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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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