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Subject: Re: pstore does not work under xen
To:     James Dingwall <james@dingwall.me.uk>,
        Kees Cook <keescook@chromium.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Cc:     Anton Vorontsov <anton@enomsg.org>,
        Colin Cross <ccross@android.com>,
        Juergen Gross <jgross@suse.com>,
        "Luck, Tony" <tony.luck@intel.com>,
        Matthias Kaehlcke <mka@chromium.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        xen-devel <xen-devel@lists.xenproject.org>
References: <20190919102643.GA9400@dingwall.me.uk>
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 <20190923154227.GA11201@dingwall.me.uk> <201909231556.7FF7A11@keescook>
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From:   Boris Ostrovsky <boris.ostrovsky@oracle.com>
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Message-ID: <c60c87d4-132d-b390-8b0e-1130fb434285@oracle.com>
Date:   Wed, 25 Sep 2019 11:38:54 -0400
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On 9/25/19 7:01 AM, James Dingwall wrote:
> 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.)

(+xen-devel which I just noticed has not been copied on this thread)

Whether a guest is rebooted is controlled by on_crash parameter in the
config file. By default it's 'destroy'.


>
> - 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.

Even thought this is now correct behavior it's a change from what we've
always had. And I think we should preserve that.

Meaning that we have to ignore if panic_timeout is set to zero either on
boot line or via sysfs.


-boris

>  - 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;
>>  }
>>
>>