Re: [PATCH v3 35/37] x86/cet: Add PTRACE interface for CET

[Edgecombe, Rick P]

On Mon, 2022-11-21 at 09:40 +0200, Mike Rapoport wrote:
> On Thu, Nov 17, 2022 at 07:57:59PM +0000, Edgecombe, Rick P wrote:
> > On Thu, 2022-11-17 at 12:25 +0000, Schimpe, Christina wrote:
> > > > Hmm, we definitely need to be able to set the SSP. Christina,
> > > > does
> > > > GDB need
> > > > anything else? I thought maybe toggling SHSTK_EN?
> > > 
> > > In addition to the SSP, we want to write the CET state. For
> > > instance
> > > for inferior calls,
> > > we want to reset the IBT bits.
> > > However, we won't write states that are disallowed by HW.
> > 
> > Sorry, I should have given more background. Peter is saying we
> > should
> > split the ptrace interface so that shadow stack and IBT are
> > separate. 
> > They would also no longer necessarily mirror the CET_U MSR format.
> > Instead the kernel would expose a kernel specific format that has
> > the
> > needed bits of shadow stack support. And a separate one later for
> > IBT.
> > 
> > So the question is what does shadow stack need to support for
> > ptrace
> > besides SSP? Is it only SSP? The other features are SHSTK_EN and
> > WRSS_EN. It might actually be nice to keep how these bits get
> > flipped
> > more controlled (remove them from ptrace). It looks like CRIU
> > didn't
> > need them.
> 
>  
> CRIU reads CET_U with ptrace(PTRACE_GETREGSET, NT_X86_CET). It's done
> before the injection of the parasite. The value of SHSTK_EN is used
> then to
> detect if shadow stack is enabled and to setup victim's shadow stack
> for
> sigreturn.

Hmm, can it read /proc/pid/status? It has some lines like this:
x86_Thread_features: shstk wrss
x86_Thread_features_locked: shstk wrss