet_user_state *state;
> + struct cet_status *cet = &tsk->thread.cet;

The tip-tree preferred ordering of variable declarations at the
beginning of a function is reverse fir tree order::

struct long_struct_name *descriptive_name;
unsigned long foo, bar;
unsigned int tmp;
int ret;

The above is faster to parse than the reverse ordering::

int ret;
unsigned int tmp;
unsigned long foo, bar;
struct long_struct_name *descriptive_name;

And even more so than random ordering::

unsigned long foo, bar;
int ret;
struct long_struct_name *descriptive_name;
unsigned int tmp;

> +
> + if (!cet->shstk_size)
> + return 0;
> +

This check needs a comment.

> + if ((clone_flags & (CLONE_VFORK | CLONE_VM)) != CLONE_VM)
> + return 0;
> +
> + state = get_xsave_addr(&tsk->thread.fpu.state.xsave,
> + XFEATURE_CET_USER);

Let that line stick out.

> +
> + if (!state)
> + return -EINVAL;
> +
> + if (stack_size == 0)

if (!stack_size)

> + return -EINVAL;

and that test needs to be done first in the function.

> +
> + /* Cap shadow stack size to 4 GB */

Why?

> + size = min_t(unsigned long long, rlimit(RLIMIT_STACK), SZ_4G);
> + size = min(size, stack_size);
> +
> + /*
> + * Compat-mode pthreads share a limited address space.
> + * If each function call takes an average of four slots
> + * stack space, allocate 1/4 of stack size for shadow stack.
> + */
> + if (in_compat_syscall())
> + size /= 4;

<---- newline here.

> + size = round_up(size, PAGE_SIZE);
> + addr = alloc_shstk(size);
> +

^ Superfluous newline.

> + if (IS_ERR_VALUE(addr)) {
> + cet->shstk_base = 0;
> + cet->shstk_size = 0;
> + return PTR_ERR((void *)addr);
> + }
> +
> + fpu__prepare_write(&tsk->thread.fpu);
> + state->user_ssp = (u64)(addr + size);

cet_user_state has u64, cet_status has unsigned longs. Make them all u64.

And since cet_status is per thread, but I had suggested struct
shstk_desc, I think now that that should be called

struct thread_shstk

or so to denote *exactly* what it is.

Thx.

--
Regards/Gruss,
Boris.