Re: WARNING in vcpu_enter_guest

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Thu, Mar 19, 2020 at 04:14:55PM +0100, Paolo Bonzini wrote:
> On 19/03/20 15:49, Sean Christopherson wrote:
> > On Thu, Mar 19, 2020 at 03:35:16AM -0700, syzbot wrote:
> >> syzbot has found a reproducer for the following crash on:
> >>
> >> HEAD commit:    5076190d mm: slub: be more careful about the double cmpxch..
> >> git tree:       upstream
> >> console output: https://syzkaller.appspot.com/x/log.txt?x=143ca61de00000
> >> kernel config:  https://syzkaller.appspot.com/x/.config?x=9f894bd92023de02
> >> dashboard link: https://syzkaller.appspot.com/bug?extid=00be5da1d75f1cc95f6b
> >> compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
> >> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=10bb4023e00000
> >>
> >> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> >> Reported-by: syzbot+00be5da1d75f1cc95f6b@syzkaller.appspotmail.com
> > Reproduced with a little tweaking of the reproducer, debug in progress.
> > 
> 
> I think the WARN_ON at x86.c:2447 is just bogus.  You can always get it
> to trigger if garbage is passed to KVM_SET_CLOCK.

Yep.  I worked through logic/math, mostly to gain a wee bit of knowledge
about the clock stuff, and it's sound.  The KVM_SET_CLOCK from syzkaller
is simply making time go backwards.