• Ladi Prosek's avatar
    KVM: nVMX: fix CR3 load if L2 uses PAE paging and EPT · 7ca29de2
    Ladi Prosek authored
    
    
    KVM does not correctly handle L1 hypervisors that emulate L2 real mode with
    PAE and EPT, such as Hyper-V. In this mode, the L1 hypervisor populates guest
    PDPTE VMCS fields and leaves guest CR3 uninitialized because it is not used
    (see 26.3.2.4 Loading Page-Directory-Pointer-Table Entries). KVM always
    dereferences CR3 and tries to load PDPTEs if PAE is on. This leads to two
    related issues:
    
    1) On the first nested vmentry, the guest PDPTEs, as populated by L1, are
    overwritten in ept_load_pdptrs because the registers are believed to have
    been loaded in load_pdptrs as part of kvm_set_cr3. This is incorrect. L2 is
    running with PAE enabled but PDPTRs have been set up by L1.
    
    2) When L2 is about to enable paging and loads its CR3, we, again, attempt
    to load PDPTEs in load_pdptrs called from kvm_set_cr3. There are no guarantees
    that this will succeed (it's just a CR3 load, paging is not enabled yet) and
    if it doesn't, kvm_set_cr3 returns early without persisting the CR3 which is
    then lost and L2 crashes right after it enables paging.
    
    This patch replaces the kvm_set_cr3 call with a simple register write if PAE
    and EPT are both on. CR3 is not to be interpreted in this case.
    Signed-off-by: default avatarLadi Prosek <lprosek@redhat.com>
    Signed-off-by: default avatarRadim Krčmář <rkrcmar@redhat.com>
    7ca29de2