Re: [PATCH v3 0/2] seccomp: pass uretprobe system call through seccomp

[Jann Horn]

On Fri, Feb 7, 2025 at 5:20 PM Eyal Birger <eyal.birger@xxxxxxxxx> wrote:
> On Fri, Feb 7, 2025 at 7:27 AM Jann Horn <jannh@xxxxxxxxxx> wrote:
> >
> > On Sun, Feb 2, 2025 at 5:29 PM Eyal Birger <eyal.birger@xxxxxxxxx> wrote:
> > > uretprobe(2) is an performance enhancement system call added to improve
> > > uretprobes on x86_64.
> > >
> > > Confinement environments such as Docker are not aware of this new system
> > > call and kill confined processes when uretprobes are attached to them.
> >
> > FYI, you might have similar issues with Syscall User Dispatch
> > (https://docs.kernel.org/admin-guide/syscall-user-dispatch.html) and
> > potentially also with ptrace-based sandboxes, depending on what kinda
> > processes you inject uprobes into. For Syscall User Dispatch, there is
> > already precedent for a bypass based on instruction pointer (see
> > syscall_user_dispatch()).
>
> Thanks. This is interesting.
>
> Do you know of confinement environments using this?

Not for Syscall User Dispatch; I think that was mostly intended for
stuff like emulating Windows syscalls in WINE. I'm not sure who
actually uses it, I just know a bit about the kernel side of it.

>From what I know, ptrace sandboxing is a technique used by some
configurations of gVisor
(https://gvisor.dev/docs/architecture_guide/platforms/#ptrace), though
now I see that that page says that this configuration is no longer
supported. I am also not sure whether you'd ever have uprobes
installed in files from which instructions are executed in this
environment.

> > > Since uretprobe is a "kernel implementation detail" system call which is
> > > not used by userspace application code directly, pass this system call
> > > through seccomp without forcing existing userspace confinement environments
> > > to be changed.
> >
> > This makes me feel kinda uncomfortable. The purpose of seccomp() is
> > that you can create a process that is as locked down as you want; you
> > can use it for some light limits on what a process can do (like in
> > Docker), or you can use it to make a process that has access to
> > essentially nothing except read(), write() and exit_group(). Even
> > stuff like restart_syscall() and rt_sigreturn() is not currently
> > excepted from that.
>
> Yes, this has been discussed at length in the threads mentioned
> in the "Link" tags.
>
> >
> > I guess your usecase is a little special in that you were already
> > calling from userspace into the kernel with SWBP before, which is also
> > not subject to seccomp; and the syscall is essentially an
> > arch-specific hack to make the SWBP a little faster.
>
> Indeed. The uretprobe mechanism wasn't enforced by seccomp before
> this syscall. This change preserves this.
>
> >
> > If we do this, we should at least ensure that there is absolutely no
> > way for anything to happen in sys_uretprobe when no uretprobes are
> > configured for the process - the first check in the syscall
> > implementation almost does that, but the implementation could be a bit
> > stricter. It checks for "regs->ip != trampoline_check_ip()", but if no
> > uprobe region exists for the process, trampoline_check_ip() returns
> > `-1 + (uretprobe_syscall_check - uretprobe_trampoline_entry)`. So
> > there is a userspace instruction pointer near the bottom of the
> > address space that is allowed to call into the syscall if uretprobes
> > are not set up. Though the mmap minimum address restrictions will
> > typically prevent creating mappings there, and
> > uprobe_handle_trampoline() will SIGILL us if we get that far without a
> > valid uretprobe.
>
> I'm not sure I understand your point. If creating mappings in that
> area is prevented, what is the issue?

It is usually prevented, not always - root can do it depending on
system configuration.

Also, in a syscall like this that will be reachable in every sandbox,
I think we should try to be more careful about edge cases and avoid
things like this offset calculation on address -1.

> also, this would be related to the
> uretprobe syscall implementation in general, no?

Yes. I just think it is relevant to the seccomp change because
excepting a syscall from seccomp makes it more important that that
syscall is robust and correct.

> To me this seems unrelated to the seccomp change.
> Jiri, do you have any input on this?
>
> Thanks!
> Eyal.