linux-kernel - Re: rseq: How to test for compat task at signal delivery

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Date:   Tue, 26 Jun 2018 14:45:11 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: rseq: How to test for compat task at signal delivery

----- On Jun 26, 2018, at 1:38 PM, Mathieu Desnoyers mathieu.desnoyers@...icios.com wrote:

> Hi Andy,
> 
> I would like to make the behavior rseq on compat tasks more robust
> by ensuring that kernel/rseq.c:rseq_get_rseq_cs() clears the high
> bits of rseq_cs->abort_ip, rseq_cs->start_ip and
> rseq_cs->post_commit_offset when a 32-bit binary is run on a 64-bit
> kernel.
> 
> The intent here is that if user-space has garbage rather than zeroes
> in its struct rseq_cs fields padding, the behavior will be the same
> whether the binary is run on 32-bit or 64 kernels.
> 
> I know that internally, the kernel is making a transition from
> is_compat_task() to in_compat_syscall().
> 
> I'm fine with using in_compat_syscall() when rseq_get_rseq_cs() is
> invoked from a system call, but is it OK to call it when it is
> invoked from signal delivery ? AFAIU, signals can be delivered
> upon return from interrupt as well.
> 
> If not, what strategy do you recommend for arch-agnostic code ?

I think what we're missing here is a new "is_compat_frame(struct ksignal *ksig)"
which I could use in the rseq code. I'll prepare a patch and we can discuss
from there.

Thanks,

Mathieu


> 
> Thanks,
> 
> Mathieu
> 
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com