linux-kernel - Re: [PATCH] x86/tsx: clear RTM and HLE when MSR_IA32_TSX

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Date:   Tue, 7 Sep 2021 15:59:12 -0700
From:   Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To:     Hao Peng <flyingpenghao@...il.com>
Cc:     tglx@...utronix.de, mingo@...hat.com,
        Borislav Petkov <bp@...en8.de>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/tsx: clear RTM and HLE when MSR_IA32_TSX_CTRL is not
 supported

On 07.09.2021 14:36, Hao Peng wrote:
>On Tue, Sep 7, 2021 at 1:13 PM Pawan Gupta
><pawan.kumar.gupta@...ux.intel.com> 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@...cent.com>
>> >---
>> > 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 made a mistake in the description before. This problem occurred
>under the qemu -cpu Icelake-server .

So looks like the real problem is with qemu feature definitions for
cpu "Icelake-Server", it is probably not exporting "taa-no".

>When I debug this problem to -cpu host, the guest can see taa-no.
>Thanks.

Thats good.