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

[Yu, Yu-cheng]

On 9/28/2020 10:37 AM, Andy Lutomirski wrote:
> On Mon, Sep 28, 2020 at 9:59 AM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote:
> > 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:
> > +
> > +               cet = get_xsave_addr(&fpu->state.xsave, XFEATURE_CET_USER);
> > +               if (!cet) {
> > +                       /*
> > +                        * This is an unlikely case where the task is
> > +                        * CET-enabled, but CET xstate is in INIT.
> > +                        */
> > +                       WARN_ONCE(1, "CET is enabled, but no xstates");
>
> "unlikely" doesn't really cover this.
>
> > +                       fpregs_unlock();
> > +                       goto sigsegv;
> > +               }
> > +
> > +               if (cet->user_ssp && ((cet->user_ssp + 8) < TASK_SIZE_MAX))
> > +                       cet->user_ssp += 8;
>
> This looks buggy.  The condition should be "if SHSTK is on, then add 8
> to user_ssp".  If the result is noncanonical, then some appropriate
> exception should be generated, probably by the FPU restore code -- see
> below.  You should be checking the SHSTK_EN bit, not SSP.

The code now checks if shadow stack is on (yes, it should check SHSTK_EN 
bit, I will fix it.), then adds 8 to user_ssp.  If the result is 
canonical, then it sets the corresponding xstate.

If the resulting address is not canonical, the kernel does not know what 
the address should be either.  I think the best action to take is doing 
nothing about the shadow stack pointer, and let the application return 
and get a control protection fault.  The application should have not got 
into such situation in the first place; if it does, it should fault.

> Also, can you point me to where any of these canonicality rules are
> documented in the SDM?  I looked and I can't find them.

The SDM is not very explicit.  It should have been.

> This reminds me: this code in extable.c needs to change.
>
> __visible bool ex_handler_fprestore(const struct exception_table_entry *fixup,
>                                      struct pt_regs *regs, int trapnr,
>                                      unsigned long error_code,
>                                      unsigned long fault_addr)
> {
>          regs->ip = ex_fixup_addr(fixup);
>
>          WARN_ONCE(1, "Bad FPU state detected at %pB, reinitializing
> FPU registers.",
>                    (void *)instruction_pointer(regs));
>
>          __copy_kernel_to_fpregs(&init_fpstate, -1);
>
> Now that we have supervisor states like CET, this is buggy.  This
> should do something intelligent like initializing all the *user* state
> and trying again.  If that succeeds, a signal should be sent rather
> than just corrupting the task.  And if it fails, then perhaps some
> actual intelligence is needed.  We certainly should not just disable
> CET because something is wrong with the CET MSRs.

Yes, but it needs more thought.  Maybe a separate patch and more discussion?

Yu-cheng