[syzbot] upstream test error: KFENCE: use-after-free in kvm_fastop_exception

[Sean Christopherson]

On Fri, Sep 17, 2021, Dmitry Vyukov wrote:
> On Fri, 17 Sept 2021 at 13:04, Marco Elver <elver at google.com> wrote:
> > > So it looks like in both cases the top fault frame is just wrong. But
> > > I would assume it's extracted by arch-dependent code, so it's
> > > suspicious that it affects both x86 and arm64...
> > >
> > > Any ideas what's happening?
> >
> > My suspicion for the x86 case is that kvm_fastop_exception is related
> > to instruction emulation and the fault occurs in an emulated
> > instruction?
> 
> Why would the kernel emulate a plain MOV?
> 2a:   4c 8b 21                mov    (%rcx),%r12
> 
> And it would also mean a broken unwind because the emulated
> instruction is in __d_lookup, so it should be in the stack trace.

kvm_fastop_exception is a red herring.  It's indeed related to emulation, and
while MOV emulation is common in KVM, that emulation is for KVM guests not for
the host kernel where this splat occurs (ignoring the fact that the "host" is
itself a guest).

kvm_fastop_exception is out-of-line fixup, and certainly shouldn't be reachable
via d_lookup.  It's also two instruction, XOR+RET, neither of which are in the
code stream.

IIRC, the unwinder gets confused when given an IP that's in out-of-line code,
e.g. exception fixup like this.  If you really want to find out what code blew
up, you might be able to objdump -D the kernel and search for unique, matching
disassembly, e.g. find "jmpq   0xf86d288c" and go from there.