Re: [PATCH v2 07/17] KVM: x86/mmu: Check PDPTRs before allocating PAE roots

[Wanpeng Li]

On Fri, 5 Mar 2021 at 09:12, Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> Check the validity of the PDPTRs before allocating any of the PAE roots,
> otherwise a bad PDPTR will cause KVM to leak any previously allocated
> roots.
>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/kvm/mmu/mmu.c | 20 ++++++++++++++------
>  1 file changed, 14 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 7ebfbc77b050..9fc2b46f8541 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3269,7 +3269,7 @@ static int mmu_alloc_direct_roots(struct kvm_vcpu *vcpu)
>  static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
>  {
>         struct kvm_mmu *mmu = vcpu->arch.mmu;
> -       u64 pdptr, pm_mask;
> +       u64 pdptrs[4], pm_mask;
>         gfn_t root_gfn, root_pgd;
>         hpa_t root;
>         int i;
> @@ -3280,6 +3280,17 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
>         if (mmu_check_root(vcpu, root_gfn))
>                 return 1;
>
> +       if (mmu->root_level == PT32E_ROOT_LEVEL) {
> +               for (i = 0; i < 4; ++i) {
> +                       pdptrs[i] = mmu->get_pdptr(vcpu, i);
> +                       if (!(pdptrs[i] & PT_PRESENT_MASK))
> +                               continue;
> +
> +                       if (mmu_check_root(vcpu, pdptrs[i] >> PAGE_SHIFT))
> +                               return 1;
> +               }
> +       }
> +

I saw this splatting:

 BUG: sleeping function called from invalid context at
arch/x86/kvm/kvm_cache_regs.h:115
 in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 3090, name:
qemu-system-x86
 3 locks held by qemu-system-x86/3090:
  #0: ffff8d499f8d00d0 (&vcpu->mutex){+.+.}-{3:3}, at:
kvm_vcpu_ioctl+0x8e/0x680 [kvm]
  #1: ffffb1b540f873e8 (&kvm->srcu){....}-{0:0}, at:
vcpu_enter_guest+0xb30/0x1b10 [kvm]
  #2: ffffb1b540f7d018 (&(kvm)->mmu_lock){+.+.}-{2:2}, at:
kvm_mmu_load+0xb5/0x540 [kvm]
 Preemption disabled at:
 [<ffffffffc0787365>] kvm_mmu_load+0xb5/0x540 [kvm]
 CPU: 2 PID: 3090 Comm: qemu-system-x86 Tainted: G        W  OE
5.12.0-rc3+ #3
 Call Trace:
  dump_stack+0x87/0xb7
  ___might_sleep+0x202/0x250
  __might_sleep+0x4a/0x80
  kvm_pdptr_read+0x20/0x60 [kvm]
  kvm_mmu_load+0x3bd/0x540 [kvm]
  vcpu_enter_guest+0x1297/0x1b10 [kvm]
  kvm_arch_vcpu_ioctl_run+0x372/0x690 [kvm]
  kvm_vcpu_ioctl+0x3ca/0x680 [kvm]
  __x64_sys_ioctl+0x27a/0x800
  do_syscall_64+0x38/0x50
  entry_SYSCALL_64_after_hwframe+0x44/0xae

There is a might_sleep() in kvm_pdptr_read(), however, the original
commit didn't explain more. I can send a formal one if the below fix
is acceptable.

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index efb41f3..f33026f 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3289,17 +3289,24 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
     if (mmu_check_root(vcpu, root_gfn))
         return 1;

+    write_unlock(&vcpu->kvm->mmu_lock);
     if (mmu->root_level == PT32E_ROOT_LEVEL) {
         for (i = 0; i < 4; ++i) {
             pdptrs[i] = mmu->get_pdptr(vcpu, i);
             if (!(pdptrs[i] & PT_PRESENT_MASK))
                 continue;

-            if (mmu_check_root(vcpu, pdptrs[i] >> PAGE_SHIFT))
+            if (mmu_check_root(vcpu, pdptrs[i] >> PAGE_SHIFT)) {
+                write_lock(&vcpu->kvm->mmu_lock);
                 return 1;
+            }
         }
     }

+    write_lock(&vcpu->kvm->mmu_lock);
+    if (make_mmu_pages_available(vcpu))
+        return -ENOSPC;
+
     /*
      * Do we shadow a long mode page table? If so we need to
      * write-protect the guests page table root.