[PATCH 3/7] x86/mm: Further restrict permissions on some virtual mappings

[Andrew Cooper <andrew.cooper3@citrix.com>]

As originally reported, the Linear Pagetable slot maps 512GB of ram as RWX,
where the guest has full read access and a lot of direct or indirect control
over the written content.  It isn't hard for a PV guest to hide shellcode
here.

Therefore, increase defence in depth by auditing our current pagetable
mappings.

 * The regular linear, shadow linear, and per-domain slots have no business
   being executable (but need to be written), so are updated to be NX.
 * The Read Only mappings of the M2P (compat and regular) don't need to be
   writeable or executable.
 * The PV GDT mappings don't need to be executable.

Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
CC: Jan Beulich <JBeulich@suse.com>
CC: Tim Deegan <tim@xen.org>
CC: George Dunlap <george.dunlap@eu.citrix.com>
---
 xen/arch/x86/domain.c          |  2 +-
 xen/arch/x86/mm.c              | 10 +++++-----
 xen/arch/x86/mm/hap/hap.c      |  4 ++--
 xen/arch/x86/mm/shadow/multi.c | 18 +++++++++---------
 xen/arch/x86/x86_64/mm.c       | 12 ++++++------
 5 files changed, 23 insertions(+), 23 deletions(-)

diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index 90e2b1f..ef8c05a 100644
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -2040,7 +2040,7 @@ static void __context_switch(void)
 
         for ( i = 0; i < NR_RESERVED_GDT_PAGES; i++ )
             l1e_write(pl1e + FIRST_RESERVED_GDT_PAGE + i,
-                      l1e_from_pfn(mfn + i, __PAGE_HYPERVISOR));
+                      l1e_from_pfn(mfn + i, __PAGE_HYPERVISOR_RW));
     }
 
     if ( need_full_gdt(pd) &&
diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index e639ce2..77b0af1 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -384,7 +384,7 @@ void __init arch_init_memory(void)
                     for ( ; i < L3_PAGETABLE_ENTRIES; ++i )
                         l3tab[i] = l3e_empty();
                     split_l4e = l4e_from_pfn(virt_to_mfn(l3tab),
-                                             __PAGE_HYPERVISOR);
+                                             __PAGE_HYPERVISOR_RW);
                 }
                 else
                     ++root_pgt_pv_xen_slots;
@@ -1588,9 +1588,9 @@ void init_guest_l4_table(l4_pgentry_t l4tab[], const struct domain *d,
             split_l4e;
 #endif
     l4tab[l4_table_offset(LINEAR_PT_VIRT_START)] =
-        l4e_from_pfn(domain_page_map_to_mfn(l4tab), __PAGE_HYPERVISOR);
+        l4e_from_pfn(domain_page_map_to_mfn(l4tab), __PAGE_HYPERVISOR_RW);
     l4tab[l4_table_offset(PERDOMAIN_VIRT_START)] =
-        l4e_from_page(d->arch.perdomain_l3_pg, __PAGE_HYPERVISOR);
+        l4e_from_page(d->arch.perdomain_l3_pg, __PAGE_HYPERVISOR_RW);
     if ( zap_ro_mpt || is_pv_32bit_domain(d) || paging_mode_refcounts(d) )
         l4tab[l4_table_offset(RO_MPT_VIRT_START)] = l4e_empty();
 }
@@ -6380,7 +6380,7 @@ int create_perdomain_mapping(struct domain *d, unsigned long va,
         }
         l2tab = __map_domain_page(pg);
         clear_page(l2tab);
-        l3tab[l3_table_offset(va)] = l3e_from_page(pg, __PAGE_HYPERVISOR);
+        l3tab[l3_table_offset(va)] = l3e_from_page(pg, __PAGE_HYPERVISOR_RW);
     }
     else
         l2tab = map_domain_page(_mfn(l3e_get_pfn(l3tab[l3_table_offset(va)])));
@@ -6422,7 +6422,7 @@ int create_perdomain_mapping(struct domain *d, unsigned long va,
                 l1tab = __map_domain_page(pg);
             }
             clear_page(l1tab);
-            *pl2e = l2e_from_page(pg, __PAGE_HYPERVISOR);
+            *pl2e = l2e_from_page(pg, __PAGE_HYPERVISOR_RW);
         }
         else if ( !l1tab )
             l1tab = map_domain_page(_mfn(l2e_get_pfn(*pl2e)));
diff --git a/xen/arch/x86/mm/hap/hap.c b/xen/arch/x86/mm/hap/hap.c
index b981432..8476269 100644
--- a/xen/arch/x86/mm/hap/hap.c
+++ b/xen/arch/x86/mm/hap/hap.c
@@ -409,11 +409,11 @@ static void hap_install_xen_entries_in_l4(struct vcpu *v, mfn_t l4mfn)
     /* Install the per-domain mappings for this domain */
     l4e[l4_table_offset(PERDOMAIN_VIRT_START)] =
         l4e_from_pfn(mfn_x(page_to_mfn(d->arch.perdomain_l3_pg)),
-                     __PAGE_HYPERVISOR);
+                     __PAGE_HYPERVISOR_RW);
 
