linux-kernel - Re: [PATCH 0/4 POC] Allow executing code and syscalls in another address space

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Date:   Wed, 14 Apr 2021 14:20:48 +0200
From:   Florian Weimer <fweimer@...hat.com>
To:     Jann Horn <jannh@...gle.com>
Cc:     Andrei Vagin <avagin@...il.com>,
        kernel list <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        linux-um@...ts.infradead.org, criu@...nvz.org,
        Andrei Vagin <avagin@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Ingo Molnar <mingo@...hat.com>, Jeff Dike <jdike@...toit.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Richard Weinberger <richard@....at>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 0/4 POC] Allow executing code and syscalls in another
 address space

* Jann Horn:

> On Wed, Apr 14, 2021 at 12:27 PM Florian Weimer <fweimer@...hat.com> wrote:
>>
>> * Andrei Vagin:
>>
>> > We already have process_vm_readv and process_vm_writev to read and write
>> > to a process memory faster than we can do this with ptrace. And now it
>> > is time for process_vm_exec that allows executing code in an address
>> > space of another process. We can do this with ptrace but it is much
>> > slower.
>> >
>> > = Use-cases =
>>
>> We also have some vaguely related within the same address space: running
>> code on another thread, without modifying its stack, while it has signal
>> handlers blocked, and without causing system calls to fail with EINTR.
>> This can be used to implement certain kinds of memory barriers.
>
> That's what the membarrier() syscall is for, right? Unless you don't
> want to register all threads for expedited membarrier use?

membarrier is not sufficiently powerful for revoking biased locks, for
example.

For the EINTR issue, <https://github.com/golang/go/issues/38836> is an
example.  I believe CIFS has since seen a few fixes (after someone
reported that tar on CIFS wouldn't work because the SIGCHLD causing
utimensat to fail—and there isn't even a signal handler for SIGCHLD!),
but the time it took to get to this point doesn't give me confidence
that it is safe to send signals to a thread that is running unknown
code.

But as you explained regarding the set*id broadcast, it seems that if we
had this run-on-another-thread functionality, we would likely encounter
issues similar to those with SA_RESTART.  We don't see the issue with
set*id today because it's a rare operation, and multi-threaded file
servers that need to change credentials frequently opt out of the set*id
broadcast anyway.  (What I have in mind is a future world where any
printf call, any malloc call, can trigger such a broadcast.)

The cross-VM CRIU scenario would probably somewhere in between (not
quite the printf/malloc level, but more frequent than set*id).

Thanks,
Florian