gnttab/ARM: don't corrupt shared GFN array

... by writing status GFNs to it. Introduce a second array instead.
Also implement gnttab_status_gmfn() properly now that the information is
suitably being tracked.

While touching it anyway, remove a misguided (but luckily benign) upper
bound check from gnttab_shared_gmfn(): We should never access beyond the
bounds of that array.

This is part of XSA-255.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

--- a/xen/arch/arm/domain.c
+++ b/xen/arch/arm/domain.c
@@ -428,19 +428,37 @@ void startup_cpu_idle_loop(void)
 struct domain *alloc_domain_struct(void)
 {
     struct domain *d;
+    unsigned int i, max_status_frames;
+
     BUILD_BUG_ON(sizeof(*d) > PAGE_SIZE);
     d = alloc_xenheap_pages(0, 0);
     if ( d == NULL )
         return NULL;
 
     clear_page(d);
-    d->arch.grant_table_gpfn = xzalloc_array(xen_pfn_t, max_grant_frames);
+
+    d->arch.grant_shared_gfn = xmalloc_array(xen_pfn_t, max_grant_frames);
+    max_status_frames = grant_to_status_frames(max_grant_frames);
+    d->arch.grant_status_gfn = xmalloc_array(xen_pfn_t, max_status_frames);
+    if ( !d->arch.grant_shared_gfn || !d->arch.grant_status_gfn )
+    {
+        free_domain_struct(d);
+        return NULL;
+    }
+
+    for ( i = 0; i < max_grant_frames; ++i )
+        d->arch.grant_shared_gfn[i] = INVALID_GFN;
+
+    for ( i = 0; i < max_status_frames; ++i )
+        d->arch.grant_status_gfn[i] = INVALID_GFN;
+
     return d;
 }
 
 void free_domain_struct(struct domain *d)
 {
-    xfree(d->arch.grant_table_gpfn);
+    xfree(d->arch.grant_shared_gfn);
+    xfree(d->arch.grant_status_gfn);
     free_xenheap_page(d);
 }
 
--- a/xen/arch/arm/mm.c
+++ b/xen/arch/arm/mm.c
@@ -1036,6 +1036,7 @@ int xenmem_add_to_physmap_one(
     int rc;
     p2m_type_t t;
     struct page_info *page = NULL;
+    bool_t status = 0;
 
     switch ( space )
     {
@@ -1053,6 +1054,7 @@ int xenmem_add_to_physmap_one(
                 mfn = virt_to_mfn(d->grant_table->status[idx]);
             else
                 mfn = INVALID_MFN;
+            status = 1;
         }
         else
         {
@@ -1068,7 +1070,10 @@ int xenmem_add_to_physmap_one(
         
         if ( mfn != INVALID_MFN )
         {
-            d->arch.grant_table_gpfn[idx] = gpfn;
+            if ( status )
+                d->arch.grant_status_gfn[idx] = gpfn;
+            else
+                d->arch.grant_shared_gfn[idx] = gpfn;
 
             t = p2m_ram_rw;
         }
--- a/xen/include/asm-arm/domain.h
+++ b/xen/include/asm-arm/domain.h
@@ -51,7 +51,8 @@ struct arch_domain
     uint64_t vttbr;
 
     struct hvm_domain hvm_domain;
-    xen_pfn_t *grant_table_gpfn;
+    xen_pfn_t *grant_shared_gfn;
+    xen_pfn_t *grant_status_gfn;
 
     struct io_handler io_handlers;
     /* Continuable domain_relinquish_resources(). */
--- a/xen/include/asm-arm/grant_table.h
+++ b/xen/include/asm-arm/grant_table.h
@@ -15,7 +15,6 @@ int replace_grant_host_mapping(unsigned
         unsigned long new_gpaddr, unsigned int flags);
 void gnttab_mark_dirty(struct domain *d, unsigned long l);
 #define gnttab_create_status_page(d, t, i) do {} while (0)
-#define gnttab_status_gmfn(d, t, i) (0)
 #define gnttab_release_host_mappings(domain) 1
 static inline int replace_grant_supported(void)
 {
@@ -30,8 +29,12 @@ static inline int replace_grant_supporte
     } while ( 0 )
 
 #define gnttab_shared_gmfn(d, t, i)                                      \
-    ( ((i >= nr_grant_frames(d->grant_table)) &&                         \
-     (i < max_grant_frames)) ? 0 : (d->arch.grant_table_gpfn[i]))
+    (((i) >= nr_grant_frames(t)) ? INVALID_GFN                           \
+                                 : (d)->arch.grant_shared_gfn[i])
+
+#define gnttab_status_gmfn(d, t, i)                                      \
+    (((i) >= nr_status_frames(t)) ? INVALID_GFN                          \
+                                  : (d)->arch.grant_status_gfn[i])
 
 #define gnttab_need_iommu_mapping(d)                    \
     (is_domain_direct_mapped(d) && need_iommu(d))
