linux-kernel - Re: [PATCH 00/10] x86: Various SYSENTER/SYSEXIT/#DB fixes and cleanups

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Date:	Mon, 29 Feb 2016 10:55:40 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Andy Lutomirski <luto@...nel.org>
Cc:	X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	Oleg Nesterov <oleg@...hat.com>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	Brian Gerst <brgerst@...il.com>
Subject: Re: [PATCH 00/10] x86: Various SYSENTER/SYSEXIT/#DB fixes and cleanups

On Sun, Feb 28, 2016 at 9:28 PM, Andy Lutomirski <luto@...nel.org> wrote:
>
> While I was doing all this, I also noticed that DR6 and BTF handling
> in do_debug was a bit off.  Two of the patches in here try to fix it
> up.

It's worth noting that do_debug is really quite screwed up with or
without this patchset applied.  For example:

    /*
     * Let others (NMI) know that the debug stack is in use
     * as we may switch to the interrupt stack.
     */
    debug_stack_usage_inc();

    /* It's safe to allow irq's after DR6 has been saved */
    preempt_disable();
    cond_local_irq_enable(regs);

This has never really been valid.  It should be guarded by an
if(user_mode(regs)).  And we need to kill the die_notifier garbage in
here -- it makes it basically impossible to understand what's going
on.

--Andy