From: David Woodhouse dwmw2@infradead.org

commit 7e2175ebd695f17860c5bd4ad7616cce12ed4591 upstream.

In commit b043138246a4 ("x86/KVM: Make sure KVM_VCPU_FLUSH_TLB flag is not missed") we switched to using a gfn_to_pfn_cache for accessing the guest steal time structure in order to allow for an atomic xchg of the preempted field. This has a couple of problems.

Firstly, kvm_map_gfn() doesn't work at all for IOMEM pages when the atomic flag is set, which it is in kvm_steal_time_set_preempted(). So a guest vCPU using an IOMEM page for its steal time would never have its preempted field set.

Secondly, the gfn_to_pfn_cache is not invalidated in all cases where it should have been. There are two stages to the GFN->PFN conversion; first the GFN is converted to a userspace HVA, and then that HVA is looked up in the process page tables to find the underlying host PFN. Correct invalidation of the latter would require being hooked up to the MMU notifiers, but that doesn't happen---so it just keeps mapping and unmapping the *wrong* PFN after the userspace page tables change.

In the !IOMEM case at least the stale page *is* pinned all the time it's cached, so it won't be freed and reused by anyone else while still receiving the steal time updates. The map/unmap dance only takes care of the KVM administrivia such as marking the page dirty.

Until the gfn_to_pfn cache handles the remapping automatically by integrating with the MMU notifiers, we might as well not get a kernel mapping of it, and use the perfectly serviceable userspace HVA that we already have. We just need to implement the atomic xchg on the userspace address with appropriate exception handling, which is fairly trivial.

Cc: stable@vger.kernel.org Fixes: b043138246a4 ("x86/KVM: Make sure KVM_VCPU_FLUSH_TLB flag is not missed") Signed-off-by: David Woodhouse dwmw@amazon.co.uk Message-Id: 3645b9b889dac6438394194bb5586a46b68d581f.camel@infradead.org [I didn't entirely agree with David's assessment of the usefulness of the gfn_to_pfn cache, and integrated the outcome of the discussion in the above commit message. - Paolo] Signed-off-by: Paolo Bonzini pbonzini@redhat.com [risbhat@amazon.com: Use the older mark_page_dirty_in_slot api without kvm argument] Signed-off-by: Rishabh Bhatnagar risbhat@amazon.com Tested-by: Allen Pais apais@linux.microsoft.com Acked-by: Sean Christopherson seanjc@google.com --- arch/x86/include/asm/kvm_host.h | 2 +- arch/x86/kvm/x86.c | 105 +++++++++++++++++++++++--------- 2 files changed, 76 insertions(+), 31 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h index 660012ab7bfa..2452c28b3e69 100644 --- a/arch/x86/include/asm/kvm_host.h +++ b/arch/x86/include/asm/kvm_host.h @@ -663,7 +663,7 @@ struct kvm_vcpu_arch { u8 preempted; u64 msr_val; u64 last_steal; - struct gfn_to_pfn_cache cache; + struct gfn_to_hva_cache cache; } st;

u64 l1_tsc_offset; diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 60bf007f4118..238abb75df11 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -3020,53 +3020,92 @@ static void kvm_vcpu_flush_tlb_guest(struct kvm_vcpu *vcpu)

static void record_steal_time(struct kvm_vcpu *vcpu) { - struct kvm_host_map map; - struct kvm_steal_time *st; + struct gfn_to_hva_cache *ghc = &vcpu->arch.st.cache; + struct kvm_steal_time __user *st; + struct kvm_memslots *slots; + u64 steal; + u32 version;

if (!(vcpu->arch.st.msr_val & KVM_MSR_ENABLED)) return;

- /* -EAGAIN is returned in atomic context so we can just return. */ - if (kvm_map_gfn(vcpu, vcpu->arch.st.msr_val >> PAGE_SHIFT, - &map, &vcpu->arch.st.cache, false)) + if (WARN_ON_ONCE(current->mm != vcpu->kvm->mm)) return;

- st = map.hva + - offset_in_page(vcpu->arch.st.msr_val & KVM_STEAL_VALID_BITS); + slots = kvm_memslots(vcpu->kvm); + + if (unlikely(slots->generation != ghc->generation || + kvm_is_error_hva(ghc->hva) || !ghc->memslot)) { + gfn_t gfn = vcpu->arch.st.msr_val & KVM_STEAL_VALID_BITS; + + /* We rely on the fact that it fits in a single page. */ + BUILD_BUG_ON((sizeof(*st) - 1) & KVM_STEAL_VALID_BITS); + + if (kvm_gfn_to_hva_cache_init(vcpu->kvm, ghc, gfn, sizeof(*st)) || + kvm_is_error_hva(ghc->hva) || !ghc->memslot) + return; + } + + st = (struct kvm_steal_time __user *)ghc->hva; + if (!user_access_begin(st, sizeof(*st))) + return;

