linux-kernel - Re: 4.14.9 with CONFIG

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Message-ID: <c1cb0fb3-ed36-771d-447d-e9f12662a240@intel.com>
Date:   Fri, 29 Dec 2017 17:04:56 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Alexander Tsoy <alexander@...y.me>, Greg KH <greg@...ah.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>
Cc:     Borislav Petkov <bp@...e.de>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Borislav Petkov <bp@...en8.de>,
        Borislav Petkov <bpetkov@...e.de>,
        Brian Gerst <brgerst@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        David Laight <David.Laight@...lab.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Eduardo Valentin <eduval@...zon.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Rik van Riel <riel@...hat.com>,
        Will Deacon <will.deacon@....com>, aliguori@...zon.com,
        daniel.gruss@...k.tugraz.at, hughd@...gle.com, keescook@...gle.com,
        Kernel Mailing List <linux-kernel@...r.kernel.org>,
        stable <stable@...r.kernel.org>
Subject: Re: 4.14.9 with CONFIG_MCORE2 fails to boot

On 12/29/2017 10:46 AM, Alexander Tsoy wrote:
> В Пт, 29/12/2017 в 09:32 -0800, Dave Hansen пишет:
>> Does anyone have the results of build that they can share?  (vmlinux,
>> vmlinuz/bzImage, System.map, .config).  That, plus a corresponding
>> serial log with an oops would be helpful.
> 
> Here you are:
> https://www.dropbox.com/s/yesupqgig3uxf73/linux-4.15-rc5%2B.tar.xz?dl=0

Alexander, thanks a bunch for the quick turnaround on this.  It is much
appreciated!

With your binary, I can reproduce this in a KVM guest.  Seems we manage
to get to paranoid_entry with a kernel GS value, but the user page
tables in place.  We don't smash the #DF stack because we reset the
stack at each new #DF.  I think the loop that we get stuck in goes
something like this:

1. Hardware does #DF, calls double_fault
2. call paranoid_entry
3. check MSR for GSBASE, see it has kernel value, skip SWAPGS and
   switch to kernel page tables
4. touch stack, try to #PF, but can't touch stack, so #DF and goto 1

The real question is where we double-faulted from in the first place
with a kernel GSBASE and user CR3.  I think I just need to disable KASLR
and do a little work in gdb to look at the stack on the first
double-fault, but we'll see.