Message-ID: <8548ea3c-7ec6-4027-927f-fce9bbd5e6a3@gmail.com>
Date: Thu, 7 Mar 2024 20:02:27 +0800
From: Like Xu <like.xu.linux@...il.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Mingwei Zhang <mizhang@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>,
 Kan Liang <kan.liang@...ux.intel.com>, Like Xu <likexu@...cent.com>,
 kvm@...r.kernel.org, linux-perf-users@...r.kernel.org,
 linux-kernel@...r.kernel.org, Zhenyu Wang <zhenyuw@...ux.intel.com>,
 Zhang Xiong <xiong.y.zhang@...el.com>, Lv Zhiyuan <zhiyuan.lv@...el.com>,
 Dapeng Mi <dapeng1.mi@...el.com>, Dapeng Mi <dapeng1.mi@...ux.intel.com>
Subject: Re: [Patch v3] KVM: x86/pmu: Manipulate FIXED_CTR_CTRL MSR with
 macros

On 7/3/2024 4:17 am, Sean Christopherson wrote:
> On Wed, Mar 06, 2024, Like Xu wrote:
>>>> @@ -595,7 +600,7 @@ static void intel_pmu_refresh(struct kvm_vcpu *vcpu)
>>>>    			pmu->reserved_bits &= ~ICL_EVENTSEL_ADAPTIVE;
>>>>    			for (i = 0; i < pmu->nr_arch_fixed_counters; i++) {
>>>>    				pmu->fixed_ctr_ctrl_mask &=
>>>> -					~(1ULL << (INTEL_PMC_IDX_FIXED + i * 4));
>>>
>>> OMG, this might just win the award for most obfuscated PMU code in KVM, which is
>>> saying something.  The fact that INTEL_PMC_IDX_FIXED happens to be 32, the same
>>> bit number as ICL_FIXED_0_ADAPTIVE, is 100% coincidence.  Good riddance.
>>>
>>> Argh, and this goofy code helped introduce a real bug.  reprogram_fixed_counters()
>>> doesn't account for the upper 32 bits of IA32_FIXED_CTR_CTRL.
>>>
>>> Wait, WTF?  Nothing in KVM accounts for the upper bits.  This can't possibly work.
>>>
>>> IIUC, because KVM _always_ sets precise_ip to a non-zero bit for PEBS events,
>>> perf will _always_ generate an adaptive record, even if the guest requested a
>>> basic record.  Ugh, and KVM will always generate adaptive records even if the
>>> guest doesn't support them.  This is all completely broken.  It probably kinda
>>> sorta works because the Basic info is always stored in the record, and generating
>>> more info requires a non-zero MSR_PEBS_DATA_CFG, but ugh.
>>
>> Yep, it works at least on machines with both adaptive and pebs_full features.
> 
> *AND* if the guest uses PEBS exactly the same way that the Linux kernel uses PEBS.
> 
>> Mingwei or others are encouraged to construct use cases in KUT::pmu_pebs.flat
>> that violate guest-pebs rules (e.g., leak host state), as we all recognize
>> that testing is the right way to condemn legacy code, not just lengthy emails.
> 
> *sigh*
> 
> diff --git a/x86/pmu_pebs.c b/x86/pmu_pebs.c
> index f7b52b90..43e7a207 100644
> --- a/x86/pmu_pebs.c
> +++ b/x86/pmu_pebs.c
> @@ -212,8 +212,12 @@ static void pebs_enable(u64 bitmask, u64 pebs_data_cfg)
>          u64 baseline_extra_ctrl = 0, fixed_ctr_ctrl = 0;
>          unsigned int idx;
>   
> -       if (has_baseline)
> -               wrmsr(MSR_PEBS_DATA_CFG, pebs_data_cfg);
> +       if (has_baseline) {
> +               if (pebs_data_cfg)
> +                       wrmsr(MSR_PEBS_DATA_CFG, pebs_data_cfg);
> +               else
> +                       wrmsr(MSR_PEBS_DATA_CFG, 0xf);
> +       }
>   
>          ds = (struct debug_store *)ds_bufer;
>          ds->pebs_index = ds->pebs_buffer_base = (unsigned long)pebs_buffer;
> @@ -224,7 +228,7 @@ static void pebs_enable(u64 bitmask, u64 pebs_data_cfg)
>          for (idx = 0; idx < pmu.nr_fixed_counters; idx++) {
>                  if (!(BIT_ULL(FIXED_CNT_INDEX + idx) & bitmask))
>                          continue;
> -               if (has_baseline)
> +               if (has_baseline && pebs_data_cfg)
>                          baseline_extra_ctrl = BIT(FIXED_CNT_INDEX + idx * 4);
>                  wrmsr(MSR_PERF_FIXED_CTRx(idx), ctr_start_val);
>                  fixed_ctr_ctrl |= (0xbULL << (idx * 4) | baseline_extra_ctrl);
> @@ -235,7 +239,7 @@ static void pebs_enable(u64 bitmask, u64 pebs_data_cfg)
>          for (idx = 0; idx < max_nr_gp_events; idx++) {
>                  if (!(BIT_ULL(idx) & bitmask))
>                          continue;
> -               if (has_baseline)
> +               if (has_baseline && pebs_data_cfg)
>                          baseline_extra_ctrl = ICL_EVENTSEL_ADAPTIVE;
>                  wrmsr(MSR_GP_EVENT_SELECTx(idx), EVNTSEL_EN | EVNTSEL_OS | EVNTSEL_USR |
>                                                   intel_arch_events[idx] | baseline_extra_ctrl);
> 
> FAIL: Multiple (0x700000055): PEBS record (written seq 0) is verified (including size, counters and cfg).
> FAIL: The pebs_record_size (488) doesn't match with MSR_PEBS_DATA_CFG (32).
> FAIL: The pebs_data_cfg (0xf) doesn't match with MSR_PEBS_DATA_CFG (0x0).
> FAIL: GP counter 0 (0xfffffffffffe): PEBS record (written seq 0) is verified (including size, counters and cfg).
> FAIL: The pebs_record_size (488) doesn't match with MSR_PEBS_DATA_CFG (32).
> FAIL: The pebs_data_cfg (0xf) doesn't match with MSR_PEBS_DATA_CFG (0x0).
> FAIL: Multiple (0x700000055): PEBS record (written seq 0) is verified (including size, counters and cfg).
> FAIL: The pebs_record_size (488) doesn't match with MSR_PEBS_DATA_CFG (32).
> FAIL: The pebs_data_cfg (0xf) doesn't match with MSR_PEBS_DATA_CFG (0x0).
> 
>>> Oh great, and it gets worse.  intel_pmu_disable_fixed() doesn't clear the upper
>>> bits either, i.e. leaves ICL_FIXED_0_ADAPTIVE set.  Unless I'm misreading the code,
>>> intel_pmu_enable_fixed() effectively doesn't clear ICL_FIXED_0_ADAPTIVE either,
>>> as it only modifies the bit when it wants to set ICL_FIXED_0_ADAPTIVE.
>>>
>>> *sigh*
>>>
>>> I'm _very_ tempted to disable KVM PEBS support for the current PMU, and make it
>>> available only when the so-called passthrough PMU is available[*].  Because I
>>> don't see how this is can possibly be functionally correct, nor do I see a way
>>> to make it functionally correct without a rather large and invasive series.
>>
>> Considering that I've tried the idea myself, I have no inclination towards
>> "passthrough PMU", and I'd like to be able to take the time to review that
>> patchset while we all wait for a clear statement from that perf-core man,
>> who don't really care about virtualization and don't want to lose control
>> of global hardware resources.
>>
>> Before we actually get to that ideal state you want, we have to deal with
>> some intermediate state and face to any users that rely on the current code,
> 
> It's not an ideal state, it's simply the only way I see to get things like adaptive
> PEBS to work safely, reliably, and correctly without taking on an absurd amount of
> complexity.

