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

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Message-ID: <6073e4c6-6fe8-0448-4586-5d04d7154164@kernel.org>
Date:   Fri, 2 Jul 2021 15:44:41 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Andrei Vagin <avagin@...il.com>, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org
Cc:     linux-um@...ts.infradead.org, criu@...nvz.org, avagin@...gle.com,
        Andrew Morton <akpm@...ux-foundation.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

On 4/13/21 10:52 PM, Andrei Vagin wrote:

> process_vm_exec has two modes:
> 
> * Execute code in an address space of a target process and stop on any
>   signal or system call.

We already have a perfectly good context switch mechanism: context
switches.  If you execute code, you are basically guaranteed to be
subject to being hijacked, which means you pretty much can't allow
syscalls.  But there's a lot of non-syscall state, and I think context
switching needs to be done with extreme care.

(Just as example, suppose you switch mms, then set %gs to point to the
LDT, then switch back.  Now you're in a weird state.  With %ss the plot
is a bit thicker.  And there are emulated vsyscalls and such.)

If you, PeterZ, and the UMCG could all find an acceptable, efficient way
to wake-and-wait so you can switch into an injected task in the target
process and switch back quickly, then I think a much nicer solution will
become available.

> 
> * Execute a system call in an address space of a target process.

I could get behind this, but there are plenty of cans of worms to watch
out for.  Serious auditing would be needed.