[PATCH v13 35/85] KVM: Disallow direct access (w/o mmu_notifier) to unpinned pfn by default

[Sean Christopherson]

Add an off-by-default module param to control whether or not KVM is allowed
to map memory that isn't pinned, i.e. that KVM can't guarantee won't be
freed while it is mapped into KVM and/or the guest.  Don't remove the
functionality entirely, as there are use cases where mapping unpinned
memory is safe (as defined by the platform owner), e.g. when memory is
hidden from the kernel and managed by userspace, in which case userspace
is already fully trusted to not muck with guest memory mappings.

But for more typical setups, mapping unpinned memory is wildly unsafe, and
unnecessary.  The APIs are used exclusively by x86's nested virtualization
support, and there is no known (or sane) use case for mapping PFN-mapped
memory a KVM guest _and_ letting the guest use it for virtualization
structures.

Tested-by: Alex Bennée <alex.bennee@xxxxxxxxxx>
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 virt/kvm/kvm_main.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index b845e9252633..6dcb4f0eed3e 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -94,6 +94,13 @@ unsigned int halt_poll_ns_shrink = 2;
 module_param(halt_poll_ns_shrink, uint, 0644);
 EXPORT_SYMBOL_GPL(halt_poll_ns_shrink);
 
+/*
+ * Allow direct access (from KVM or the CPU) without MMU notifier protection
+ * to unpinned pages.
+ */
+static bool allow_unsafe_mappings;
+module_param(allow_unsafe_mappings, bool, 0444);
+
 /*
  * Ordering of locks:
  *
@@ -2811,6 +2818,9 @@ static kvm_pfn_t kvm_resolve_pfn(struct kvm_follow_pfn *kfp, struct page *page,
 	 * reference to such pages would cause KVM to prematurely free a page
 	 * it doesn't own (KVM gets and puts the one and only reference).
 	 * Don't allow those pages until the FIXME is resolved.
+	 *
+	 * Don't grab a reference for pins, callers that pin pages are required
+	 * to check refcounted_page, i.e. must not blindly release the pfn.
 	 */
 	if (map) {
 		pfn = map->pfn;
@@ -2929,6 +2939,14 @@ static int hva_to_pfn_remapped(struct vm_area_struct *vma,
 	bool write_fault = kfp->flags & FOLL_WRITE;
 	int r;
 
+	/*
+	 * Remapped memory cannot be pinned in any meaningful sense.  Bail if
+	 * the caller wants to pin the page, i.e. access the page outside of
+	 * MMU notifier protection, and unsafe umappings are disallowed.
+	 */
+	if (kfp->pin && !allow_unsafe_mappings)
+		return -EINVAL;
+
 	r = follow_pfnmap_start(&args);
 	if (r) {
 		/*
-- 
2.47.0.rc1.288.g06298d1525-goog