linux-kernel - Re: [RFC PATCH v2 6/6] x86/entry/pti: don't switch PGD on when pti

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Message-ID: <af80ba81-f8bc-156e-a286-0825aa47f134@linux.intel.com>
Date:   Thu, 11 Jan 2018 07:51:45 -0800
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Willy Tarreau <w@....eu>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [RFC PATCH v2 6/6] x86/entry/pti: don't switch PGD on when
 pti_disable is set

On 01/11/2018 07:44 AM, Willy Tarreau wrote:
>> I think we also need to be able to dump the actual
>> CR3 value that we entered the kernel with before we start doing too much
>> other funky stuff with the entry code.
> When you say dump, you mean save it somewhere in a per_cpu variable ?

Yeah, I think a per-cpu variable is fine for now.  But, that only gives
you a dump from a single entry to the kernel.  Ideally, it would be nice
to have a stack of them so you could do things like debug
syscall->fault->oops.  Was it the syscall entry or the fault entry that
lead to the oops?

But, the stack gets really fun because of NMIs.

I'm sure Andy Lutomirski has some ideas too.