Re: [PATCH] x86/tsx: clear RTM and HLE when MSR_IA32_TSX_CTRL is not supported

[Pawan Gupta]

On 08.09.2021 13:06, Hao Peng wrote:
> On Wed, Sep 8, 2021 at 6:57 AM Pawan Gupta
> <pawan.kumar.gupta@xxxxxxxxxxxxxxx> wrote:
> > On 07.09.2021 14:36, Hao Peng wrote:
> > >On Tue, Sep 7, 2021 at 1:13 PM Pawan Gupta
> > ><pawan.kumar.gupta@xxxxxxxxxxxxxxx> wrote:
> > >>
> > >> On 06.09.2021 10:46, Hao Peng wrote:
> > >> >If hypervisor does not support MSR_IA32_TSX_CTRL, but guest supports
> > >> >RTM and HLE features, it will affect TAA mitigation.
> > >>
> > >> Guests are on purpose not allowed to control TSX via MSR_IA32_TSX_CTRL,
> > >> otherwise a malicious guest can enable TSX and attack host or other
> > >> guests. The TAA mitigation within a guest is same as MDS i.e.
> > >> micro-architectural buffer clear using VERW instruction. Support for
> > >> VERW is added by the microcode update and enumerate by
> > >> MSR_ARCH_CAP[MD_CLEAR] bit.
> > >>
> > >> >Signed-off-by: Peng Hao <flyingpeng@xxxxxxxxxxx>
> > >> >---
> > >> > arch/x86/kernel/cpu/tsx.c | 7 +++++++
> > >> > 1 file changed, 7 insertions(+)
> > >> >
> > >> >diff --git a/arch/x86/kernel/cpu/tsx.c b/arch/x86/kernel/cpu/tsx.c
> > >> >index 9c7a5f049292..5e852c14fef2 100644
> > >> >--- a/arch/x86/kernel/cpu/tsx.c
> > >> >+++ b/arch/x86/kernel/cpu/tsx.c
> > >> >@@ -122,6 +122,13 @@ void __init tsx_init(void)
> > >> >
> > >> >        if (!tsx_ctrl_is_supported()) {
> > >> >                tsx_ctrl_state = TSX_CTRL_NOT_SUPPORTED;
> > >> >+
> > >> >+               /* If hypervisor does not support MSR_IA32_TSX_CTRL emulation,
> > >> >+                * but guest supports RTM and HLE features, it will affect TAA
> > >> >+                * （tsx_async_abort）mitigation.
> > >> >+                */
> > >> >+               setup_clear_cpu_cap(X86_FEATURE_RTM);
> > >> >+               setup_clear_cpu_cap(X86_FEATURE_HLE);
> > >>
> > >> This is not correct. TSX feature can exist without TSX_CTRL MSR.
> > >> Moreover, clearing the cached bits with setup_clear_cpu_cap() doesn't
> > >> disable the TSX feature in CPU.
> > >>
> > >After applying this patch, the output of
> > >/sys/devices/system/cpu/vulnerabilities/tsx_async_abort
> > >becomes “Mitigation: TSX disabled”.Do you mean that tsx is still
> > >enabled in this case in guest?
> >
> > If the host has TSX enabled, guest can use TSX instructions irrespective
> > of what cpu capabilities in the guest says.
> I understand that guest cannot produce any actual effects on the hardware,
> so if the host has resolved the TAA bug on the hardware, does the guest actually
> have no vulnerability no matter what TAA status is displayed?

Yes, if the host does not have TAA bug, guest also does not have it.