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Message-ID: <6c4a78e8-8237-b604-d047-b1ea010cf655@redhat.com>
Date: Fri, 13 Nov 2020 21:58:09 +0100
From: Paolo Bonzini <pbonzini@...hat.com>
To: Ben Gardon <bgardon@...gle.com>, linux-kernel@...r.kernel.org,
kvm@...r.kernel.org
Cc: Sean Christopherson <sean.j.christopherson@...el.com>,
Peter Shier <pshier@...gle.com>,
Jim Mattson <jmattson@...gle.com>,
Zdenek Kaspar <zkaspar82@...il.com>
Subject: Re: [PATCH] kvm: x86/mmu: Fix is_tdp_mmu_check when using PAE
On 11/11/20 19:53, Ben Gardon wrote:
> When PAE is in use, the root_hpa will not have a shadow page assoicated
> with it. In this case the kernel will crash with a NULL pointer
> dereference. Add checks to ensure is_tdp_mmu_root works as intended even
> when using PAE.
>
> Tested: compiles
>
> Fixes: 02c00b3a2f7e ("kvm: x86/mmu: Allocate and free TDP MMU roots")
> Reported-by: Zdenek Kaspar <zkaspar82@...il.com>
> Signed-off-by: Ben Gardon <bgardon@...gle.com>
> ---
> arch/x86/kvm/mmu/tdp_mmu.c | 10 ++++++++++
> 1 file changed, 10 insertions(+)
>
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 27e381c9da6c..13013f4d98ad 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -49,8 +49,18 @@ bool is_tdp_mmu_root(struct kvm *kvm, hpa_t hpa)
> {
> struct kvm_mmu_page *sp;
>
> + if (WARN_ON(!VALID_PAGE(hpa)))
> + return false;
> +
> sp = to_shadow_page(hpa);
>
> + /*
> + * If this VM is being run with PAE, the TDP MMU will not be enabled
> + * and the root HPA will not have a shadow page associated with it.
> + */
> + if (!sp)
> + return false;
> +
> return sp->tdp_mmu_page && sp->root_count;
> }
>
>
If this was just PAE, it would be easier to test "if (shadow_root_level
>= PT64_ROOT_4LEVEL)"---and more correct too, because using the
page_private of __pa(vcpu->arch.mmu->pae_root) is a bit untidy; we
should only use page_private for pages that we know have a shadow page.
In Jamie's case however, it is x86_64 (so kvm_mmu_get_tdp_level(vcpu) ==
4 and therefore the "if (shadow_root_level >= PT64_ROOT_4LEVEL)" would
be true) but without EPT. In that case we go through
vcpu->arch.mmu->root_hpa = __pa(vcpu->arch.mmu->lm_root);
but lm_root is allocated with get_zeroed_page and therefore
to_shadow_page is NULL.
I am thinking of testing simply "if (tdp_enabled)" so that we can see if
there are other cases with to_shadow_page(hpa) == NULL and we don't
sweep them under the rug. Or test "if (tdp_enabled)" and also WARN if
!sp. What do you think?
Paolo
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