[Xen-devel] [PATCH] x86/xpti: really hide almost all of Xen image

["Jan Beulich" <JBeulich@xxxxxxxx>]

Commit 422588e885 ("x86/xpti: Hide almost all of .text and all
.data/.rodata/.bss mappings") carefully limited the Xen image cloning to
just entry code, but then overwrote the just allocated and populated L3
entry with the normal one again covering both Xen image and stubs.

Drop the respective code in favor of an explicit clone_mapping()
invocation. This in turn now requires setup_cpu_root_pgt() to run after
stub setup in all cases. Additionally, with (almost) no unintended
mappings left, the BSP's IDT now also needs to be page aligned.

Note that the removed BUILD_BUG_ON()s don't get replaced by anything -
there already is a suitable ASSERT() in xen.lds.S.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
What should we do with the TSS? Currently together with it we expose
almost a full page of other per-CPU data. A simple (but slightly
hackish) option would be to use one of the two unused stack slots.

Talking of the stack: While APs properly have the guard page mirrored
into the cloned page tables, this is a debug-build only thing _and_
doesn't cover the BSP. Should we perhaps add code to fully mirror the
guard pages, including on release builds?

--- a/xen/arch/x86/smpboot.c
+++ b/xen/arch/x86/smpboot.c
@@ -622,9 +622,6 @@ unsigned long alloc_stub_page(unsigned i
         unmap_domain_page(memset(__map_domain_page(pg), 0xcc, PAGE_SIZE));
     }
 
-    /* Confirm that all stubs fit in a single L3 entry. */
-    BUILD_BUG_ON(NR_CPUS * PAGE_SIZE > (1u << L3_PAGETABLE_SHIFT));
-
     stub_va = XEN_VIRT_END - (cpu + 1) * PAGE_SIZE;
     if ( map_pages_to_xen(stub_va, mfn_x(page_to_mfn(pg)), 1,
                           PAGE_HYPERVISOR_RX | MAP_SMALL_PAGES) )
@@ -786,8 +783,6 @@ static int setup_cpu_root_pgt(unsigned i
     /* One-time setup of common_pgt, which maps .text.entry and the stubs. */
     if ( unlikely(!root_get_intpte(common_pgt)) )
     {
-        unsigned long stubs_linear = XEN_VIRT_END - 1;
-        l3_pgentry_t *stubs_main, *stubs_shadow;
         const char *ptr;
 
         for ( rc = 0, ptr = _stextentry;
@@ -797,16 +792,6 @@ static int setup_cpu_root_pgt(unsigned i
         if ( rc )
             return rc;
 
-        /* Confirm that all stubs fit in a single L3 entry. */
-        BUILD_BUG_ON(NR_CPUS * PAGE_SIZE > (1u << L3_PAGETABLE_SHIFT));
-
-        stubs_main = l4e_to_l3e(idle_pg_table[l4_table_offset(stubs_linear)]);
-        stubs_shadow = l4e_to_l3e(rpt[l4_table_offset(stubs_linear)]);
-
-        /* Splice into the regular L2 mapping the stubs. */
-        stubs_shadow[l3_table_offset(stubs_linear)] =
-            stubs_main[l3_table_offset(stubs_linear)];
-
         common_pgt = rpt[root_table_offset(XEN_VIRT_START)];
     }
 
@@ -820,6 +805,8 @@ static int setup_cpu_root_pgt(unsigned i
         rc = clone_mapping(idt_tables[cpu], rpt);
     if ( !rc )
         rc = clone_mapping(&per_cpu(init_tss, cpu), rpt);
+    if ( !rc )
+        rc = clone_mapping((void *)per_cpu(stubs.addr, cpu), rpt);
 
     return rc;
 }
@@ -968,11 +955,6 @@ static int cpu_smpboot_alloc(unsigned in
     memcpy(idt_tables[cpu], idt_table, IDT_ENTRIES * sizeof(idt_entry_t));
     disable_each_ist(idt_tables[cpu]);
 
-    rc = setup_cpu_root_pgt(cpu);
-    if ( rc )
-        goto out;
-    rc = -ENOMEM;
-
     for ( stub_page = 0, i = cpu & ~(STUBS_PER_PAGE - 1);
           i < nr_cpu_ids && i <= (cpu | (STUBS_PER_PAGE - 1)); ++i )
         if ( cpu_online(i) && cpu_to_node(i) == node )
@@ -986,6 +968,11 @@ static int cpu_smpboot_alloc(unsigned in
         goto out;
     per_cpu(stubs.addr, cpu) = stub_page + STUB_BUF_CPU_OFFS(cpu);
 
+    rc = setup_cpu_root_pgt(cpu);
+    if ( rc )
+        goto out;
+    rc = -ENOMEM;
+
     if ( secondary_socket_cpumask == NULL &&
          (secondary_socket_cpumask = xzalloc(cpumask_t)) == NULL )
         goto out;
--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -102,7 +102,8 @@ DEFINE_PER_CPU_READ_MOSTLY(struct desc_s
 DEFINE_PER_CPU_READ_MOSTLY(struct desc_struct *, compat_gdt_table);
 
 /* Master table, used by CPU0. */
-idt_entry_t idt_table[IDT_ENTRIES];
+idt_entry_t __section(".data.page_aligned") __aligned(PAGE_SIZE)
+    idt_table[IDT_ENTRIES];
 
 /* Pointer to the IDT of every CPU. */
 idt_entry_t *idt_tables[NR_CPUS] __read_mostly;



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