linux-kernel - Re: [patch V4 part 1 29/36] x86/mce: Send #MC singal from task work

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Date:   Thu, 14 May 2020 09:19:32 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        x86 <x86@...nel.org>, paulmck <paulmck@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Petr Mladek <pmladek@...e.com>, rostedt <rostedt@...dmis.org>,
        "Joel Fernandes, Google" <joel@...lfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <JGross@...e.com>,
        Brian Gerst <brgerst@...il.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Tony Luck <tony.luck@...el.com>
Subject: Re: [patch V4 part 1 29/36] x86/mce: Send #MC singal from task work



> On May 14, 2020, at 9:03 AM, Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:
> 
> ----- On May 14, 2020, at 10:17 AM, Borislav Petkov bp@...en8.de wrote:
> 
>> + Tony.
>> 
>>> On Tue, May 05, 2020 at 03:16:31PM +0200, Thomas Gleixner wrote:
>>> From: Peter Zijlstra <peterz@...radead.org>
>>> 
>>> Convert #MC over to using task_work_add(); it will run the same code
>>> slightly later, on the return to user path of the same exception.
>>> 
>>> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
>>> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
>>> Reviewed-by: Frederic Weisbecker <frederic@...nel.org>
>>> ---
>>> arch/x86/kernel/cpu/mce/core.c |   56 ++++++++++++++++++++++-------------------
>>> include/linux/sched.h          |    6 ++++
>>> 2 files changed, 37 insertions(+), 25 deletions(-)
>> 
>> I like this:
>> 
>> Reviewed-by: Borislav Petkov <bp@...e.de>
> 
> What I am not fully grasping here is whether this patch preserves the instruction
> pointer (and possibly other relevant information for siginfo_t) triggering the
> exception in a scenario where we have:
> 
> - #MC triggered, queuing task work,
> - unrelated signal happens to be delivered to task,
> - exit to usermode loop handles do_signal first,
> - then it runs task work.

If anyone wants to ponder this, I suspect that we have lots of delightful bugs in our handling of cr2, trapnr, and error_code in signals. We should move them to the sigcontext, at least in kernel, and fix up ucontext when we deliver the signal.  The current code can’t possibly be correct.