Messages in this thread | | | Subject | Re: [NEEDS-REVIEW] Re: [PATCH v11 25/25] x86/cet/shstk: Add arch_prctl functions for shadow stack | From | "Yu, Yu-cheng" <> | Date | Mon, 14 Sep 2020 13:44:50 -0700 |
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On 9/14/2020 11:31 AM, Andy Lutomirski wrote: >> On Sep 14, 2020, at 7:50 AM, Dave Hansen <dave.hansen@intel.com> 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)?
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