linux-kernel - Re: [PATCH] x86/mm: Don't try to change poison pages to uncacheable in a guest

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Date:   Sat, 16 May 2020 08:54:31 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Tony Luck <tony.luck@...el.com>
Cc:     Jue Wang <juew@...gle.com>,
        Dan Williams <dan.j.williams@...el.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/mm: Don't try to change poison pages to uncacheable
 in a guest

On Tue, May 05, 2020 at 11:46:48AM -0700, Tony Luck wrote:
> An interesting thing happened when a guest Linux instance took
> a machine check. The VMM unmapped the bad page from guest physical
> space and passed the machine check to the guest.
> 
> Linux took all the normal actions to offline the page from the process
> that was using it. But then guest Linux crashed because it said there
> was a second machine check inside the kernel with this stack trace:
> 
> do_memory_failure
>     set_mce_nospec
>          set_memory_uc
>               _set_memory_uc
>                    change_page_attr_set_clr
>                         cpa_flush
>                              clflush_cache_range_opt

Maybe I don't see it but how can clflush_cache_range_opt() call
cpa_flush() ?

> This was odd, because a CLFLUSH instruction shouldn't raise a machine
> check (it isn't consuming the data). Further investigation showed that
> the VMM had passed in another machine check because is appeared that the
> guest was accessing the bad page.

This is where you lost me - if the VMM unmaps the page during the first
MCE, how can the guest even attempt to touch it and do this stack trace
above?

/me is confused.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette