On Mon, 2021-11-22 at 18:21 -0500, Paolo Bonzini wrote:
> An uninitialized gfn_to_hva_cache has ghc->len == 0, which causes
> the accessors to croak very loudly. While a BUG_ON is definitely
> _too_ loud and a bug on its own, there is indeed an issue of using
> the caches in such a way that they could not have been initialized,
> because ghc->gpa == 0 might match and thus kvm_gfn_to_hva_cache_init
> would not be called.
Hm, in real usage, I thought an initialized-to-zeroes cache would have
been considered invalid regardless of the gpa, because ghc->generation
wouldn't have matched slots->generation.
But in the syzbot case perhaps it hadn't actually added any memslots
yet, so slots->generation == 0 and that doesn't help. Although....
would it even have got that far if there weren't any memslots at all?
> For the vmcs12_cache, the solution is simply to invoke
> kvm_gfn_to_hva_cache_init unconditionally: we already know
> that the cache does not match the current VMCS pointer.
> For the shadow_vmcs12_cache, there is no similar condition
> that checks the VMCS link pointer, so invalidate the cache
> on VMXON.
>
> Fixes: cee66664dcd6 ("KVM: nVMX: Use a gfn_to_hva_cache for vmptrld")
> Cc: David Woodhouse <dwmw@xxxxxxxxxxxx>
> Reported-by: syzbot+7b7db8bb4db6fd5e157b@xxxxxxxxxxxxxxxxxxxxxxxxx
>
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Acked-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
Thanks, I was just staring at the report when I realised you'd already
fixed it.
> ---
> arch/x86/kvm/vmx/nested.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index 1e2f66951566..315fa456d368 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -4857,6 +4857,7 @@ static int enter_vmx_operation(struct kvm_vcpu
> *vcpu)
> if (!vmx->nested.cached_vmcs12)
> goto out_cached_vmcs12;
>
> + vmx->nested.shadow_vmcs12_cache.gpa = INVALID_GPA;
> vmx->nested.cached_shadow_vmcs12 = kzalloc(VMCS12_SIZE,
> GFP_KERNEL_ACCOUNT);
> if (!vmx->nested.cached_shadow_vmcs12)
> goto out_cached_shadow_vmcs12;
> @@ -5289,8 +5290,7 @@ static int handle_vmptrld(struct kvm_vcpu
> *vcpu)
> struct gfn_to_hva_cache *ghc = &vmx-
> >nested.vmcs12_cache;
> struct vmcs_hdr hdr;
>
> - if (ghc->gpa != vmptr &&
> - kvm_gfn_to_hva_cache_init(vcpu->kvm, ghc, vmptr,
> VMCS12_SIZE)) {
> + if (kvm_gfn_to_hva_cache_init(vcpu->kvm, ghc, vmptr,
> VMCS12_SIZE)) {
> /*
> * Reads from an unbacked page return all 1s,
> * which means that the 32 bits located at the
>
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