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

Vitaly Kuznetsov
2018-07-13T17:11:00Z

Sean Christopherson <sean.j.christopherson@intel.com> writes:

> On Wed, Jul 11, 2018 at 07:37:18PM +0200, Vitaly Kuznetsov 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.
>>
>> Reported-by: Dmitry V. Levin <ldv@altlinux.org>
>> Bisected-by: Masatake YAMATO <yamato@redhat.com>
>> Fixes: 42b933b59721 ("x86/kvm/vmx: read MSR_{FS,KERNEL_GS}_BASE from current->thread")
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
>> ---
>> arch/x86/kvm/vmx.c | 25 +++++++++++++++++--------
>> 1 file changed, 17 insertions(+), 8 deletions(-)
>>
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index 559a12b6184d..65968649b365 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -2560,6 +2560,7 @@ static void vmx_save_host_state(struct kvm_vcpu *vcpu)
>> struct vcpu_vmx *vmx = to_vmx(vcpu);
>> #ifdef CONFIG_X86_64
>> int cpu = raw_smp_processor_id();
>> + unsigned long fsbase, kernel_gsbase;
>
> Because bikeshedding is fun, what do you think about using fs_base and
> kernel_gs_base for these names? I have a series that touches this
> code and also adds local variables for {FS,GS}.base and {FS,GS}.sel.
> I used {fs,gs}_base and {fs,gs}_sel to be consistent with the
> vmx->host_state nomenclature (the local variables are used to update
> the associated vmx->host_state variables), but I'll change my patches
> if you have a strong preference for omitting the underscore.
>

I have nothing against underscores :-)

Hope this small change can be done by Paolo/Radim upon commit. Or I'll
send v2 if needed.

--
Vitaly