Re: PANIC: double fault, error_code: 0x0 in 4.0.0-rc3-2, kvm related?

[Andy Lutomirski <luto@amacapital.net>]

On Wed, Mar 18, 2015 at 3:22 PM, Jiri Kosina <jkosina@suse.cz> wrote:
> On Wed, 18 Mar 2015, Andy Lutomirski wrote:
>
>> sysret64 can only fail with #GP, and we're totally screwed if that
>> happens,
>
> But what if the GPF handler pagefaults afterwards? It'd be operating on
> user stack already.

Good point.

Stefan, can you try changing the first "jne
opportunistic_sysret_failed" to "jmp opportunistic_sysret_failed" in
entry_64.S and seeing if you can reproduce this?  (Is it easy enough
to reproduce that this would tell us anything?)

It's a shame that double_fault doesn't record what gs was on entry.
If we did sysret -> general_protection -> page_fault -> double_fault,
then we'd enter double_fault with usergs, whereas syscall ->
page_fault -> double_fault would enter double_fault with kernelgs.

Hmm.  We may be able to answer this more directly.  Stefan, can you
dump a couple hundred bytes starting at 0x00007fffa55eafb8 (i.e. your
page_fault stack at the time of the failure)?  That will tell us the
faulting address.  If that fails, try starting at 00007fffa55eb000
instead.

--Andy