Re: pstore does not work under xen

[James Dingwall <james@dingwall.me.uk>]

On Mon, Sep 23, 2019 at 08:41:05PM -0400, Boris Ostrovsky wrote:
> On 9/23/19 6:59 PM, Kees Cook wrote:
> > On Mon, Sep 23, 2019 at 03:42:27PM +0000, James Dingwall wrote:
> >> On Thu, Sep 19, 2019 at 12:37:40PM -0400, Boris Ostrovsky wrote:
> >>> On 9/19/19 12:14 PM, James Dingwall wrote:
> >>>> On Thu, Sep 19, 2019 at 03:51:33PM +0000, Luck, Tony wrote:
> >>>>>> I have been investigating a regression in our environment where pstore 
> >>>>>> (efi-pstore specifically but I suspect this would affect all 
> >>>>>> implementations) no longer works after upgrading from a 4.4 to 5.0 
> >>>>>> kernel when running under xen.  (This is an Ubuntu kernel but I don't 
> >>>>>> think there are patches which affect this area.)
> >>>>> I don't have any answer for this ... but want to throw out the idea that
> >>>>> VMM systems could provide some hypercalls to guests to save/return
> >>>>> some blob of memory (perhaps the "save" triggers automagically if the
> >>>>> guest crashes?).
> >>>>>
> >>>>> That would provide a much better pstore back end than relying on emulation
> >>>>> of EFI persistent variables (which have severe contraints on size, and don't
> >>>>> support some pstore modes because you can't dynamically update EFI variables
> >>>>> hundreds of times per second).
> >>>>>
> >>>> For clarification this is a dom0 crash rather than an HVM guest with EFI.  I
> >>>> should probably have also mentioned the xen verion has changed from 4.8.4 to
> >>>> 4.11.2 in case its behaviour on detection of crashed domain has changed.
> >>>>
> >>>> (For capturing guest crashes we have enabled xenconsole logging so the
> >>>> hvc0 log is available in dom0.)
> >>>
> >>> Do you only see this difference between 4.4 and 5.0 when you crash via
> >>> sysrq?
> >>>
> >>> Because that's where things changed. On 4.4 we seem to be forcing an
> >>> oops, which eventually calls kmsg_dump() and then panic. On 5.0 we call
> >>> panic() directly from sysrq handler. And because Xen's panic notifier
> >>> doesn't return we never get a chance to call kmsg_dump().
> >>>
> >> Ok, I see that change in 8341f2f222d729688014ce8306727fdb9798d37e.  I 
> >> hadn't tested it any other way before.  Using the null pointer 
> >> de-reference module code at [1] a pstore record is generated as expected 
> >> when the module is loaded (panic_on_oops=1).
> > This change looks correct -- it just gets us directly to the panic()
> > state instead of exercising the various exception handlers.
> >
> >> I have also tested swapping the kmsg_dump() / 
> >> atomic_notifier_call_chain() around in panic.c and this also results in 
> >> a pstore record being created with sysrq-c.  I don't know if that would 
> >> be an acceptable solution though since it may break behaviour that other 
> >> things depend on.
> > I don't think reordering these is a good idea: as the comments say,
> > there might be work done in the notifier chain that kmsg_dump() will
> > want to capture (e.g. the KASLR base offset).
> >
> > The situation seems to be that notifier callbacks must return -- I think
> > Xen needs fixing here.
> >
> 
> I only had one quick sanity test with a PV guest so this needs more
> testing. James, can you give it a try?

I have tested the patch in a pv domU and in dom0.  The kernel was built 
with a default Ubuntu .config which sets CONFIG_PANIC_TIMEOUT=0. uname 
-r = 5.0.0-27-generic.

In the domU (no custom kernel parameters):

- sysrq-c: I saw my debug printk messages about kmsg_dump() being 
invoked after the traceback and the domain became listed as crashed in 
the xl status output.

- halt -p: clean shutdown

- shutdown -r now: clean reboot


In the domU with panic=15 in kernel parameters:

- sysrq-c: as without panic=15 then final message "Rebooting in 15 
seconds.." printed but domain never rebooted.  Without this patch the 
domain doesn't reboot or print the Rebooting message presumably because 
of the non-returning panic handler.  If that message is reached then I 
think I would expect a reboot.  (In our Linux 4.4 / Xen 4.8.4 
environment no value of panic resulted in reboot.)

- halt -p: clean shutdown

- shutdown -r now: clean reboot


In dom0 with oops=panic panic=15 in the kernel parameters:

- sysrq-c: kmsg_dump() debug messages printed, last linux message 
"Rebooting in 15 seconds..", after 15s "(XEN) Hardware Dom0 crashed: 
rebooting machine in 5 seconds.", after 5s system rebooted.  On the 
next start a pstore record is present as expected.

- halt -p: clean shutdown, no pstore record present on next start.

- shutdown -r now: clean reboot, no pstore record present on next 
start.


In dom0 with panic=0:

- sysrq-c: dom0 crashes, no reboot messages printed from Xen or kernel 
and system hangs.  A pstore record is present on next start.

- halt -p: clean shutdown, no pstore record present on next start.

- shutdown -r now: clean reboot, no pstore record present on next 
start.


In my opinion this patch:
 - fixes the original issue of no pstore record being generated for a 
dom0 panic.
 - respects the dom0 panic=... value.  This is a change in behaviour in 
how xen handles the crashed dom0 from always rebooting to only rebooting 
if panic > 0.
 - causes a Rebooting message to be printed in a crashed pv domU when 
panic > 0 but the domain does not reboot when it should.

James

> 
> 
> diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
> index 750f46ad018a..d88f118028b4 100644
> --- a/arch/x86/xen/enlighten.c
> +++ b/arch/x86/xen/enlighten.c
> @@ -269,16 +269,17 @@ void xen_reboot(int reason)
>                 BUG();
>  }
>  
> +static int reboot_reason = SHUTDOWN_reboot;
>  void xen_emergency_restart(void)
>  {
> -       xen_reboot(SHUTDOWN_reboot);
> +       xen_reboot(reboot_reason);
>  }
>  
>  static int
>  xen_panic_event(struct notifier_block *this, unsigned long event, void
> *ptr)
>  {
>         if (!kexec_crash_loaded())
> -               xen_reboot(SHUTDOWN_crash);
> +               reboot_reason = SHUTDOWN_crash;
>         return NOTIFY_DONE;
>  }
>  
> @@ -290,6 +291,8 @@ static struct notifier_block xen_panic_block = {
>  int xen_panic_handler_init(void)
>  {
>         atomic_notifier_chain_register(&panic_notifier_list,
> &xen_panic_block);
> +       if (panic_timeout == 0)
> +               set_arch_panic_timeout(-1, CONFIG_PANIC_TIMEOUT);
>         return 0;
>  }
> 
>