Re: [Xen-devel] 100% reliable oops on Xen 4.1.3 (initially reported on 4.0.1)

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 29.01.13 at 12:56, David Vrabel <david.vrabel@xxxxxxxxxx> wrote:
> On 28/01/13 19:17, Peter Moody wrote:
>> TL;DR, the domU crash I reported over the summer on Xen 4.0.1 can be
>> reproduced on 4.1.3 and on more processor families and with out the
>> special memory/cpu configurations I previously reported.
>> 
>> Longer version:
>> apropos of this thread [1] from last summer, I've managed to test for
>> this bug on a more recent version of Xen and I can confirm that it
>> exists in at least 4.1.3. Also, based on the release notes for 4.0.1
>> [2] (the original version of Xen where I encountered this issue), I
>> reproduced the bug on an AMD Athlon processor in case the interrupts
>> issue mentioned had an effect.
>> 
>> The patch I posted to the audit list didn't actually fix the problem.
>> 
>> Steps I used to reproduce:
>> 1) installed Xen from ubuntu packages and boot into Xen enabled system.
>> 2) installed ubuntu 12.10 domU using 20G flat file as disk (the
>> previous system used drbd).
>> 3) installed auditd and inserted any syscall rule (audit on chmod's
>> for example).
>> 4) compiled the attached sample program as a 32 bit binary.
>> 5) ran it (works as a normal user).
> 
> The BUG is because irqs_disabled().
> 
> The call to __audit_syscall_exit is from ia32_sysenter_target in
> arch/x86/ia32/ia32entry.S which attempts to enable interrupts prior to
> the call with an sti instruction.
> 
> I don't think this works as expected with a PV kernel and I'm surprised
> that this doesn't cause a #GP fault.

It does, but it gets dealt with by the hypervisor. Just that the
code handling this is commented out (i.e. both STI and CLI are
effectively NOPs), because of the inconsistency their emulation
would cause with PUSHF/POPF. See the respective cases in
arch/x86/traps.c:emulate_privileged_op().

> Jan (Cc'd) is more familar with these low-level bits but does (untested)
> this patch help?

Apart from the title and description, the one I had sent a couple
of minutes earlier than you sent yours, is identical, so I guess
both of us having come to the same conclusion is a good sign...

Jan


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel