Re: [PATCH v6 1/2] ptrace,syscall_user_dispatch: Implement Syscall User Dispatch Suspension

[Andrei Vagin]

On Thu, Jan 26, 2023 at 7:07 AM Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
>
> On 01/25, Andrei Vagin wrote:
> >
> > On Wed, Jan 25, 2023 at 4:30 PM Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
> > >
> > > On 01/24, Gregory Price wrote:
> > > >
> > > > Adds PTRACE_O_SUSPEND_SYSCALL_USER_DISPATCH to ptrace options, and
> > > > modify Syscall User Dispatch to suspend interception when enabled.
> > > >
> > > > This is modeled after the SUSPEND_SECCOMP feature, which suspends
> > > > SECCOMP interposition.  Without doing this, software like CRIU will
> > > > inject system calls into a process and be intercepted by Syscall
> > > > User Dispatch, either causing a crash (due to blocked signals) or
> > > > the delivery of those signals to a ptracer (not the intended behavior).
> > >
> > > Cough... Gregory, I am sorry ;)
> > >
> > > but can't we drop this patch to ?
> > >
> > > CRIU needs to do PTRACE_SET_SYSCALL_USER_DISPATCH_CONFIG and check
> > > config->mode anyway as we discussed.
> > >
> > > Then it can simply set *config->selector = SYSCALL_DISPATCH_FILTER_ALLOW
> > > with the same effect, no?
> >
> > Oleg,
> >
> > PTRACE_O_SUSPEND_SYSCALL_USER_DISPATCH is automatically cleared when
> > a tracer detaches. It is critical when tracers detach due to unexpected
> > reasons
>
> IIUC, PTRACE_O_SUSPEND_SYSCALL_USER_DISPATCH is needed to run the injected
> code, and this also needs to change the state of the traced process. If
> the tracer (CRIU) dies while the tracee runs this code, I guess the tracee
> will have other problems?

Our injected code can reheal itself if something goes wrong. The hack
here is that we inject
the code with a signal frame and it calls rt_segreturn to resume the process.

We want to have this functionality for most cases. I don't expect that
the syscall user dispatch
is used by many applications, so I don't strongly insist on
PTRACE_O_SUSPEND_SYSCALL_USER_DISPATCH. In addition, if we know a user dispatch
memory region, it can be enough to inject our code out of this region
without disabling SUD.

Thanks,
Andrei