Re: [PATCH] KVM: x86: restore IP after all far jump failures

["Radim Krčmář" <rkrcmar@redhat.com>]

2016-11-22 11:43-0800, Nadav Amit:
> I admit my wrongdoings, but I still think the fix should have been to
> remove the entire recovery logic and just return X86EMUL_UNHANDLEABLE if
> something goes wrong (exception). This will kill the misbehaving process
> but keep the VM running.

X86EMUL_UNHANDLEABLE will kill the whole VM (on QEMU, other userspaces
might handle the instruction and resume KVM).

The recovery path is in the spec, which means that nothing goes wrong.
I think we implement the spec quite well now, so keeping the #GP and CS
recovery is slightly better, although not safer.

> Otherwise, a malicious VM process, which can somehow control descriptors
> (LDT?) may modify the descriptor during the emulation and get the system
> to inconsistent state and prevent the VM-entry.

We restore the original CS -- malicious guest would get killed on a
failed VM entry anyway, so the difference is only in KVM internal error
code (assuming there are no other bugs).

Am I misunderstanding something?