[PATCH][4.15] x86: mirror compat argument translation area for 32-bit PV

[Jan Beulich <jbeulich@suse.com>]

Now that we guard the entire Xen VA space against speculative abuse
through hypervisor accesses to guest memory, the argument translation
area's VA also needs to live outside this range, at least for 32-bit PV
guests. To avoid extra is_hvm_*() conditionals, use the alternative VA
uniformly.

While this could be conditionalized upon CONFIG_PV32 &&
CONFIG_SPECULATIVE_HARDEN_GUEST_ACCESS, omitting such extra conditionals
keeps the code more legible imo.

Fixes: 4dc181599142 ("x86/PV: harden guest memory accesses against speculative abuse")
Signed-off-by: Jan Beulich <jbeulich@suse.com>

--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -1727,6 +1727,11 @@ void init_xen_l4_slots(l4_pgentry_t *l4t
                (ROOT_PAGETABLE_FIRST_XEN_SLOT + slots -
                 l4_table_offset(XEN_VIRT_START)) * sizeof(*l4t));
     }
+
+    /* Slot 511: Per-domain mappings mirror. */
+    if ( !is_pv_64bit_domain(d) )
+        l4t[l4_table_offset(PERDOMAIN2_VIRT_START)] =
+            l4e_from_page(d->arch.perdomain_l3_pg, __PAGE_HYPERVISOR_RW);
 }
 
 bool fill_ro_mpt(mfn_t mfn)
--- a/xen/include/asm-x86/config.h
+++ b/xen/include/asm-x86/config.h
@@ -159,11 +159,11 @@ extern unsigned char boot_edid_info[128]
  *    1:1 direct mapping of all physical memory.
 #endif
  *  0xffff880000000000 - 0xffffffffffffffff [120TB,             PML4:272-511]
- *    PV: Guest-defined use.
+ *    PV (64-bit): Guest-defined use.
  *  0xffff880000000000 - 0xffffff7fffffffff [119.5TB,           PML4:272-510]
  *    HVM/idle: continuation of 1:1 mapping
  *  0xffffff8000000000 - 0xffffffffffffffff [512GB, 2^39 bytes  PML4:511]
- *    HVM/idle: unused
+ *    HVM / 32-bit PV: Secondary per-domain mappings.
  *
  * Compatibility guest area layout:
  *  0x0000000000000000 - 0x00000000f57fffff [3928MB,            PML4:0]
@@ -242,6 +242,9 @@ extern unsigned char boot_edid_info[128]
 #endif
 #define DIRECTMAP_VIRT_END      (DIRECTMAP_VIRT_START + DIRECTMAP_SIZE)
 
+/* Slot 511: secondary per-domain mappings (for compat xlat area accesses). */
+#define PERDOMAIN2_VIRT_START   (PML4_ADDR(511))
+
 #ifndef __ASSEMBLY__
 
 #ifdef CONFIG_PV32
--- a/xen/include/asm-x86/x86_64/uaccess.h
+++ b/xen/include/asm-x86/x86_64/uaccess.h
@@ -1,7 +1,17 @@
 #ifndef __X86_64_UACCESS_H
 #define __X86_64_UACCESS_H
 
-#define COMPAT_ARG_XLAT_VIRT_BASE ((void *)ARG_XLAT_START(current))
+/*
+ * With CONFIG_SPECULATIVE_HARDEN_GUEST_ACCESS (apparent) PV guest accesses
+ * are prohibited to touch the Xen private VA range.  The compat argument
+ * translation area, therefore, can't live within this range.  Domains
+ * (potentially) in need of argument translation (32-bit PV, possibly HVM) get
+ * a secondary mapping installed, which needs to be used for such accesses in
+ * the PV case, and will also be used for HVM to avoid extra conditionals.
+ */
+#define COMPAT_ARG_XLAT_VIRT_BASE ((void *)ARG_XLAT_START(current) + \
+                                   (PERDOMAIN2_VIRT_START - \
+                                    PERDOMAIN_VIRT_START))
 #define COMPAT_ARG_XLAT_SIZE      (2*PAGE_SIZE)
 struct vcpu;
 int setup_compat_arg_xlat(struct vcpu *v);