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    Subject[PATCH 4.19 111/120] KVM: do not assume PTE is writable after follow_pfn
    Date
    From: Paolo Bonzini <pbonzini@redhat.com>

    commit bd2fae8da794b55bf2ac02632da3a151b10e664c upstream.

    In order to convert an HVA to a PFN, KVM usually tries to use
    the get_user_pages family of functinso. This however is not
    possible for VM_IO vmas; in that case, KVM instead uses follow_pfn.

    In doing this however KVM loses the information on whether the
    PFN is writable. That is usually not a problem because the main
    use of VM_IO vmas with KVM is for BARs in PCI device assignment,
    however it is a bug. To fix it, use follow_pte and check pte_write
    while under the protection of the PTE lock. The information can
    be used to fail hva_to_pfn_remapped or passed back to the
    caller via *writable.

    Usage of follow_pfn was introduced in commit add6a0cd1c5b ("KVM: MMU: try to fix
    up page faults before giving up", 2016-07-05); however, even older version
    have the same issue, all the way back to commit 2e2e3738af33 ("KVM:
    Handle vma regions with no backing page", 2008-07-20), as they also did
    not check whether the PFN was writable.

    Fixes: 2e2e3738af33 ("KVM: Handle vma regions with no backing page")
    Reported-by: David Stevens <stevensd@google.com>
    Cc: 3pvd@google.com
    Cc: Jann Horn <jannh@google.com>
    Cc: Jason Gunthorpe <jgg@ziepe.ca>
    Cc: stable@vger.kernel.org
    Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
    [OP: backport to 4.19, adjust follow_pte() -> follow_pte_pmd()]
    Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    virt/kvm/kvm_main.c | 15 ++++++++++++---
    1 file changed, 12 insertions(+), 3 deletions(-)

    --- a/virt/kvm/kvm_main.c
    +++ b/virt/kvm/kvm_main.c
    @@ -1495,9 +1495,11 @@ static int hva_to_pfn_remapped(struct vm
    kvm_pfn_t *p_pfn)
    {
    unsigned long pfn;
    + pte_t *ptep;
    + spinlock_t *ptl;
    int r;

    - r = follow_pfn(vma, addr, &pfn);
    + r = follow_pte_pmd(vma->vm_mm, addr, NULL, NULL, &ptep, NULL, &ptl);
    if (r) {
    /*
    * get_user_pages fails for VM_IO and VM_PFNMAP vmas and does
    @@ -1512,14 +1514,19 @@ static int hva_to_pfn_remapped(struct vm
    if (r)
    return r;

    - r = follow_pfn(vma, addr, &pfn);
    + r = follow_pte_pmd(vma->vm_mm, addr, NULL, NULL, &ptep, NULL, &ptl);
    if (r)
    return r;
    + }

    + if (write_fault && !pte_write(*ptep)) {
    + pfn = KVM_PFN_ERR_RO_FAULT;
    + goto out;
    }

    if (writable)
    - *writable = true;
    + *writable = pte_write(*ptep);
    + pfn = pte_pfn(*ptep);

    /*
    * Get a reference here because callers of *hva_to_pfn* and
    @@ -1534,6 +1541,8 @@ static int hva_to_pfn_remapped(struct vm
    */
    kvm_get_pfn(pfn);

    +out:
    + pte_unmap_unlock(ptep, ptl);
    *p_pfn = pfn;
    return 0;
    }

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