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

[Gregory Price]

On Thu, Jan 26, 2023 at 09:45:39AM -0800, Andrei Vagin wrote:
> 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

The region is exclusive, so syscalls *outside* [offset, offset+len]
produce a dispatch. That means you would have to inject into that region.

That's what's problematic for injection.  Even rt_sigreturn itself
may/will be intercepted.