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

Yu, Yu-cheng
2021-05-10T22:58:09Z

On 5/10/2021 7:15 AM, Borislav Petkov wrote:
> On Tue, Apr 27, 2021 at 01:43:08PM -0700, Yu-cheng Yu wrote:

[...]

>> 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?
>

This is not necessary. I will make it just stack_size, which is passed
in from copy_thread().

>> + 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.
>

So this struct will be:

struct thread_shstk {
u64 shstk_base;
u64 shstk_size;
u64 locked:1;
u64 ibt:1;
};

Ok?

Thanks,
Yu-cheng