Re: [NEEDS-REVIEW] Re: [PATCH v11 25/25] x86/cet/shstk: Add arch_prctl functions for shadow stack

[Yu, Yu-cheng]

On 9/14/2020 11:31 AM, Andy Lutomirski wrote:
> > On Sep 14, 2020, at 7:50 AM, Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
> >
> > On 9/11/20 3:59 PM, Yu-cheng Yu wrote:
> > ...
> > > Here are the changes if we take the mprotect(PROT_SHSTK) approach.
> > > Any comments/suggestions?
> >
> > I still don't like it. :)
> >
> > I'll also be much happier when there's a proper changelog to accompany
> > this which also spells out the alternatives any why they suck so much.
>
> Let’s take a step back here. Ignoring the precise API, what exactly is
> a shadow stack from the perspective of a Linux user program?
>
> The simplest answer is that it’s just memory that happens to have
> certain protections.  This enables all kinds of shenanigans.  A
> program could map a memfd twice, once as shadow stack and once as
> non-shadow-stack, and change its control flow.  Similarly, a program
> could mprotect its shadow stack, modify it, and mprotect it back.  In

What if we do the following:

- If the mapping has VM_SHARED, it cannot be turned to shadow stack. 
Shadow stack cannot be shared anyway.

- Only allow an anonymous mapping to be converted to shadow stack, but 
not the other way.

> some threat models, though could be seen as a WRSS bypass.  (Although
> if an attacker can coerce a process to call mprotect(), the game is
> likely mostly over anyway.)
>
> But we could be more restrictive, or perhaps we could allow user code
> to opt into more restrictions.  For example, we could have shadow
> stacks be special memory that cannot be written from usermode by any
> means other than ptrace() and friends, WRSS, and actual shadow stack
> usage.
>
> What is the goal?

There primary goal is to allocate/mmap a shadow stack from user space.

> No matter what we do, the effects of calling vfork() are going to be a
> bit odd with SHSTK enabled.  I suppose we could disallow this, but
> that seems likely to cause its own issues.

Do you mean vfork() has issues with call/return?  That is taken care of 
in GLIBC.  Or do you mean it has issues with mprotect(PROT_SHSTK)?