KVM: x86/mmu: avoid NULL-pointer dereference on page freeing bugs

BugLink: https://bugs.launchpad.net/bugs/1978240
[ Upstream commit 9191b8f0745e63edf519e4a54a4aaae1d3d46fbd ]

WARN and bail if KVM attempts to free a root that isn't backed by a shadow
page.  KVM allocates a bare page for "special" roots, e.g. when using PAE
paging or shadowing 2/3/4-level page tables with 4/5-level, and so root_hpa
will be valid but won't be backed by a shadow page.  It's all too easy to
blindly call mmu_free_root_page() on root_hpa, be nice and WARN instead of
crashing KVM and possibly the kernel.

Reviewed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 62addcb98be791faf20339e37d467fe330ae5562..cd06b1da4e29ae83190ac36ab45508140a09ae80 100644 (file)
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3314,6 +3314,8 @@ static void mmu_free_root_page(struct kvm *kvm, hpa_t *root_hpa,
                return;
 
        sp = to_shadow_page(*root_hpa & PT64_BASE_ADDR_MASK);
+       if (WARN_ON(!sp))
+               return;
 
        if (is_tdp_mmu_page(sp))
                kvm_tdp_mmu_put_root(kvm, sp, false);