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; > } > >