Re: extending ucontext (Re: [PATCH v26 25/30] x86/cet/shstk: Handle signals for shadow stack)

From: Andy Lutomirski
Date: Thu Apr 29 2021 - 10:44:23 EST

On Thu, Apr 29, 2021 at 12:28 AM Cyrill Gorcunov <gorcunov@xxxxxxxxx> wrote:
> On Wed, Apr 28, 2021 at 04:03:55PM -0700, Andy Lutomirski wrote:
> > On Tue, Apr 27, 2021 at 1:44 PM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote:
> > >
> > > When shadow stack is enabled, a task's shadow stack states must be saved
> > > along with the signal context and later restored in sigreturn. However,
> > > currently there is no systematic facility for extending a signal context.
> > > There is some space left in the ucontext, but changing ucontext is likely
> > > to create compatibility issues and there is not enough space for further
> > > extensions.
> > >
> > > Introduce a signal context extension struct 'sc_ext', which is used to save
> > > shadow stack restore token address. The extension is located above the fpu
> > > states, plus alignment. The struct can be extended (such as the ibt's
> > > wait_endbr status to be introduced later), and sc_ext.total_size field
> > > keeps track of total size.
> >
> > I still don't like this.
> >
> > Here's how the signal layout works, for better or for worse:
> >
> > The kernel has:
> >
> > struct rt_sigframe {
> > char __user *pretcode;
> > struct ucontext uc;
> > struct siginfo info;
> > /* fp state follows here */
> > };
> >
> > This is roughly the actual signal frame. But userspace does not have
> > this struct declared, and user code does not know the sizes of the
> > fields. So it's accessed in a nonsensical way. The signal handler
> Well, not really. While indeed this is not declared as a part of API
> the structure is widely used for rt_sigreturn syscall (and we're using
> it inside criu thus any change here will simply break the restore
> procedure). Sorry out of time right now, I'll read your mail more
> carefully once time permit.

I skimmed the CRIU code. You appear to declare struct rt_sigframe,
and you use the offset from the start of rt_sigframe to uc. You also
use the offset to the /* fp state follows here */ part, but that's
unnecessary -- you could just as easily have put the fp state at any
other address -- the kernel will happily follow the pointer you supply
regardless of where it points. So the only issues I can see are if
you write the fp state on top of something else or if you
inadvertently fill in the proposed extension part of uc_flags. Right
now you seem to be ignoring uc_flags, which I presume means that you
are filling it in as zero. Even if the offset of the fp state in the
kernel rt_sigframe changes, the kernel should still successfully parse
the signal frame you generate.

I suppose there is another potential issue: would CRIU have issues if
the *save* runs on a kernel that uses this proposed extension
mechanism? Are you doing something with the saved state that would
get confused?