Re: [PATCH] x86/kvm/vmx: don't read current->thread.{fs,gs}base of legacy tasks

Vitaly Kuznetsov
2018-07-12T09:31:48Z

Wanpeng Li <kernellwp@gmail.com> writes:

> On Thu, 12 Jul 2018 at 08:07, Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
>>
>> When we switched from doing rdmsr() to reading FS/GS base values from
>> current->thread we completely forgot about legacy 32-bit userspaces which
>> we still support in KVM (why?). task->thread.{fsbase,gsbase} are only
>> synced for 64-bit processes, calling save_fsgs_for_kvm() and using
>> its result from current is illegal for legacy processes.
>>
>> There's no ARCH_SET_FS/GS prctls for legacy applications. Base MSRs are,
>> however, not always equal to zero. Intel's manual says (3.4.4 Segment
>> Loading Instructions in IA-32e Mode):
>>
>> "In order to set up compatibility mode for an application, segment-load
>> instructions (MOV to Sreg, POP Sreg) work normally in 64-bit mode. An
>> entry is read from the system descriptor table (GDT or LDT) and is loaded
>> in the hidden portion of the segment register.
>> ...
>> The hidden descriptor register fields for FS.base and GS.base are
>> physically mapped to MSRs in order to load all address bits supported by
>> a 64-bit implementation.
>> "
>>
>> The issue was found by strace test suite where 32-bit ioctl_kvm_run test
>> started segfaulting.
>
> Test suite: MSR switch
> PASS: VM entry MSR load
> PASS: VM exit MSR store
> PASS: VM exit MSR load
> FAIL: VM entry MSR load: try to load FS_BASE
> SUMMARY: 4 tests, 1 unexpected failures
>
> kvm-unit-tests fails w/ and w/o the patch, maybe it is another issue,
> i didn't dig further, you can have a look if you are interested in. :)

The patch only changes the behavior for legacy userspaces and I can
reproduce the failure on native x86_64, it is something different. I'm,
however, interested so stay tuned :-)

--
Vitaly