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

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Date:   Thu, 3 Sep 2020 02:33:53 +0200
From:   Jann Horn <jannh@...gle.com>
To:     "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
Cc:     "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 Thu, Sep 3, 2020 at 12:13 AM Yu, Yu-cheng <yu-cheng.yu@...el.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@...el.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.