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    SubjectRe: [PATCH 2/2] KVM: MMU: fix relaxing permission
    On 05/26/2010 05:46 AM, Xiao Guangrong wrote:
    > There is a relaxing permission operation in set_spte():
    >
    > if guest's CR0.WP is not set and R/W #PF occurs in supervisor-level,
    > the mapping path might set to writable, then user can allow to write.
    >
    > @@ -1859,8 +1859,7 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
    >
    > spte |= (u64)pfn<< PAGE_SHIFT;
    >
    > - if ((pte_access& ACC_WRITE_MASK)
    > - || (write_fault&& !is_write_protection(vcpu)&& !user_fault)) {
    > + if (pte_access& ACC_WRITE_MASK) {
    >
    >

    The host always sets cr0.wp (in shadow mode) so we can write protect
    page tables. So when the guest clears cr0.wp, we emulate a gpte with
    gpte.w=0 and gpte.u=1 in two ways:

    - spte.w=1, spte.u=0: this will allow the guest kernel to write but trap
    on guest user access
    - spte.w=0, spte.u=1: allows guest user access but traps on guest kernel
    writes

    If the guest attempts an access that is currently disallowed, we switch
    to the other spte encoding.

    --
    error compiling committee.c: too many arguments to function



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