Re: [PATCH v2] x86/speculation, KVM: only IBPB for switch_mm_always_ibpb on vCPU load

[Jon Kohler <jon@nutanix.com>]

> On Apr 21, 2022, at 11:20 AM, Sean Christopherson <seanjc@google.com> wrote:
> 
> On Mon, Apr 18, 2022, Jon Kohler wrote:
>> On vmx_vcpu_load_vmcs and svm_vcpu_load, respect user controlled
>> configuration for conditional IBPB and only attempt IBPB MSR when
>> switching between different guest vCPUs IFF switch_mm_always_ibpb,
>> which fixes a situation where the kernel will issue IBPB
>> unconditionally even when conditional IBPB is enabled.
>> 
>> If a user has spectre_v2_user mitigation enabled, in any
>> configuration, and the underlying processor supports X86_FEATURE_IBPB,
>> X86_FEATURE_USE_IBPB is set and any calls to
>> indirect_branch_prediction_barrier() will issue IBPB MSR.
>> 
>> Depending on the spectre_v2_user configuration, either
>> switch_mm_always_ibpb key or switch_mm_cond_ibpb key will be set.
>> 
>> Both switch_mm_always_ibpb and switch_mm_cond_ibpb are handled by
>> switch_mm() -> cond_mitigation(), which works well in cases where
>> switching vCPUs (i.e. switching tasks) also switches mm_struct;
>> however, this misses a paranoid case where user space may be running
>> multiple guests in a single process (i.e. single mm_struct).
>> 
>> This paranoid case is already covered by vmx_vcpu_load_vmcs and
>> svm_vcpu_load; however, this is done by calling
>> indirect_branch_prediction_barrier() and thus the kernel
>> unconditionally issues IBPB if X86_FEATURE_USE_IBPB is set.
> 
> The changelog should call out that switch_mm_cond_ibpb is intentionally "ignored"
> for the virt case, and explain why it's nonsensical to emit IBPB in that scenario.

Ok will do, thanks

> 
>> Fix by using intermediary call to x86_virt_guest_switch_ibpb(), which
>> gates IBPB MSR IFF switch_mm_always_ibpb is true. This is useful for
>> security paranoid VMMs in either single process or multi-process VMM
>> configurations.
> 
> Multi-process VMM?  KVM doesn't allow "sharing" a VM across processes.  Userspace
> can share guest memory across processes, but that's not relevant to an IBPB on
> guest switch.  I suspect you're loosely referring to all of userspace as a single
> VMM.  That's inaccurate, or at least unnecessarily confusing, from a kernel
> perspective.  I am not aware of a VMM that runs as a monolithic "daemon" and forks
> a new process for every VM.  And even in such a case, I would argue that most
> people would refer to each process as a separate VMM.
> 
> If there's a blurb about the switch_mm_cond_ibpb case being nonsensical, there's
> probably a good segue into stating the new behavior.

Yea, thats what I was getting at but failed to wordsmith it nicely. I’ll sharpen it up
and integrate your feedback into a v3

> 
>> switch_mm_always_ibpb key is user controlled via spectre_v2_user and
>> will be true for the following configurations:
>>  spectre_v2_user=on
>>  spectre_v2_user=prctl,ibpb
>>  spectre_v2_user=seccomp,ibpb
>> 
>> Signed-off-by: Jon Kohler <jon@nutanix.com>
>> Cc: Sean Christopherson <seanjc@google.com>
>> Cc: Andrea Arcangeli <aarcange@redhat.com>
>> Cc: Kees Cook <keescook@chromium.org>
>> Cc: Josh Poimboeuf <jpoimboe@redhat.com>
>> Cc: Waiman Long <longman@redhat.com>
>> ---
>> v1 -> v2:
>> - Addressed comments on approach from Sean.
>> 
>> arch/x86/include/asm/spec-ctrl.h | 15 +++++++++++++++
>> arch/x86/kernel/cpu/bugs.c       |  6 +++++-
>> arch/x86/kvm/svm/svm.c           |  2 +-
>> arch/x86/kvm/vmx/vmx.c           |  2 +-
>> 4 files changed, 22 insertions(+), 3 deletions(-)
>> 
>> diff --git a/arch/x86/include/asm/spec-ctrl.h b/arch/x86/include/asm/spec-ctrl.h
>> index 5393babc0598..1ad140b17ad7 100644
>> --- a/arch/x86/include/asm/spec-ctrl.h
>> +++ b/arch/x86/include/asm/spec-ctrl.h
>> @@ -85,4 +85,19 @@ static inline void speculative_store_bypass_ht_init(void) { }
>> extern void speculation_ctrl_update(unsigned long tif);
>> extern void speculation_ctrl_update_current(void);
>> 
>> +/*
>> + * Issue IBPB when switching guest vCPUs IFF if switch_mm_always_ibpb.
> 
> Extra "if" there.
> 
>> + * Primarily useful for security paranoid (or naive) user space VMMs
>> + * that may run multiple VMs within a single process.
>> + * For multi-process VMMs, switching vCPUs, i.e. switching tasks,
> 
> As above, "multi-process VMMs" is very confusing, they're really just separate VMMs.
> Something like this?
> 
> * For the more common case of running VMs in their own dedicated process,
> * switching vCPUs that belong to different VMs, i.e. switching tasks, will also
> * ...
> 
>> + * will also switch mm_structs and thus do IPBP via cond_mitigation();
>> + * however, in the always_ibpb case, take a paranoid approach and issue
>> + * IBPB on both switch_mm() and vCPU switch.
>> + */
>> +static inline void x86_virt_guest_switch_ibpb(void)
>> +{
>> +	if (static_branch_unlikely(&switch_mm_always_ibpb))
>> +		indirect_branch_prediction_barrier();
>> +}
>> +
>> #endif
>> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
>> index 6296e1ebed1d..6aafb0279cbc 100644