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

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Message-ID: <CALCETrXJkXXDF=tdu3KBHgzDO+E-HhqMk9Ttixgk4WX_PLPDJw@mail.gmail.com>
Date:   Thu, 3 Sep 2020 09:32:36 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        "Yu, Yu-cheng" <yu-cheng.yu@...el.com>,
        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>,
        "open list:DOCUMENTATION" <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>,
        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 Thu, Sep 3, 2020 at 9:25 AM Dave Hansen <dave.hansen@...el.com> wrote:
>
> On 9/3/20 9:15 AM, Andy Lutomirski wrote:
> > On Thu, Sep 3, 2020 at 9:12 AM Dave Hansen <dave.hansen@...el.com> wrote:
> >>
> >> On 9/3/20 9:09 AM, Yu, Yu-cheng wrote:
> >>> 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 this is strange, then why do we even _implement_ writes?
> >
> > Well, if gdb wants to force a tracee to return early from a function,
> > wouldn't it want the ability to modify SSP?
>
> That's true.
>
> Yu-cheng, can you take a look through and see for the other setregs
> users what they do for logically crazy, strange things?  Is there
> precedent for refusing a write which is possible but illogical or
> strange?  If so, which error code do they use?
>
> Taking the config register out of the init state is illogical, as is
> writing to SSP while the config register is in its init state.

What's so special about the INIT state?  It's optimized by XSAVES, but
it's just a number, right?  So taking the register out of the INIT
state is kind of like saying "gdb wanted to set xmm0 to (0,0,0,1), but
it was in the INIT state to begin with", right?