The upstream guest_pebs feature has always been broken from my perspective,
but unfortunately I didn't get enough time to polish it (slow path, host/guest 
multiplexing,
live migration support, security defense), so I'm really glad that Google is 
seriously
considering using this or that PMU feature.

> 
>> you had urged to merge in a KVM document for vPMU, not sure how far
>> along that part of the work is.
> 
>>> Ouch.  And after chatting with Mingwei, who asked the very good question of
>>> "can this leak host state?", I am pretty sure that yes, this can leak host state.
>>
>> The Basic Info has a tsc field, I suspect it's the host-state-tsc.
> 
> It's not, the CPU offsets it correctly, at least on ICX (I haven't check scaling).

And one more to check, the tsc fields form the guest Intel_PT or Timed LBR if any.

> 
>>> When PERF_CAP_PEBS_BASELINE is enabled for the guest, i.e. when the guest has
>>> access to adaptive records, KVM gives the guest full access to MSR_PEBS_DATA_CFG
>>>
>>> 	pmu->pebs_data_cfg_mask = ~0xff00000full;
>>>
>>> which makes sense in a vacuum, because AFAICT the architecture doesn't allow
>>> exposing a subset of the four adaptive controls.
>>>
>>> GPRs and XMMs are always context switched and thus benign, but IIUC, Memory Info
>>> provides data that might now otherwise be available to the guest, e.g. if host
>>> userspace has disallowed equivalent events via KVM_SET_PMU_EVENT_FILTER.
>>
>> Indeed, KVM_SET_PMU_EVENT_FILTER doesn't work in harmony with
>> guest-pebs, and I believe there is a big problem here, especially with the
>> lack of targeted testing.
>>
>> One reason for this is that we don't use this cockamamie API in our
> 
> Libeling APIs because they aren't useful for _your_ security goals doesn't mean
> you get to ignore their existence when contributing upstream.  New features don't
> necessarilly have to fully support existing capabilities, e.g. I would be a-ok
> making KVM_SET_PMU_EVENT_FILTER mututally exclusive with exposing adapative PEBS
> to the guest.  That way the user can at least know that filtering won't work if
> adapative PEBS is exposed to the guest.

Yes, that should be considered.

> 
>> large-scale production environments, and users of vPMU want to get real
>> runtime information about physical cpus, not just virtualised hardware
>> architecture interfaces.
>>
>>>
>>> And unless I'm missing something, LBRs are a full leak of host state.  Nothing
>>> in the SDM suggests that PEBS records honor MSR intercepts, so unless KVM is
>>> also passing through LBRs, i.e. is context switching all LBR MSRs, the guest can
>>> use PEBS to read host LBRs at will.
>>
>> KVM is also passing through LBRs when guest uses LBR but not at the
>> granularity of vm-exit/entry. I'm not sure if the LBR_EN bit is required
>> to get LBR information via PEBS, also not confirmed whether PEBS-lbr
>> can be enabled at the same time as independent LBR;
>>
>> I recall that PEBS-assist, per cpu-arch, would clean up this part of the
>> record when crossing root/non-root boundaries, or not generate record.
> 
> Nope.  The MSRs definitely leak to the guest.  The only hard part was figuring
> out how to get perf to utilize LBRs without consuming every counter (`perf top`
> was the extent of my knowledge, until now...).
> 
> E.g.
> 
>    perf record -b -e instructions

Consulting "Ian Rogers" is the fastest route.

> 
> and
> 