/* * Doing a TLB flush here, on the guest's behalf, can avoid * expensive IPIs. */ if (guest_pv_has(vcpu, KVM_FEATURE_PV_TLB_FLUSH)) { - u8 st_preempted = xchg(&st->preempted, 0); + u8 st_preempted = 0; + int err = -EFAULT; + + asm volatile("1: xchgb %0, %2\n" + "xor %1, %1\n" + "2:\n" + _ASM_EXTABLE_UA(1b, 2b) + : "+r" (st_preempted), + "+&r" (err) + : "m" (st->preempted)); + if (err) + goto out; + + user_access_end(); + + vcpu->arch.st.preempted = 0;

trace_kvm_pv_tlb_flush(vcpu->vcpu_id, st_preempted & KVM_VCPU_FLUSH_TLB); if (st_preempted & KVM_VCPU_FLUSH_TLB) kvm_vcpu_flush_tlb_guest(vcpu); + + if (!user_access_begin(st, sizeof(*st))) + goto dirty; } else { - st->preempted = 0; + unsafe_put_user(0, &st->preempted, out); + vcpu->arch.st.preempted = 0; }

- vcpu->arch.st.preempted = 0; - - if (st->version & 1) - st->version += 1; /* first time write, random junk */ + unsafe_get_user(version, &st->version, out); + if (version & 1) + version += 1; /* first time write, random junk */

- st->version += 1; + version += 1; + unsafe_put_user(version, &st->version, out);

smp_wmb();

- st->steal += current->sched_info.run_delay - + unsafe_get_user(steal, &st->steal, out); + steal += current->sched_info.run_delay - vcpu->arch.st.last_steal; vcpu->arch.st.last_steal = current->sched_info.run_delay; + unsafe_put_user(steal, &st->steal, out);

- smp_wmb(); - - st->version += 1; + version += 1; + unsafe_put_user(version, &st->version, out);

- kvm_unmap_gfn(vcpu, &map, &vcpu->arch.st.cache, true, false); + out: + user_access_end(); + dirty: + mark_page_dirty_in_slot(ghc->memslot, gpa_to_gfn(ghc->gpa)); }

int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info) @@ -4051,8 +4090,10 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)

static void kvm_steal_time_set_preempted(struct kvm_vcpu *vcpu) { - struct kvm_host_map map; - struct kvm_steal_time *st; + struct gfn_to_hva_cache *ghc = &vcpu->arch.st.cache; + struct kvm_steal_time __user *st; + struct kvm_memslots *slots; + static const u8 preempted = KVM_VCPU_PREEMPTED;

if (!(vcpu->arch.st.msr_val & KVM_MSR_ENABLED)) return; @@ -4060,16 +4101,23 @@ static void kvm_steal_time_set_preempted(struct kvm_vcpu *vcpu) if (vcpu->arch.st.preempted) return;

- if (kvm_map_gfn(vcpu, vcpu->arch.st.msr_val >> PAGE_SHIFT, &map, - &vcpu->arch.st.cache, true)) + /* This happens on process exit */ + if (unlikely(current->mm != vcpu->kvm->mm)) return;

- st = map.hva + - offset_in_page(vcpu->arch.st.msr_val & KVM_STEAL_VALID_BITS); + slots = kvm_memslots(vcpu->kvm); + + if (unlikely(slots->generation != ghc->generation || + kvm_is_error_hva(ghc->hva) || !ghc->memslot)) + return;

- st->preempted = vcpu->arch.st.preempted = KVM_VCPU_PREEMPTED; + st = (struct kvm_steal_time __user *)ghc->hva; + BUILD_BUG_ON(sizeof(st->preempted) != sizeof(preempted));

- kvm_unmap_gfn(vcpu, &map, &vcpu->arch.st.cache, true, true); + if (!copy_to_user_nofault(&st->preempted, &preempted, sizeof(preempted))) + vcpu->arch.st.preempted = KVM_VCPU_PREEMPTED; + + mark_page_dirty_in_slot(ghc->memslot, gpa_to_gfn(ghc->gpa)); }

void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu) @@ -10244,11 +10292,8 @@ void kvm_arch_vcpu_postcreate(struct kvm_vcpu *vcpu)

void kvm_arch_vcpu_destroy(struct kvm_vcpu *vcpu) { - struct gfn_to_pfn_cache *cache = &vcpu->arch.st.cache; int idx;

- kvm_release_pfn(cache->pfn, cache->dirty, cache); - kvmclock_reset(vcpu);

kvm_x86_ops.vcpu_free(vcpu);

From: Sean Christopherson seanjc@google.com

commit 19979fba9bfaeab427a8e106d915f0627c952828 upstream.

