Re: [RFC PATCH v9 01/27] Documentation/x86: Add CET description

Yu-cheng Yu
2020-03-09T19:27:32Z

On Mon, 2020-03-09 at 10:21 -0700, Dave Hansen wrote:
> On 3/9/20 10:00 AM, Yu-cheng Yu wrote:
> > On Wed, 2020-02-26 at 09:57 -0800, Dave Hansen wrote:
> > > > index ade4e6ec23e0..8b69ebf0baed 100644
> > > > --- a/Documentation/admin-guide/kernel-parameters.txt
> > > > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > > > @@ -3001,6 +3001,12 @@
> > > > noexec=on: enable non-executable mappings (default)
> > > > noexec=off: disable non-executable mappings
> > > >
> > > > + no_cet_shstk [X86-64] Disable Shadow Stack for user-mode
> > > > + applications
> > >
> > > If we ever add kernel support, "no_cet_shstk" will mean "no cet shstk
> > > for userspace"?
> >
> > What about no_user_shstk, no_kernel_shstk?

[...]

> > > > +Note:
> > > > + There is no CET-enabling arch_prctl function. By design, CET is
> > > > + enabled automatically if the binary and the system can support it.
> > >
> > > This is kinda interesting. It means that a JIT couldn't choose to
> > > protect the code it generates and have different rules from itself?
> >
> > JIT needs to be updated for CET first. Once that is done, it runs with CET
> > enabled. It can use the NOTRACK prefix, for example.
>
> Am I missing something?
>
> What's the direct connection between shadow stacks and Indirect Branch
> Tracking other than Intel marketing umbrellas?

What I meant is that JIT code needs to be updated first; if it skips RETs,
it needs to unwind the stack, and if it does indirect JMPs somewhere it
needs to fix up the branch target or use NOTRACK.

> > > > + The parameters passed are always unsigned 64-bit. When an IA32
> > > > + application passing pointers, it should only use the lower 32 bits.
> > >
> > > Won't a 32-bit app calling prctl() use the 32-bit ABI? How would it
> > > even know it's running on a 64-bit kernel?
> >
> > The 32-bit app is passing only a pointer to an array of 64-bit numbers.
>
> Well, the documentation just talked about pointers and I naively assume
> it means the "unsigned long *" you had in there.
>
> Rather than make suggestions, just say that the ABI is universally
> 64-bit. Saying that the pointers must be valid is just kinda silly.
> It's also not 100% clear what an "IA32 application" *MEANS* given fun
> things like x32.

Ok, I will update the text.

>
> Also, I went to go find this implementation in your series. I couldn't
> find it. Did I miss a patch? Or are you documenting things you didn't
> even implement?

In patch #27: Add arch_prctl functions for Shadow Stack.

Yu-cheng