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

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Date:   Wed, 25 Jan 2023 20:30:32 -0500
From:   Gregory Price <gregory.price@...verge.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Gregory Price <gourry.memverge@...il.com>,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        avagin@...il.com, peterz@...radead.org, luto@...nel.org,
        krisman@...labora.com, tglx@...utronix.de, corbet@....net,
        shuah@...nel.org
Subject: Re: [PATCH v6 1/2] ptrace,syscall_user_dispatch: Implement Syscall
 User Dispatch Suspension

On Thu, Jan 26, 2023 at 01:30:08AM +0100, Oleg Nesterov 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.
> 

The selector is optional, but the core idea seems reasonable.

Though I think this complicates the quiesce vs checkpoint phases a bit.

My best understanding of CRIU is there are (at least) two checkpoint
phases: quiesce and checkpoint. The intent of patch 1/2 is to aid the
quiesce phase, not the checkpoint phase.

In both phases the `compel` code is used to inject system calls, so
turning SUD off is required.  That can obviously be achieved via saving
with get_config, and just clearing it entirely with set_config.

I'm NOT sure whether the `compel` code can save settings that the
`cr-check` code then saves to disc, or if `compel` is standalone. I will
go check this and report back.

The only other concern is one of how it's restored, and in what order
compared to SECCOMP - for the absolute insane case of someone running a
SUD task inside a locked down cgroup? Technically possible (TM)!

We may find that the suspend flag is "just easier" but not required.

I do think more-simple-is-more-better, though, so I will investigate.

~Gregory