Re: [PATCH v28 25/32] x86/cet/shstk: Handle thread shadow stack

Yu, Yu-cheng
2021-07-23T17:30:39Z

On 7/22/2021 2:05 PM, Dave Hansen wrote:
> On 7/22/21 1:52 PM, Yu-cheng Yu wrote:
>> + if (!stack_size)
>> + stack_size = min_t(unsigned long long, rlimit(RLIMIT_STACK), SZ_4G);
>> +
>> + if (!shstk->size)
>> + return 0;
>> +
>> + /*
>> + * For CLONE_VM, except vfork, the child needs a separate shadow
>> + * stack.
>> + */
>> + if ((clone_flags & (CLONE_VFORK | CLONE_VM)) != CLONE_VM)
>> + return 0;
>> +
>> + /*
>> + * This is in clone() syscall and fpu__copy() already copies xstates
>> + * from the parent. If get_xsave_addr() returns null, then XFEATURE_
>> + * CET_USER is still in init state, which certainly is an error.
>> + */
>> + state = get_xsave_addr(&tsk->thread.fpu.state.xsave, XFEATURE_CET_USER);
>> + if (!state)
>> + return -EINVAL;
>
> I don't care much for that comment.
>
> This code is meant to copy shadow stack config information into children
> when it is already enabled. We *just* checked for that above in the
> "shstk->size" check. The fact that this is called from clone() is
> irrelevant. The shadow stack enabling status *is*.
>
> I think I'd rather this be more along the lines of:
>
> /*
> * 'tsk' is configured with a shadow stack and the fpu.state is
> * up to date since it was just copied from the parent. There
> * must be a valid non-init CET state location in the buffer.
> */
>
> There is also a strong enough assumption violation that I'd probably
> WARN() in addition to returning -EINVAL.
>

Yes, I will update the comment and put in the WARN().

Thanks,
Yu-cheng