Re: [PATCH v11 6/9] x86/cet: Add PTRACE interface for CET

["Yu, Yu-cheng" <yu-cheng.yu@intel.com>]

On 9/2/2020 5:33 PM, Jann Horn wrote:
> On Thu, Sep 3, 2020 at 12:13 AM Yu, Yu-cheng <yu-cheng.yu@intel.com> wrote:
>> On 9/2/2020 1:03 PM, Jann Horn wrote:
>>> On Tue, Aug 25, 2020 at 2:30 AM Yu-cheng Yu <yu-cheng.yu@intel.com> wrote:
>>>> Add REGSET_CET64/REGSET_CET32 to get/set CET MSRs:
>>>>
>>>>       IA32_U_CET (user-mode CET settings) and
>>>>       IA32_PL3_SSP (user-mode Shadow Stack)
>>> [...]
>>>> diff --git a/arch/x86/kernel/fpu/regset.c b/arch/x86/kernel/fpu/regset.c
>>> [...]
>>>> +int cetregs_get(struct task_struct *target, const struct user_regset *regset,
>>>> +               struct membuf to)
>>>> +{
>>>> +       struct fpu *fpu = &target->thread.fpu;
>>>> +       struct cet_user_state *cetregs;
>>>> +
>>>> +       if (!boot_cpu_has(X86_FEATURE_SHSTK))
>>>> +               return -ENODEV;
>>>> +
>>>> +       fpu__prepare_read(fpu);
>>>> +       cetregs = get_xsave_addr(&fpu->state.xsave, XFEATURE_CET_USER);
>>>> +       if (!cetregs)
>>>> +               return -EFAULT;
>>>
>>> Can this branch ever be hit without a kernel bug? If yes, I think
>>> -EFAULT is probably a weird error code to choose here. If no, this
>>> should probably use WARN_ON(). Same thing in cetregs_set().
>>>
>>
>> When a thread is not CET-enabled, its CET state does not exist.  I
>> looked at EFAULT, and it means "Bad address".  Maybe this can be ENODEV,
>> which means "No such device"?
> 
> Yeah, I guess ENODEV might fit reasonably well.
> 

I will update it.  Thanks!