     /* Install a linear mapping */
     l4e[l4_table_offset(LINEAR_PT_VIRT_START)] =
-        l4e_from_pfn(mfn_x(l4mfn), __PAGE_HYPERVISOR);
+        l4e_from_pfn(mfn_x(l4mfn), __PAGE_HYPERVISOR_RW);
 
     unmap_domain_page(l4e);
 }
diff --git a/xen/arch/x86/mm/shadow/multi.c b/xen/arch/x86/mm/shadow/multi.c
index 2fb0125..f65ffc6 100644
--- a/xen/arch/x86/mm/shadow/multi.c
+++ b/xen/arch/x86/mm/shadow/multi.c
@@ -1475,7 +1475,7 @@ void sh_install_xen_entries_in_l4(struct domain *d, mfn_t gl4mfn, mfn_t sl4mfn)
     /* Install the per-domain mappings for this domain */
     sl4e[shadow_l4_table_offset(PERDOMAIN_VIRT_START)] =
         shadow_l4e_from_mfn(page_to_mfn(d->arch.perdomain_l3_pg),
-                            __PAGE_HYPERVISOR);
+                            __PAGE_HYPERVISOR_RW);
 
     if ( !shadow_mode_external(d) && !is_pv_32bit_domain(d) &&
          !VM_ASSIST(d, m2p_strict) )
@@ -1489,7 +1489,7 @@ void sh_install_xen_entries_in_l4(struct domain *d, mfn_t gl4mfn, mfn_t sl4mfn)
      * monitor pagetable structure, which is built in make_monitor_table
      * and maintained by sh_update_linear_entries. */
     sl4e[shadow_l4_table_offset(SH_LINEAR_PT_VIRT_START)] =
-        shadow_l4e_from_mfn(sl4mfn, __PAGE_HYPERVISOR);
+        shadow_l4e_from_mfn(sl4mfn, __PAGE_HYPERVISOR_RW);
 
     /* Self linear mapping.  */
     if ( shadow_mode_translate(d) && !shadow_mode_external(d) )
@@ -1501,7 +1501,7 @@ void sh_install_xen_entries_in_l4(struct domain *d, mfn_t gl4mfn, mfn_t sl4mfn)
     else
     {
         sl4e[shadow_l4_table_offset(LINEAR_PT_VIRT_START)] =
-            shadow_l4e_from_mfn(gl4mfn, __PAGE_HYPERVISOR);
+            shadow_l4e_from_mfn(gl4mfn, __PAGE_HYPERVISOR_RW);
     }
 
     unmap_domain_page(sl4e);
@@ -1654,12 +1654,12 @@ sh_make_monitor_table(struct vcpu *v)
             m3mfn = shadow_alloc(d, SH_type_monitor_table, 0);
             mfn_to_page(m3mfn)->shadow_flags = 3;
             l4e[shadow_l4_table_offset(SH_LINEAR_PT_VIRT_START)]
-                = l4e_from_pfn(mfn_x(m3mfn), __PAGE_HYPERVISOR);
+                = l4e_from_pfn(mfn_x(m3mfn), __PAGE_HYPERVISOR_RW);
 
             m2mfn = shadow_alloc(d, SH_type_monitor_table, 0);
             mfn_to_page(m2mfn)->shadow_flags = 2;
             l3e = map_domain_page(m3mfn);
-            l3e[0] = l3e_from_pfn(mfn_x(m2mfn), __PAGE_HYPERVISOR);
+            l3e[0] = l3e_from_pfn(mfn_x(m2mfn), __PAGE_HYPERVISOR_RW);
             unmap_domain_page(l3e);
 
             if ( is_pv_32bit_domain(d) )
@@ -1668,7 +1668,7 @@ sh_make_monitor_table(struct vcpu *v)
                  * area into its usual VAs in the monitor tables */
                 m3mfn = shadow_alloc(d, SH_type_monitor_table, 0);
                 mfn_to_page(m3mfn)->shadow_flags = 3;
-                l4e[0] = l4e_from_pfn(mfn_x(m3mfn), __PAGE_HYPERVISOR);
+                l4e[0] = l4e_from_pfn(mfn_x(m3mfn), __PAGE_HYPERVISOR_RW);
 
