patches/kernel/0013-KVM-x86-mmu-Fix-an-sign-extension-bug-with-mmu_seq-t.patch

   1 From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
   2 From: Sean Christopherson <seanjc@google.com>
   3 Date: Wed, 23 Aug 2023 18:01:04 -0700
   4 Subject: [PATCH] KVM: x86/mmu: Fix an sign-extension bug with mmu_seq that
   5  hangs vCPUs
   6 MIME-Version: 1.0
   7 Content-Type: text/plain; charset=UTF-8
   8 Content-Transfer-Encoding: 8bit
   9
  10 Upstream commit ba6e3fe25543 ("KVM: x86/mmu: Grab mmu_invalidate_seq in
  11 kvm_faultin_pfn()") unknowingly fixed the bug in v6.3 when refactoring
  12 how KVM tracks the sequence counter snapshot.
  13
  14 Take the vCPU's mmu_seq snapshot as an "unsigned long" instead of an "int"
  15 when checking to see if a page fault is stale, as the sequence count is
  16 stored as an "unsigned long" everywhere else in KVM.  This fixes a bug
  17 where KVM will effectively hang vCPUs due to always thinking page faults
  18 are stale, which results in KVM refusing to "fix" faults.
  19
  20 mmu_invalidate_seq (née mmu_notifier_seq) is a sequence counter used when
  21 KVM is handling page faults to detect if userspace mappings relevant to
  22 the guest were invalidated between snapshotting the counter and acquiring
  23 mmu_lock, i.e. to ensure that the userspace mapping KVM is using to
  24 resolve the page fault is fresh.  If KVM sees that the counter has
  25 changed, KVM simply resumes the guest without fixing the fault.
  26
  27 What _should_ happen is that the source of the mmu_notifier invalidations
  28 eventually goes away, mmu_invalidate_seq becomes stable, and KVM can once
  29 again fix guest page fault(s).
  30
  31 But for a long-lived VM and/or a VM that the host just doesn't particularly
  32 like, it's possible for a VM to be on the receiving end of 2 billion (with
  33 a B) mmu_notifier invalidations.  When that happens, bit 31 will be set in
  34 mmu_invalidate_seq.  This causes the value to be turned into a 32-bit
  35 negative value when implicitly cast to an "int" by is_page_fault_stale(),
  36 and then sign-extended into a 64-bit unsigned when the signed "int" is
  37 implicitly cast back to an "unsigned long" on the call to
  38 mmu_invalidate_retry_hva().
  39
  40 As a result of the casting and sign-extension, given a sequence counter of
  41 e.g. 0x8002dc25, mmu_invalidate_retry_hva() ends up doing
  42
  43         if (0x8002dc25 != 0xffffffff8002dc25)
  44
  45 and signals that the page fault is stale and needs to be retried even
  46 though the sequence counter is stable, and KVM effectively hangs any vCPU
  47 that takes a page fault (EPT violation or #NPF when TDP is enabled).
  48
  49 Reported-by: Brian Rak <brak@vultr.com>
  50 Reported-by: Amaan Cheval <amaan.cheval@gmail.com>
  51 Reported-by: Eric Wheeler <kvm@lists.ewheeler.net>
  52 Closes: https://lore.kernel.org/all/f023d927-52aa-7e08-2ee5-59a2fbc65953@gameservers.com
  53 Fixes: a955cad84cda ("KVM: x86/mmu: Retry page fault if root is invalidated by memslot update")
  54 Signed-off-by: Sean Christopherson <seanjc@google.com>
  55 Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
  56 (cherry-picked from commit 82d811ff566594de3676f35808e8a9e19c5c864c in stable v6.1.51)
  57 Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
  58 ---
  59  arch/x86/kvm/mmu/mmu.c | 3 ++-
  60  1 file changed, 2 insertions(+), 1 deletion(-)
  61
  62 diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
  63 index 3220c1285984..c42ba5cde7a4 100644
  64 --- a/arch/x86/kvm/mmu/mmu.c
  65 +++ b/arch/x86/kvm/mmu/mmu.c
  66 @@ -4261,7 +4261,8 @@ static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
  67   * root was invalidated by a memslot update or a relevant mmu_notifier fired.
  68   */
  69  static bool is_page_fault_stale(struct kvm_vcpu *vcpu,
  70 -                               struct kvm_page_fault *fault, int mmu_seq)
  71 +                               struct kvm_page_fault *fault,
  72 +                               unsigned long mmu_seq)
  73  {
  74         struct kvm_mmu_page *sp = to_shadow_page(vcpu->arch.mmu->root.hpa);
  75