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From:   Lai Jiangshan <jiangshanlai+lkml@gmail.com>
Date:   Sun, 10 May 2020 12:33:33 +0800
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Subject: Re: [patch V4 part 5 02/31] x86/entry: Provide helpers for execute on irqstack
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     LKML <linux-kernel@vger.kernel.org>, x86@kernel.org,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
        Alexandre Chartre <alexandre.chartre@oracle.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Petr Mladek <pmladek@suse.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Joel Fernandes <joel@joelfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Juergen Gross <jgross@suse.com>,
        Brian Gerst <brgerst@gmail.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Will Deacon <will@kernel.org>
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On Tue, May 5, 2020 at 10:19 PM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> Device interrupt handlers and system vector handlers are executed on the
> interrupt stack. The stack switch happens in the low level assembly entry
> code. This conflicts with the efforts to consolidate the exit code in C to
> ensure correctness vs. RCU and tracing.
>
> As there is no way to move #DB away from IST due to the MOV SS issue, the
> requirements vs. #DB and NMI for switching to the interrupt stack do not
> exist anymore. The only requirement is that interrupts are disabled.

Hi, tglx and Andy Lutomirski,

Is there any information about "no way to move #DB away from IST
due to the MOV SS issue"? IST-based #DB results to ist_shift(for
nested #DB) and debug_idt(for #NMI vs. #DB) which are somewhat
ugly. If IST-less #DB should work, debug stack should be switched
in software manner like interrupt stack.

There was a "POP/MOV SS" CVE/issue about #BP which lead to
moving #BP to IST-less by d8ba61ba58c8
(x86/entry/64: Don't use IST entry for #BP stack)

#DB #BP are considered as #NMI due to their super-interrupt
ability. But the kernel has much more control over #DB and #BP
which can be disabled by putting the code snip into non-instrument
sections like __entry noinstr etc.

Is it possible to implement IST-less #DB?

Thanks,
Lai