>    FAIL: PEBS LBR record 0 isn't empty, got from = 'ffffffffc0a00ccb', to = 'ffffffffc0a010af', info = '2'
>    FAIL: PEBS LBR record 1 isn't empty, got from = 'ffffffffc0a010aa', to = 'ffffffffc0a00cb0', info = '6'
>    FAIL: PEBS LBR record 2 isn't empty, got from = 'ffffffffc0a00e06', to = 'ffffffffc0a01090', info = '1'
>    FAIL: PEBS LBR record 3 isn't empty, got from = 'ffffffffc0a00df4', to = 'ffffffffc0a00e00', info = '2'
>    FAIL: PEBS LBR record 4 isn't empty, got from = 'ffffffffc0a00dbc', to = 'ffffffffc0a00de0', info = '1'
>    FAIL: PEBS LBR record 5 isn't empty, got from = 'ffffffffc0a00f63', to = 'ffffffffc0a00db5', info = '1'
>    FAIL: PEBS LBR record 6 isn't empty, got from = 'ffffffffc0903f23', to = 'ffffffffc0a00f61', info = '11'
>    FAIL: PEBS LBR record 7 isn't empty, got from = 'ffffffffc0a00f5c', to = 'ffffffffc0903f10', info = '1'
>    FAIL: PEBS LBR record 8 isn't empty, got from = 'ffffffffc0a00db0', to = 'ffffffffc0a00f55', info = '1'
>    FAIL: PEBS LBR record 9 isn't empty, got from = 'ffffffff8f6b2c23', to = 'ffffffffc0a00da6', info = 'a'
>    FAIL: PEBS LBR record 10 isn't empty, got from = 'ffffffffc0a00da1', to = 'ffffffff8f6b2b60', info = '7'
>    FAIL: PEBS LBR record 11 isn't empty, got from = 'ffffffff8eba1b85', to = 'ffffffffc0a00d9a', info = '6'
>    FAIL: PEBS LBR record 12 isn't empty, got from = 'ffffffff8eba1b8c', to = 'ffffffff8eba1b3f', info = '1'
>    FAIL: PEBS LBR record 13 isn't empty, got from = 'ffffffff8eba1b2d', to = 'ffffffff8eba1b87', info = 'e'
>    FAIL: PEBS LBR record 14 isn't empty, got from = 'ffffffff8eba1aff', to = 'ffffffff8eba1b18', info = '7'
>    FAIL: PEBS LBR record 15 isn't empty, got from = 'ffffffff8eb996e0', to = 'ffffffff8eba1aeb', info = '2'
>    FAIL: PEBS LBR record 16 isn't empty, got from = 'ffffffff8eb9963a', to = 'ffffffff8eb996cc', info = '1'
>    FAIL: PEBS LBR record 17 isn't empty, got from = 'ffffffff8eb995ef', to = 'ffffffff8eb9962f', info = '1'
>    FAIL: PEBS LBR record 18 isn't empty, got from = 'ffffffff8f6b30a1', to = 'ffffffff8eb995df', info = '4'
>    FAIL: PEBS LBR record 19 isn't empty, got from = 'ffffffff8eb995da', to = 'ffffffff8f6b3070', info = '2'
>    FAIL: PEBS LBR record 20 isn't empty, got from = 'ffffffff8eba1ae6', to = 'ffffffff8eb995c0', info = '6'
>    FAIL: PEBS LBR record 21 isn't empty, got from = 'ffffffffc0a00d95', to = 'ffffffff8eba1a90', info = '2'
>    FAIL: PEBS LBR record 22 isn't empty, got from = 'ffffffff8eb69135', to = 'ffffffffc0a00d89', info = '2'
>    FAIL: PEBS LBR record 23 isn't empty, got from = 'ffffffff8eb690d6', to = 'ffffffff8eb69104', info = '2'
>    FAIL: PEBS LBR record 24 isn't empty, got from = 'ffffffff8eb69102', to = 'ffffffff8eb690c3', info = '1'
>    FAIL: PEBS LBR record 25 isn't empty, got from = 'ffffffff8eb6f442', to = 'ffffffff8eb69100', info = '2'
>    FAIL: PEBS LBR record 26 isn't empty, got from = 'ffffffff8eb6f3f3', to = 'ffffffff8eb6f429', info = '5'
>    FAIL: PEBS LBR record 27 isn't empty, got from = 'ffffffff8eb6f3a6', to = 'ffffffff8eb6f3c3', info = '2'
>    FAIL: PEBS LBR record 28 isn't empty, got from = 'ffffffff8eb690fb', to = 'ffffffff8eb6f390', info = '2'
>    FAIL: PEBS LBR record 29 isn't empty, got from = 'ffffffff8eb690c1', to = 'ffffffff8eb690d8', info = '2'
>    FAIL: PEBS LBR record 30 isn't empty, got from = 'ffffffff8eb6907b', to = 'ffffffff8eb690b1', info = '1'
>    FAIL: PEBS LBR record 31 isn't empty, got from = 'ffffffff8eb690af', to = 'ffffffff8eb6906e', info = '1'

