linux-kernel - Re: [PATCH 8/9] x86/dumpstack: Save first regs set for the executive summary

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Date:   Fri, 16 Mar 2018 12:40:49 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Borislav Petkov <bp@...en8.de>, X86 ML <x86@...nel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 8/9] x86/dumpstack: Save first regs set for the executive
 summary

On Fri, Mar 16, 2018 at 10:22:29AM -0700, Linus Torvalds wrote:
> On Fri, Mar 16, 2018 at 4:48 AM, Borislav Petkov <bp@...en8.de> wrote:
> > On Thu, Mar 15, 2018 at 02:01:32PM -0500, Josh Poimboeuf wrote:
> >> no_context() has the following line, right before it calls oops_end():
> >>
> >>       /* Executive summary in case the body of the oops scrolled away */
> >>       printk(KERN_DEFAULT "CR2: %016lx\n", address);
> >>
> >> I think that line can now be removed, since the executive summary
> >> __show_regs() will include CR2.
> >
> > Good idea. Done.
> 
> NOOOO!
> 
> Guys, %cr2 CAN AND DOES CHANGE!
> 
> The reason we do that
> 
>         printk(KERN_DEFAULT "CR2: %016lx\n", address);
> 
> is because WE ARE NOT PRINTING OUT THE CURRENT CR2 REGISTER!

Good point.  I missed the fact that no_context() isn't printing the
current CR2.

> This is really damn important.
> 
> The "address" register contains the CR2 value as it was read *very*
> early in the page fault case, before we enabled interrupts, and before
> we did various random things that can cause further page faults and
> change CR2!
> 
> So the executive summary that does __show_regs() may end up showing
> something completely different than the actual faulting address,
> because we might have taken a vmalloc-space exception in the meantime,
> for example.
> 
> Do *NOT* get rid of that thing.
> 
> You're better off getting rid of the CR2 line from __show_regs(),
> because it can be dangerously confusing. It's not actually part of the
> saved register state at all, it's something entirely different. It's
> like showing the current eflags rather than the eflags saved on the
> faulting stack.

True, it's probably best to remove it.  The only time we need CR2's
value is presumably when it would have already been printed in
no_context(), and so it primarily just adds confusion as you said.

-- 
Josh