KVM: PPC: Book3S HV: Fix KSM memory corruption

Testing with KSM active in the host showed occasional corruption of
guest memory.  Typically a page that should have contained zeroes
would contain values that look like the contents of a user process
stack (values such as 0x0000_3fff_xxxx_xxx).

Code inspection in kvmppc_h_protect revealed that there was a race
condition with the possibility of granting write access to a page
which is read-only in the host page tables.  The code attempts to keep
the host mapping read-only if the host userspace PTE is read-only, but
if that PTE had been temporarily made invalid for any reason, the
read-only check would not trigger and the host HPTE could end up
read-write.  Examination of the guest HPT in the failure situation
revealed that there were indeed shared pages which should have been
read-only that were mapped read-write.

To close this race, we don't let a page go from being read-only to
being read-write, as far as the real HPTE mapping the page is
concerned (the guest view can go to read-write, but the actual mapping
stays read-only).  When the guest tries to write to the page, we take
an HDSI and let kvmppc_book3s_hv_page_fault take care of providing a
writable HPTE for the page.

This eliminates the occasional corruption of shared pages
that was previously seen with KSM active.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Alexander Graf <agraf@suse.de>

diff --git a/arch/powerpc/kvm/book3s_hv_rm_mmu.c b/arch/powerpc/kvm/book3s_hv_rm_mmu.c
index 084ad54c73cd6e9918a73f8a6da2be9abda65311..411720f5964393d1728c14a953002fcaf08a339c 100644 (file)
--- a/arch/powerpc/kvm/book3s_hv_rm_mmu.c
+++ b/arch/powerpc/kvm/book3s_hv_rm_mmu.c
@@ -667,40 +667,30 @@ long kvmppc_h_protect(struct kvm_vcpu *vcpu, unsigned long flags,
                rev->guest_rpte = r;
                note_hpte_modification(kvm, rev);
        }
-       r = (be64_to_cpu(hpte[1]) & ~mask) | bits;
 
        /* Update HPTE */
        if (v & HPTE_V_VALID) {
-               rb = compute_tlbie_rb(v, r, pte_index);
-               hpte[0] = cpu_to_be64(v & ~HPTE_V_VALID);
-               do_tlbies(kvm, &rb, 1, global_invalidates(kvm, flags), true);
                /*
-                * If the host has this page as readonly but the guest
-                * wants to make it read/write, reduce the permissions.
-                * Checking the host permissions involves finding the
-                * memslot and then the Linux PTE for the page.
+                * If the page is valid, don't let it transition from
+                * readonly to writable.  If it should be writable, we'll
+                * take a trap and let the page fault code sort it out.
                 */
-               if (hpte_is_writable(r) && kvm->arch.using_mmu_notifiers) {
-                       unsigned long psize, gfn, hva;
-                       struct kvm_memory_slot *memslot;
-                       pgd_t *pgdir = vcpu->arch.pgdir;
-                       pte_t pte;
-
-                       psize = hpte_page_size(v, r);
-                       gfn = ((r & HPTE_R_RPN) & ~(psize - 1)) >> PAGE_SHIFT;
-                       memslot = __gfn_to_memslot(kvm_memslots_raw(kvm), gfn);
-                       if (memslot) {
-                               hva = __gfn_to_hva_memslot(memslot, gfn);
-                               pte = lookup_linux_pte_and_update(pgdir, hva,
-                                                                 1, &psize);
-                               if (pte_present(pte) && !pte_write(pte))
-                                       r = hpte_make_readonly(r);
-                       }
+               pte = be64_to_cpu(hpte[1]);
+               r = (pte & ~mask) | bits;
+               if (hpte_is_writable(r) && kvm->arch.using_mmu_notifiers &&
+                   !hpte_is_writable(pte))
+                       r = hpte_make_readonly(r);
+               /* If the PTE is changing, invalidate it first */
+               if (r != pte) {
+                       rb = compute_tlbie_rb(v, r, pte_index);
+                       hpte[0] = cpu_to_be64((v & ~HPTE_V_VALID) |
+                                             HPTE_V_ABSENT);
+                       do_tlbies(kvm, &rb, 1, global_invalidates(kvm, flags),
+                                 true);
+                       hpte[1] = cpu_to_be64(r);
                }
        }
-       hpte[1] = cpu_to_be64(r);
-       eieio();
-       hpte[0] = cpu_to_be64(v & ~HPTE_V_HVLOCK);
+       unlock_hpte(hpte, v & ~HPTE_V_HVLOCK);
        asm volatile("ptesync" : : : "memory");
        return H_SUCCESS;
 }