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

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Message-ID: <88261152-2de1-fe8d-7ab0-acb108e97e04@intel.com>
Date:   Thu, 3 Sep 2020 09:09:01 -0700
From:   "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
To:     Dave Hansen <dave.hansen@...el.com>,
        Andy Lutomirski <luto@...capital.net>
Cc:     Jann Horn <jannh@...gle.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        kernel list <linux-kernel@...r.kernel.org>,
        linux-doc@...r.kernel.org, Linux-MM <linux-mm@...ck.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H.J. Lu" <hjl.tools@...il.com>, Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Dave Martin <Dave.Martin@....com>,
        Weijiang Yang <weijiang.yang@...el.com>
Subject: Re: [PATCH v11 6/9] x86/cet: Add PTRACE interface for CET

On 9/3/2020 7:26 AM, Dave Hansen wrote:
> On 9/2/20 9:35 PM, Andy Lutomirski wrote:
>>>>>>> +       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"?
>> Having read the code, I’m unconvinced. It looks like a get_xsave_addr() failure means “state not saved; task sees INIT state”.  So *maybe* it’s reasonable -ENODEV this, but I’m not really convinced. I tend to think we should return the actual INIT state and that we should permit writes and handle them correctly.
> 
> PTRACE is asking for access to the values in the *registers*, not for
> the value in the kernel XSAVE buffer.  We just happen to only have the
> kernel XSAVE buffer around.

When get_xsave_addr() returns NULL, there are three possibilities:
- XSAVE is not enabled or not supported;
- The kernel does not support the requested feature;
- The requested feature is in INIT state.

If the debugger is going to write an MSR, only in the third case would 
this make a slight sense.  For example, if the system has CET enabled, 
but the task does not have CET enabled, and GDB is writing to a CET MSR. 
  But still, this is strange to me.

> 
> If we want to really support PTRACE we have to allow the registers to be
> get/set, regardless of what state they are in, INIT state or not.  So,
> yeah I agree with Andy.
> 

GDB does not have a WRMSR mechanism.  If GDB is going to write an MSR, 
it will call arch_prctl or an assembly routine in memory.

Yu-cheng