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

[Andrei Vagin]

On Wed, Apr 14, 2021 at 08:46:40AM +0200, Jann Horn wrote:
> On Wed, Apr 14, 2021 at 7:59 AM Andrei Vagin <avagin at gmail.com> wrote:
> > We already have process_vm_readv and process_vm_writev to read and write
> > to a process memory faster than we can do this with ptrace. And now it
> > is time for process_vm_exec that allows executing code in an address
> > space of another process. We can do this with ptrace but it is much
> > slower.
> >
> > = Use-cases =
> 
> It seems to me like your proposed API doesn't really fit either one of
> those usecases well...
> 
> > Here are two known use-cases. The first one is “application kernel”
> > sandboxes like User-mode Linux and gVisor. In this case, we have a
> > process that runs the sandbox kernel and a set of stub processes that
> > are used to manage guest address spaces. Guest code is executed in the
> > context of stub processes but all system calls are intercepted and
> > handled in the sandbox kernel. Right now, these sort of sandboxes use
> > PTRACE_SYSEMU to trap system calls, but the process_vm_exec can
> > significantly speed them up.
> 
> In this case, since you really only want an mm_struct to run code
> under, it seems weird to create a whole task with its own PID and so
> on. It seems to me like something similar to the /dev/kvm API would be
> more appropriate here? Implementation options that I see for that
> would be:
> 
> 1. mm_struct-based:
>       a set of syscalls to create a new mm_struct,
>       change memory mappings under that mm_struct, and switch to it

I like the idea to have a handle for mm. Instead of pid, we will pass
this handle to process_vm_exec. We have pidfd for processes and we can
introduce mmfd for mm_struct.


> 2. pagetable-mirroring-based:
>       like /dev/kvm, an API to create a new pagetable, mirror parts of
>       the mm_struct's pagetables over into it with modified permissions
>       (like KVM_SET_USER_MEMORY_REGION),
>       and run code under that context.
>       page fault handling would first handle the fault against mm->pgd
>       as normal, then mirror the PTE over into the secondary pagetables.
>       invalidation could be handled with MMU notifiers.
>

I found this idea interesting and decided to look at it more closely.
After reading the kernel code for a few days, I realized that it would
not be easy to implement something like this, but more important is that
I don’t understand what problem it solves. Will it simplify the
user-space code? I don’t think so. Will it improve performance? It is
unclear for me too.

First, in the KVM case, we have a few big linear mappings and need to
support one “shadow” address space. In the case of sandboxes, we can
have a tremendous amount of mappings and many address spaces that we
need to manage.  Memory mappings will be mapped with different addresses
in a supervisor address space and “guest” address spaces. If guest
address spaces will not have their mm_structs, we will need to reinvent
vma-s in some form. If guest address spaces have mm_structs, this will
look similar to https://lwn.net/Articles/830648/.

Second, each pagetable is tied up with mm_stuct. You suggest creating
new pagetables that will not have their mm_struct-s (sorry if I
misunderstood something). I am not sure that it will be easy to
implement. How many corner cases will be there?

As for page faults in a secondary address space, we will need to find a
fault address in the main address space, handle the fault there and then
mirror the PTE to the secondary pagetable. Effectively, it means that
page faults will be handled in two address spaces. Right now, we use
memfd and shared mappings. It means that each fault is handled only in
one address space, and we map a guest memory region to the supervisor
address space only when we need to access it. A large portion of guest
anonymous memory is never mapped to the supervisor address space.
Will an overhead of mirrored address spaces be smaller than memfd shared
mappings? I am not sure.

Third, this approach will not get rid of having process_vm_exec. We will
need to switch to a guest address space with a specified state and
switch back on faults or syscalls. If the main concern is the ability to
run syscalls on a remote mm, we can think about how to fix this. I see
two ways what we can do here:

* Specify the exact list of system calls that are allowed. The first
three candidates are mmap, munmap, and vmsplice.

* Instead of allowing us to run system calls, we can implement this in
the form of commands. In the case of sandboxes, we need to implement
only two commands to create and destroy memory mappings in a target
address space.

Thanks,
Andrei