Remove the disabling of page faults across kvm_steal_time_set_preempted() as KVM now accesses the steal time struct (shared with the guest) via a cached mapping (see commit b043138246a4, "x86/KVM: Make sure KVM_VCPU_FLUSH_TLB flag is not missed".) The cache lookup is flagged as atomic, thus it would be a bug if KVM tried to resolve a new pfn, i.e. we want the splat that would be reached via might_fault().

Signed-off-by: Sean Christopherson seanjc@google.com Message-Id: 20210123000334.3123628-2-seanjc@google.com Signed-off-by: Paolo Bonzini pbonzini@redhat.com Signed-off-by: Rishabh Bhatnagar risbhat@amazon.com Tested-by: Allen Pais apais@linux.microsoft.com Acked-by: Sean Christopherson seanjc@google.com --- arch/x86/kvm/x86.c | 10 ---------- 1 file changed, 10 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index b0379ff4b743..4e77233880b7 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -4127,15 +4127,6 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu) if (vcpu->preempted) vcpu->arch.preempted_in_kernel = !kvm_x86_ops.get_cpl(vcpu);

- /* - * Disable page faults because we're in atomic context here. - * kvm_write_guest_offset_cached() would call might_fault() - * that relies on pagefault_disable() to tell if there's a - * bug. NOTE: the write to guest memory may not go through if - * during postcopy live migration or if there's heavy guest - * paging. - */ - pagefault_disable(); /* * kvm_memslots() will be called by * kvm_write_guest_offset_cached() so take the srcu lock. @@ -4143,7 +4134,6 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu) idx = srcu_read_lock(&vcpu->kvm->srcu); kvm_steal_time_set_preempted(vcpu); srcu_read_unlock(&vcpu->kvm->srcu, idx); - pagefault_enable(); kvm_x86_ops.vcpu_put(vcpu); vcpu->arch.last_host_tsc = rdtsc(); /*

From: Paolo Bonzini pbonzini@redhat.com

commit 6cd88243c7e03845a450795e134b488fc2afb736 upstream.

If a vCPU is outside guest mode and is scheduled out, it might be in the process of making a memory access. A problem occurs if another vCPU uses the PV TLB flush feature during the period when the vCPU is scheduled out, and a virtual address has already been translated but has not yet been accessed, because this is equivalent to using a stale TLB entry.

To avoid this, only report a vCPU as preempted if sure that the guest is at an instruction boundary. A rescheduling request will be delivered to the host physical CPU as an external interrupt, so for simplicity consider any vmexit *not* instruction boundary except for external interrupts.

It would in principle be okay to report the vCPU as preempted also if it is sleeping in kvm_vcpu_block(): a TLB flush IPI will incur the vmentry/vmexit overhead unnecessarily, and optimistic spinning is also unlikely to succeed. However, leave it for later because right now kvm_vcpu_check_block() is doing memory accesses. Even though the TLB flush issue only applies to virtual memory address, it's very much preferrable to be conservative.

Reported-by: Jann Horn jannh@google.com Signed-off-by: Paolo Bonzini pbonzini@redhat.com [risbhat@amazon.com: Use VCPU_STAT to expose preemption_reported and preemption_other debugfs entries.] Signed-off-by: Rishabh Bhatnagar risbhat@amazon.com Tested-by: Allen Pais apais@linux.microsoft.com Acked-by: Sean Christopherson seanjc@google.com --- arch/x86/include/asm/kvm_host.h | 3 +++ arch/x86/kvm/svm/svm.c | 2 ++ arch/x86/kvm/vmx/vmx.c | 1 + arch/x86/kvm/x86.c | 22 ++++++++++++++++++++++ 4 files changed, 28 insertions(+)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h index 2452c28b3e69..3e9f1c820edb 100644 --- a/arch/x86/include/asm/kvm_host.h +++ b/arch/x86/include/asm/kvm_host.h @@ -553,6 +553,7 @@ struct kvm_vcpu_arch { u64 ia32_misc_enable_msr; u64 smbase; u64 smi_count; + bool at_instruction_boundary; bool tpr_access_reporting; bool xsaves_enabled; u64 ia32_xss; @@ -1061,6 +1062,8 @@ struct kvm_vcpu_stat { u64 req_event; u64 halt_poll_success_ns; u64 halt_poll_fail_ns; + u64 preemption_reported; + u64 preemption_other; };

struct x86_instruction_info; diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c index 5775983fec56..7b2b61309d8a 100644 --- a/arch/x86/kvm/svm/svm.c +++ b/arch/x86/kvm/svm/svm.c @@ -3983,6 +3983,8 @@ static int svm_check_intercept(struct kvm_vcpu *vcpu,

