x86/shadow: Correct guest behaviour when creating PTEs above maxphysaddr

XSA-173 (c/s 8b1764833) introduces gfn_bits, and an upper limit which might be
lower than the real maxphysaddr, to avoid overflowing the superpage shadow
backpointer.

However, plenty of hardware has a physical address width less that 44 bits,
and the code added in shadow_domain_init() is a straight assignment.  This
causes gfn_bits to be increased beyond the physical address width on most
Intel consumer hardware (typically a width of 39, which is the number reported
to the guest via CPUID).

If the guest intentionally creates a PTE referencing a physical address
between 39 and 44 bits, the result should be #PF[RSVD] for using the virtual
address.  However, the shadow code accepts the PTE, shadows it, and the
virtual address works normally.

Introduce paging_max_paddr_bits() to calculate the largest guest physical
address supportable by the paging infrastructure, and update
recalculate_cpuid_policy() to take this into account when clamping the guests
cpuid_policy to reality.

There is an existing gfn_valid() in guest_pt.h but it is unused in the
codebase.  Repurpose it to perform a guest-specific maxphysaddr check, which
replaces the users of gfn_bits.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: George Dunlap <george.dunlap@citrix.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Tim Deegan <tim@xen.org>

xen/arch/x86/cpuid.c

@@ -6,6 +6,7 @@
 #include <asm/hvm/nestedhvm.h>
 #include <asm/hvm/svm/svm.h>
 #include <asm/hvm/vmx/vmcs.h>
+#include <asm/paging.h>
 #include <asm/processor.h>
 #include <asm/xstate.h>
 
@@ -504,7 +505,7 @@ void recalculate_cpuid_policy(struct domain *d)
 
     p->extd.maxphysaddr = min(p->extd.maxphysaddr, max->extd.maxphysaddr);
     p->extd.maxphysaddr = min_t(uint8_t, p->extd.maxphysaddr,
-                                d->arch.paging.gfn_bits + PAGE_SHIFT);
+                                paging_max_paddr_bits(d));
     p->extd.maxphysaddr = max_t(uint8_t, p->extd.maxphysaddr,
                                 (p->basic.pae || p->basic.pse36) ? 36 : 32);
 

xen/arch/x86/hvm/vmx/vvmx.c

@@ -1420,8 +1420,7 @@ int nvmx_handle_vmxon(struct cpu_user_regs *regs)
         return X86EMUL_OKAY;
     }
 
-    if ( (gpa & ~PAGE_MASK) ||
-         (gpa >> (v->domain->arch.paging.gfn_bits + PAGE_SHIFT)) )
+    if ( (gpa & ~PAGE_MASK) || !gfn_valid(v->domain, _gfn(gpa >> PAGE_SHIFT)) )
     {
         vmfail_invalid(regs);
         return X86EMUL_OKAY;

xen/arch/x86/mm/guest_walk.c

@@ -434,8 +434,7 @@ set_ad:
 
     /* If this guest has a restricted physical address space then the
      * target GFN must fit within it. */
-    if ( !(rc & _PAGE_PRESENT)
-         && gfn_x(guest_l1e_get_gfn(gw->l1e)) >> d->arch.paging.gfn_bits )
+    if ( !(rc & _PAGE_PRESENT) && !gfn_valid(d, guest_l1e_get_gfn(gw->l1e)) )
         rc |= _PAGE_INVALID_BITS;
 
     return rc;

xen/arch/x86/mm/hap/hap.c

@@ -446,8 +446,6 @@ void hap_domain_init(struct domain *d)
 {
     INIT_PAGE_LIST_HEAD(&d->arch.paging.hap.freelist);
 
-    d->arch.paging.gfn_bits = hap_paddr_bits - PAGE_SHIFT;
-
     /* Use HAP logdirty mechanism. */
     paging_log_dirty_init(d, hap_enable_log_dirty,
                           hap_disable_log_dirty,

xen/arch/x86/mm/p2m.c

@@ -1784,7 +1784,7 @@ void *map_domain_gfn(struct p2m_domain *p2m, gfn_t gfn, mfn_t *mfn,
 {
     struct page_info *page;
 