Guilty Guilty Guilty

> 
>>> diff --git a/arch/x86/kvm/vmx/capabilities.h b/arch/x86/kvm/vmx/capabilities.h
>>> index 41a4533f9989..a2f827fa0ca1 100644
>>> --- a/arch/x86/kvm/vmx/capabilities.h
>>> +++ b/arch/x86/kvm/vmx/capabilities.h
>>> @@ -392,7 +392,7 @@ static inline bool vmx_pt_mode_is_host_guest(void)
>>>    static inline bool vmx_pebs_supported(void)
>>>    {
>>> -       return boot_cpu_has(X86_FEATURE_PEBS) && kvm_pmu_cap.pebs_ept;
>>> +       return false;
>>
>> As you know, user-space VMM may disable guest-pebs by filtering out the
>> MSR_IA32_PERF_CAPABILITIE.PERF_CAP_PEBS_FORMAT or CPUID.PDCM.
> 
> Relying on userspace to fudge around KVM bugs is not acceptable for upstream.
> PMU virtualization is already a bit dicey in that KVM relies on userspace to
> filter out sensitive events, but leaking host addresses and failing to correctly
> virtualize the architecture is an entirely different level of wrong.

The leak is real and I would do the same thing (drop ADAPTIVE) in the kernel side.

> 
>> In the end, if our great KVM maintainers insist on doing this,
>> there is obviously nothing I can do about it.
> 
> Bullshit.  There is plenty you could have done to prevent this.  It took me what,
> 6 hours total?  To (a) realize the code is buggy, (b) type up an email, and (c)
> modify tests to confirm the bugs.  And at least half of that time was due to me
> floundering around trying to generate LBR records because I know nothing about
> the perf tool.
> 
> If you were more interested in ensuring KVM is maintainble, secure, and functionally
> correct, we wouldn't be where we are today.

I was actually really trying to communicate my todo list to my successors
in this direction, but it took a long time for them to warm up.

Very pleased to know that you have so much manpower invested in vPMU,
which was unimaginable a few years ago when I was the only one doing
these things.

We have benefited from your efforts here or there, and hope to still have
the opportunity to contribute back. Thank you, Sean.

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