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SubjectRe: [PATCH] KVM: x86: restore IP after all far jump failures
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> On Nov 22, 2016, at 12:56 PM, Radim Krčmář <rkrcmar@redhat.com> wrote:
>
> 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).

I don’t think so. If CPL is not 0, handle_emulation_failure() will be called
and will inject #UD.

>
> 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?

Most likely you are right, but after doing one mistake, I don’t want
to be accountable for another.

Note there is another problematic case in em_ret_far(). If an exception
occurs when RIP is assigned, the RSP updates (of emulate_pop() ) are not
going to be reverted. Can it be used for anything malicious? I don’t know.

Nadav
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