static void svm_handle_exit_irqoff(struct kvm_vcpu *vcpu) { + if (to_svm(vcpu)->vmcb->control.exit_code == SVM_EXIT_INTR) + vcpu->arch.at_instruction_boundary = true; }

static void svm_sched_in(struct kvm_vcpu *vcpu, int cpu) diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c index 2c5d8b9f9873..16943e923902 100644 --- a/arch/x86/kvm/vmx/vmx.c +++ b/arch/x86/kvm/vmx/vmx.c @@ -6510,6 +6510,7 @@ static void handle_external_interrupt_irqoff(struct kvm_vcpu *vcpu) return;

handle_interrupt_nmi_irqoff(vcpu, gate_offset(desc)); + vcpu->arch.at_instruction_boundary = true; }

static void vmx_handle_exit_irqoff(struct kvm_vcpu *vcpu) diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 80231f62e129..116a225fb26e 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -231,6 +231,8 @@ struct kvm_stats_debugfs_item debugfs_entries[] = { VCPU_STAT("l1d_flush", l1d_flush), VCPU_STAT("halt_poll_success_ns", halt_poll_success_ns), VCPU_STAT("halt_poll_fail_ns", halt_poll_fail_ns), + VCPU_STAT("preemption_reported", preemption_reported), + VCPU_STAT("preemption_other", preemption_other), VM_STAT("mmu_shadow_zapped", mmu_shadow_zapped), VM_STAT("mmu_pte_write", mmu_pte_write), VM_STAT("mmu_pde_zapped", mmu_pde_zapped), @@ -4095,6 +4097,19 @@ static void kvm_steal_time_set_preempted(struct kvm_vcpu *vcpu) struct kvm_memslots *slots; static const u8 preempted = KVM_VCPU_PREEMPTED;

+ /* + * The vCPU can be marked preempted if and only if the VM-Exit was on + * an instruction boundary and will not trigger guest emulation of any + * kind (see vcpu_run). Vendor specific code controls (conservatively) + * when this is true, for example allowing the vCPU to be marked + * preempted if and only if the VM-Exit was due to a host interrupt. + */ + if (!vcpu->arch.at_instruction_boundary) { + vcpu->stat.preemption_other++; + return; + } + + vcpu->stat.preemption_reported++; if (!(vcpu->arch.st.msr_val & KVM_MSR_ENABLED)) return;

@@ -9399,6 +9414,13 @@ static int vcpu_run(struct kvm_vcpu *vcpu) vcpu->arch.l1tf_flush_l1d = true;

for (;;) { + /* + * If another guest vCPU requests a PV TLB flush in the middle + * of instruction emulation, the rest of the emulation could + * use a stale page translation. Assume that any code after + * this point can start executing an instruction. + */ + vcpu->arch.at_instruction_boundary = false; if (kvm_vcpu_running(vcpu)) { r = vcpu_enter_guest(vcpu); } else {

From: Paolo Bonzini pbonzini@redhat.com

commit c3c28d24d910a746b02f496d190e0e8c6560224b upstream.

Commit 7e2175ebd695 ("KVM: x86: Fix recording of guest steal time / preempted status", 2021-11-11) open coded the previous call to kvm_map_gfn, but in doing so it dropped the comparison between the cached guest physical address and the one in the MSR. This cause an incorrect cache hit if the guest modifies the steal time address while the memslots remain the same. This can happen with kexec, in which case the preempted bit is written at the address used by the old kernel instead of the old one.

Cc: David Woodhouse dwmw@amazon.co.uk Cc: stable@vger.kernel.org Fixes: 7e2175ebd695 ("KVM: x86: Fix recording of guest steal time / preempted status") Signed-off-by: Paolo Bonzini pbonzini@redhat.com Signed-off-by: Rishabh Bhatnagar risbhat@amazon.com Tested-by: Allen Pais apais@linux.microsoft.com Acked-by: Sean Christopherson seanjc@google.com --- arch/x86/kvm/x86.c | 2 ++ 1 file changed, 2 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 0503bb8b64e5..8461aa63c251 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -4096,6 +4096,7 @@ static void kvm_steal_time_set_preempted(struct kvm_vcpu *vcpu) struct kvm_steal_time __user *st; struct kvm_memslots *slots; static const u8 preempted = KVM_VCPU_PREEMPTED; + gpa_t gpa = vcpu->arch.st.msr_val & KVM_STEAL_VALID_BITS;

/* * The vCPU can be marked preempted if and only if the VM-Exit was on @@ -4123,6 +4124,7 @@ static void kvm_steal_time_set_preempted(struct kvm_vcpu *vcpu) slots = kvm_memslots(vcpu->kvm);

if (unlikely(slots->generation != ghc->generation || + gpa != ghc->gpa || kvm_is_error_hva(ghc->hva) || !ghc->memslot)) return;