-    if ( gfn_x(gfn) >> p2m->domain->arch.paging.gfn_bits )
+    if ( !gfn_valid(p2m->domain, gfn) )
     {
         *rc = _PAGE_INVALID_BIT;
         return NULL;

xen/arch/x86/mm/shadow/common.c

@@ -51,16 +51,6 @@ int shadow_domain_init(struct domain *d, unsigned int domcr_flags)
     INIT_PAGE_LIST_HEAD(&d->arch.paging.shadow.freelist);
     INIT_PAGE_LIST_HEAD(&d->arch.paging.shadow.pinned_shadows);
 
-    d->arch.paging.gfn_bits = paddr_bits - PAGE_SHIFT;
-#ifndef CONFIG_BIGMEM
-    /*
-     * Shadowed superpages store GFNs in 32-bit page_info fields.
-     * Note that we cannot use guest_supports_superpages() here.
-     */
-    if ( !is_pv_domain(d) || opt_allow_superpage )
-        d->arch.paging.gfn_bits = 32;
-#endif
-
     /* Use shadow pagetables for log-dirty support */
     paging_log_dirty_init(d, sh_enable_log_dirty,
                           sh_disable_log_dirty, sh_clean_dirty_bitmap);

xen/arch/x86/mm/shadow/multi.c

@@ -537,7 +537,7 @@ _sh_propagate(struct vcpu *v,
 
     /* Check there's something for the shadows to map to */
     if ( (!p2m_is_valid(p2mt) && !p2m_is_grant(p2mt))
-         || gfn_x(target_gfn) >> d->arch.paging.gfn_bits )
+         || !gfn_valid(d, target_gfn) )
     {
         *sp = shadow_l1e_empty();
         goto done;

xen/include/asm-x86/domain.h

@@ -195,9 +195,6 @@ struct paging_domain {
     /* log dirty support */
     struct log_dirty_domain log_dirty;
 
-    /* Number of valid bits in a gfn. */
-    unsigned int gfn_bits;
-
     /* preemption handling */
     struct {
         const struct domain *dom;

xen/include/asm-x86/guest_pt.h

@@ -34,12 +34,6 @@
 
 #define VALID_GFN(m) (m != gfn_x(INVALID_GFN))
 
-static inline int
-valid_gfn(gfn_t m)
-{
-    return VALID_GFN(gfn_x(m));
-}
-
 static inline paddr_t
 gfn_to_paddr(gfn_t gfn)
 {

xen/include/asm-x86/paging.h

@@ -360,6 +360,27 @@ void paging_dump_vcpu_info(struct vcpu *v);
 int paging_set_allocation(struct domain *d, unsigned int pages,
                           bool *preempted);
 
+/* Is gfn within maxphysaddr for the domain? */
+static inline bool gfn_valid(const struct domain *d, gfn_t gfn)
+{
+    return !(gfn_x(gfn) >> (d->arch.cpuid->extd.maxphysaddr - PAGE_SHIFT));
+}
+
+/* Maxphysaddr supportable by the paging infrastructure. */
+static inline unsigned int paging_max_paddr_bits(const struct domain *d)
+{
+    unsigned int bits = paging_mode_hap(d) ? hap_paddr_bits : paddr_bits;
+
+    if ( !IS_ENABLED(BIGMEM) && paging_mode_shadow(d) &&
+         (!is_pv_domain(d) || opt_allow_superpage) )
+    {
+        /* Shadowed superpages store GFNs in 32-bit page_info fields. */
+        bits = min(bits, 32U + PAGE_SHIFT);
+    }
+
+    return bits;
+}
+
 #endif /* XEN_PAGING_H */
 
 /*