From: Paul Mackerras <paulus@xxxxxxxxx>
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@xxxxxxxxx>
Signed-off-by: Alexander Graf <agraf@xxxxxxx>
---
arch/powerpc/kvm/book3s_hv_rm_mmu.c | 44 ++++++++++++++-----------------------
1 file changed, 17 insertions(+), 27 deletions(-)
diff --git a/arch/powerpc/kvm/book3s_hv_rm_mmu.c b/arch/powerpc/kvm/book3s_hv_rm_mmu.c
index 084ad54..411720f 100644
--- 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;
}
--
1.8.1.4
--
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