Re: [PATCH 0/4 POC] Allow executing code and syscalls in another address space

From: Andrei Vagin
Date: Sat Jul 17 2021 - 21:38:46 EST

On Fri, Jul 02, 2021 at 03:44:41PM -0700, Andy Lutomirski wrote:
> On 4/13/21 10:52 PM, Andrei Vagin wrote:
> > process_vm_exec has two modes:
> >
> > * Execute code in an address space of a target process and stop on any
> > signal or system call.
> We already have a perfectly good context switch mechanism: context
> switches. If you execute code, you are basically guaranteed to be
> subject to being hijacked, which means you pretty much can't allow
> syscalls. But there's a lot of non-syscall state, and I think context
> switching needs to be done with extreme care.
> (Just as example, suppose you switch mms, then set %gs to point to the
> LDT, then switch back. Now you're in a weird state. With %ss the plot
> is a bit thicker. And there are emulated vsyscalls and such.)
> If you, PeterZ, and the UMCG could all find an acceptable, efficient way
> to wake-and-wait so you can switch into an injected task in the target
> process and switch back quickly, then I think a much nicer solution will
> become available.

I know about umcg and I even did a prototype that used fuxet_swap (the
previous attempt of umcg). Here are a few problems and maybe you will
have some ideas on how to solve them.

The main question is how to hijack a stub process where a guest code is
executing. We need to trap system calls, memory faults, and other
exceptions and handle them in the Sentry (supervisor/kernel). All
interested events except system calls generate signals. We can use
seccomp to get signals on system calls too. In my prototype, a guest
code is running in stub processes. One stub process is for each guest
address space. In a stub process, I set a signal handler for SIGSEGV,
SIGBUS, SIGFPE, SIGSYS, SIGILL, set an alternate signal stack, and set
seccomp rules. The signal handler communicates with the Sentry
(supervisor/kernel) via shared memory and uses futex_swap to make fast
switches to the Sentry and back to a stub process.

Here are a few problems. First, we have a signal handler code, its
stack, and a shared memory region in a guest address space, and we need
to guarantee that a guest code will not be able to use them to do
something unexpected.

The second problem is performance. It is much faster if we compare it
with the ptrace platform, but it is still a few times slower than
process_vm_exec. Signal handling is expensive. The kernel has to
generate a signal frame, execute a signal handler, and then it needs to
call rt_sigreturn. Futex_swap makes fast context switches, but it is
still slower than process_vm_exec. UMCG should be faster because it
doesn’t have a futex overhead.

Andy, what do you think about the idea to rework process_vm_exec so that
it executes code and syscalls in the context of a target process?
Maybe you see other ways how we can “hijack” a remote process?


> >
> > * Execute a system call in an address space of a target process.
> I could get behind this, but there are plenty of cans of worms to watch
> out for. Serious auditing would be needed.