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

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Date:   Thu, 26 Jan 2023 16:07:25 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Andrei Vagin <avagin@...il.com>
Cc:     Gregory Price <gourry.memverge@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Gabriel Krisman Bertazi <krisman@...labora.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jonathan Corbet <corbet@....net>,
        Shuah Khan <shuah@...nel.org>,
        Gregory Price <gregory.price@...verge.com>
Subject: Re: [PATCH v6 1/2] ptrace,syscall_user_dispatch: Implement Syscall
 User Dispatch Suspension

On 01/25, Andrei Vagin wrote:
>
> On Wed, Jan 25, 2023 at 4:30 PM Oleg Nesterov <oleg@...hat.com> 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?

Oleg.