                 m2mfn = shadow_alloc(d, SH_type_monitor_table, 0);
                 mfn_to_page(m2mfn)->shadow_flags = 2;
@@ -3838,7 +3838,7 @@ sh_update_linear_entries(struct vcpu *v)
         {
             __linear_l4_table[l4_linear_offset(SH_LINEAR_PT_VIRT_START)] =
                 l4e_from_pfn(pagetable_get_pfn(v->arch.shadow_table[0]),
-                             __PAGE_HYPERVISOR);
+                             __PAGE_HYPERVISOR_RW);
         }
         else
         {
@@ -3846,7 +3846,7 @@ sh_update_linear_entries(struct vcpu *v)
             ml4e = map_domain_page(pagetable_get_mfn(v->arch.monitor_table));
             ml4e[l4_table_offset(SH_LINEAR_PT_VIRT_START)] =
                 l4e_from_pfn(pagetable_get_pfn(v->arch.shadow_table[0]),
-                             __PAGE_HYPERVISOR);
+                             __PAGE_HYPERVISOR_RW);
             unmap_domain_page(ml4e);
         }
     }
@@ -3902,7 +3902,7 @@ sh_update_linear_entries(struct vcpu *v)
             ml2e[i] =
                 (shadow_l3e_get_flags(sl3e[i]) & _PAGE_PRESENT)
                 ? l2e_from_pfn(mfn_x(shadow_l3e_get_mfn(sl3e[i])),
-                               __PAGE_HYPERVISOR)
+                               __PAGE_HYPERVISOR_RW)
                 : l2e_empty();
         }
 
diff --git a/xen/arch/x86/x86_64/mm.c b/xen/arch/x86/x86_64/mm.c
index 34f3250..ac358a8 100644
--- a/xen/arch/x86/x86_64/mm.c
+++ b/xen/arch/x86/x86_64/mm.c
@@ -470,7 +470,7 @@ static int setup_m2p_table(struct mem_hotadd_info *info)
                 clear_page(l2_ro_mpt);
                 l3e_write(&l3_ro_mpt[l3_table_offset(va)],
                           l3e_from_paddr(__pa(l2_ro_mpt),
-                                         __PAGE_HYPERVISOR | _PAGE_USER));
+                                         __PAGE_HYPERVISOR_RO | _PAGE_USER));
                 l2_ro_mpt += l2_table_offset(va);
             }
 
@@ -515,7 +515,7 @@ void __init paging_init(void)
             l3_ro_mpt = page_to_virt(l3_pg);
             clear_page(l3_ro_mpt);
             l4e_write(&idle_pg_table[l4_table_offset(va)],
-                      l4e_from_page(l3_pg, __PAGE_HYPERVISOR));
+                      l4e_from_page(l3_pg, __PAGE_HYPERVISOR_RW));
         }
     }
 
@@ -525,7 +525,7 @@ void __init paging_init(void)
     l3_ro_mpt = page_to_virt(l3_pg);
     clear_page(l3_ro_mpt);
     l4e_write(&idle_pg_table[l4_table_offset(RO_MPT_VIRT_START)],
-              l4e_from_page(l3_pg, __PAGE_HYPERVISOR | _PAGE_USER));
+              l4e_from_page(l3_pg, __PAGE_HYPERVISOR_RO | _PAGE_USER));
 
     /*
      * Allocate and map the machine-to-phys table.
@@ -612,7 +612,7 @@ void __init paging_init(void)
             l2_ro_mpt = page_to_virt(l2_pg);
             clear_page(l2_ro_mpt);
             l3e_write(&l3_ro_mpt[l3_table_offset(va)],
-                      l3e_from_page(l2_pg, __PAGE_HYPERVISOR | _PAGE_USER));
+                      l3e_from_page(l2_pg, __PAGE_HYPERVISOR_RO | _PAGE_USER));
             ASSERT(!l2_table_offset(va));
         }
         /* NB. Cannot be GLOBAL: guest user mode should not see it. */
@@ -634,7 +634,7 @@ void __init paging_init(void)
     compat_idle_pg_table_l2 = l2_ro_mpt;
     clear_page(l2_ro_mpt);
     l3e_write(&l3_ro_mpt[l3_table_offset(HIRO_COMPAT_MPT_VIRT_START)],
-              l3e_from_paddr(__pa(l2_ro_mpt), __PAGE_HYPERVISOR));
+              l3e_from_paddr(__pa(l2_ro_mpt), __PAGE_HYPERVISOR_RO));
     l2_ro_mpt += l2_table_offset(HIRO_COMPAT_MPT_VIRT_START);
     /* Allocate and map the compatibility mode machine-to-phys table. */
     mpt_size = (mpt_size >> 1) + (1UL << (L2_PAGETABLE_SHIFT - 1));
@@ -679,7 +679,7 @@ void __init paging_init(void)
 
     /* Set up linear page table mapping. */
     l4e_write(&idle_pg_table[l4_table_offset(LINEAR_PT_VIRT_START)],
-              l4e_from_paddr(__pa(idle_pg_table), __PAGE_HYPERVISOR));
+              l4e_from_paddr(__pa(idle_pg_table), __PAGE_HYPERVISOR_RW));
     return;
 
  nomem:
-- 
2.1.4


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