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

[Yu, Yu-cheng]

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@xxxxxxxxx> 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@xxxxxxxxx> 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!