Re: [PATCH v2 7/9] x86: Use current USER_CS to setup correct context on vmx entry

[Gabriel Krisman Bertazi <krisman@collabora.com>]

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

> On Thu, Oct 01, 2020 at 02:52:32PM -0700, Andy Lutomirski wrote:
>> On Thu, Oct 1, 2020 at 1:59 PM Gabriel Krisman Bertazi
>> <krisman@collabora.com> wrote:
>> >
>> > vmx_prepare_switch_to_guest shouldn't use is_64bit_mm, which has a
>> > very specific use in uprobes.  Use the user_64bit_mode helper instead.
>> > This reduces the usage of is_64bit_mm, which is awkward, since it relies
>> > on the personality at load time, which is fine for uprobes, but doesn't
>> > seem fine here.
>> >
>> > I tested this by running VMs with 64 and 32 bits payloads from 64/32
>> > programs.
>> >
>> > Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>
>> > ---
>> >  arch/x86/kvm/vmx/vmx.c | 2 +-
>> >  1 file changed, 1 insertion(+), 1 deletion(-)
>> >
>> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>> > index 7b2a068f08c1..b5aafd9e5f5d 100644
>> > --- a/arch/x86/kvm/vmx/vmx.c
>> > +++ b/arch/x86/kvm/vmx/vmx.c
>> > @@ -1172,7 +1172,7 @@ void vmx_prepare_switch_to_guest(struct kvm_vcpu *vcpu)
>> >         savesegment(es, host_state->es_sel);
>> >
>> >         gs_base = cpu_kernelmode_gs_base(cpu);
>> > -       if (likely(is_64bit_mm(current->mm))) {
>> > +       if (likely(user_64bit_mode(current_pt_regs()))) {
>> >                 current_save_fsgs();
>> >                 fs_sel = current->thread.fsindex;
>> >                 gs_sel = current->thread.gsindex;
>> 
>> I disagree with this one.  This whole code path is nonsense.  Can you
>> just remove the condition entirely and use the 64-bit path
>> unconditionally?
>
> I finally came back to this one with fresh eyes.  I've read through the code
> a bajllion times and typed up half a dozen responses.  I think, finally, I
> understand what's broken.
>
> I believe your original assertion that the bug was misdiagnosed is correct
> (can't link because LKML wasn't in the loop).  I'm pretty sure your analysis
> that KVM's handling of things works mostly by coincidence is also correct.
>
> The coincidence is that "real" VMMs all use arch_prctl(), and
> do_arch_prctl_64() ensures thread.{fs,gs}base are accurate.  save_base_legacy()
> detects sel==0 and intentionally does nothing, knowing the the base is already
> accurate.
>
> Userspaces that don't use arch_prctl(), in the bug report case a 32-bit compat
> test, may or may not have accurate thread.{fs,gs}base values.  This is
> especially true if sel!=0 as save_base_legacy() explicitly zeros the base in
> this case, as load_seg_legacy() will restore the seg on the backend.
>
> KVM on the other hand assumes thread.{fs,gs}base are always fresh.  When that
> didn't hold true for userspace that didn't use arch_prctl(), the fix of
> detecting a !64-bit mm just so happened to work because all 64-bit VMMs use
> arch_prctl().
>
> It's tempting to just open code this and use RD{FS,GS}BASE when possible,
> i.e. avoid any guesswork.  Maybe with a module param that userspace can set
> to tell KVM it doesn't do anything fancy with FS/GS base (will userspace still
> use arch_prctl() even if FSGSABSE is available?).
>
>         savesegment(fs, fs_sel);
>         savesegment(gs, gs_sel);
> 	if (use_current_fsgs_base) {
> 		fs_base = current->thread.fsbase;
> 		vmx->msr_host_kernel_gs_base = current->thread.gsbase;
> 	} else if (static_cpu_has(X86_FEATURE_FSGSBASE)) {
>                 fs_base = rdfsbase()
>                 vmx->msr_host_kernel_gs_base = __rdgsbase_inactive();
>         } else {
>                 fs_base = read_msr(MSR_FS_BASE);
>                 vmx->msr_host_kernel_gs_base = read_msr(MSR_KERNEL_GS_BASE);
>         }
>
> I'll revisit this on Monday and run a patch by Paolo.

If this is the case, I will just drop the current patch from my series
and leave it to you.  Given that is_64bit_mm() is not going away, this
use case can be fixed separately.

-- 
Gabriel Krisman Bertazi