linux-kernel - Re: [PATCH] rseq: x86: Fix rseq_cs get cleared when returning from signal handler

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Date:   Tue, 21 Jun 2022 16:05:12 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Derek Bruening <bruening@...gle.com>,
        Kevin Malachowski <chowski@...gle.com>,
        Alexander Mikhalitsyn <alexander@...alicyn.com>,
        Florian Weimer <fw@...eb.enyo.de>,
        Carlos O'Donell <carlos@...hat.com>,
        Paul Turner <pjt@...gle.com>,
        Simon Marchi <simon.marchi@...icios.com>,
        Peter Oskolkov <posk@...k.io>,
        Chris Kennelly <ckennelly@...gle.com>,
        Pedro Alves <palves@...hat.com>,
        Bui Quang Minh <minhquangbui99@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        x86 <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Kees Cook <keescook@...omium.org>,
        "Chang S. Bae" <chang.seok.bae@...el.com>,
        Brian Gerst <brgerst@...il.com>,
        Andrei Vagin <avagin@...il.com>,
        Pavel Tikhomirov <ptikhomirov@...tuozzo.com>
Subject: Re: [PATCH] rseq: x86: Fix rseq_cs get cleared when returning from
 signal handler

----- On Jun 21, 2022, at 3:48 PM, Eric W. Biederman ebiederm@...ssion.com wrote:

> Derek Bruening <bruening@...gle.com> writes:
> 
>> From the viewpoint of dynamic binary translation/instrumentation and
>> memtrace (go/memtrace), removing those RSEQ_CS_FLAG_NO_RESTART_ON_* flags
>> is a good thing as it reduces complexity and makes it easier to handle rseq
>> (which is painful enough to handle already).
> 
> It sounds like there is consensus.
> 
> Does someone want to code up a simple patch that detects when
> RSEQ_CS_NO_RESTART_ON_SIGNAL and does a WARN_ON_ONCE and fails if
> someone uses so it can be set to Linus in the next merge window.
> 
> After no one screams at that patch it should be safe to remove the
> functionality, because you have empirical proof that no one uses
> that functionality.

Sure, I can whip up something.

I'll send it to Peter Zijlstra shortly.

I plan to, as you suggest, WARN_ON_ONCE() when this happens, and return
an error when the rseq flags or rseq_cs flags contain either of the
RSEQ_CS_FLAG_NO_RESTART_ON_* flags. This error is handled by forcing a
killing the process with sigsegv:

__rseq_handle_notify_resume()
[...]
error:
        sig = ksig ? ksig->sig : 0;
        force_sigsegv(sig);

Does it look acceptable ?

Thanks,

Mathieu

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