Re: [PATCH v13 8/8] x86/vsyscall/64: Fixup Shadow Stack and Indirect Branch Tracking for vsyscall emulation

[Yu-cheng Yu]

On Fri, 2020-09-25 at 09:51 -0700, Andy Lutomirski wrote:
> > On Sep 25, 2020, at 9:48 AM, Yu, Yu-cheng <yu-cheng.yu@xxxxxxxxx> wrote:
> > 
> > On 9/25/2020 9:31 AM, Andy Lutomirski wrote:
> > > > On Fri, Sep 25, 2020 at 7:58 AM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote:
> > > > 
> > 
> > [...]
> > 
> > > > @@ -286,6 +289,37 @@ bool emulate_vsyscall(unsigned long error_code,
> > > >         /* Emulate a ret instruction. */
> > > >         regs->ip = caller;
> > > >         regs->sp += 8;
> > > > +
> > > > +#ifdef CONFIG_X86_CET
> > > > +       if (tsk->thread.cet.shstk_size || tsk->thread.cet.ibt_enabled) {
> > > > +               struct cet_user_state *cet;
> > > > +               struct fpu *fpu;
> > > > +
> > > > +               fpu = &tsk->thread.fpu;
> > > > +               fpregs_lock();
> > > > +
> > > > +               if (!test_thread_flag(TIF_NEED_FPU_LOAD)) {
> > > > +                       copy_fpregs_to_fpstate(fpu);
> > > > +                       set_thread_flag(TIF_NEED_FPU_LOAD);
> > > > +               }
> > > > +
> > > > +               cet = get_xsave_addr(&fpu->state.xsave, XFEATURE_CET_USER);
> > > > +               if (!cet) {
> > > > +                       fpregs_unlock();
> > > > +                       goto sigsegv;
> > > I *think* your patchset tries to keep cet.shstk_size and
> > > cet.ibt_enabled in sync with the MSR, in which case it should be
> > > impossible to get here, but a comment and a warning would be much
> > > better than a random sigsegv.
> > 
> > Yes, it should be impossible to get here.  I will add a comment and a warning, but still do sigsegv.  Should this happen, and the function return, the app gets a control-protection fault.  Why not let it fail early?
> 
> I’m okay with either approach as long as we get a comment and warning.
> 

Here is the updated patch.  I can also re-send the whole series as v14.  Thanks!

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