x86/spec-ctrl: Introduce an option to control L1TF mitigation for PV guests

Shadowing a PV guest is only available when shadow paging is compiled in.
When shadow paging isn't available, guests can be crashed instead as
mitigation from Xen's point of view.

Ideally, dom0 would also be potentially-shadowed-by-default, but dom0 has
never been shadowed before, and there are some stability issues under
investigation.

This is part of XSA-273 / CVE-2018-3620.

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

--- a/docs/misc/xen-command-line.markdown
+++ b/docs/misc/xen-command-line.markdown
@@ -1377,6 +1377,27 @@ do; there may be other custom operating
 certain you don't plan on having PV guests which use this feature,
 turning it off can reduce the attack surface.
 
+### pv-l1tf (x86)
+> `= List of [ <bool>, dom0=<bool>, domu=<bool> ]`
+
+> Default: `false` on believed-unaffected hardware.
+>          `domu`  on believed-affected hardware.
+
+Mitigations for L1TF / XSA-273 / CVE-2018-3620 for PV guests.
+
+For backwards compatibility, we may not alter an architecturally-legitimate
+pagetable entry a PV guest chooses to write.  We can however force such a
+guest into shadow mode so that Xen controls the PTEs which are reachable by
+the CPU pagewalk.
+
+Shadowing is performed at the point where a PV guest first tries to write an
+L1TF-vulnerable PTE.  Therefore, a PV guest kernel which has been updated with
+its own L1TF mitigations will not trigger shadow mode if it is well behaved.
+
+If CONFIG\_SHADOW\_PAGING is not compiled in, this mitigation instead crashes
+the guest when an L1TF-vulnerable PTE is written, which still allows updated,
+well-behaved PV guests to run, despite Shadow being compiled out.
+
 ### reboot
 > `= t[riple] | k[bd] | a[cpi] | p[ci] | P[ower] | e[fi] | n[o] [, [w]arm | [c]old]`
 
--- a/xen/arch/x86/Kconfig
+++ b/xen/arch/x86/Kconfig
@@ -61,6 +61,7 @@ config SHADOW_PAGING
             * Running HVM guests on hardware lacking hardware paging support
               (First-generation Intel VT-x or AMD SVM).
             * Live migration of PV guests.
+            * L1TF sidechannel mitigation for PV guests.
 
           Under a small number of specific workloads, shadow paging may be
           deliberately used as a performance optimisation.
--- a/xen/arch/x86/spec_ctrl.c
+++ b/xen/arch/x86/spec_ctrl.c
@@ -187,6 +187,55 @@ static int __init parse_spec_ctrl(char *
 }
 custom_param("spec-ctrl", parse_spec_ctrl);
 
+int8_t __read_mostly opt_pv_l1tf = -1;
+
+static __init int parse_pv_l1tf(char *s)
+{
+    char *ss;
+    int val, rc = 0;
+
+    /* Inhibit the defaults as an explicit choice has been given. */
+    if ( opt_pv_l1tf == -1 )
+        opt_pv_l1tf = 0;
+
+    /* Interpret 'pv-l1tf' alone in its positive boolean form. */
+    if ( *s == '\0' )
+        opt_pv_l1tf = OPT_PV_L1TF_DOM0 | OPT_PV_L1TF_DOMU;
+
+    do {
+        ss = strchr(s, ',');
+        if ( ss )
+            *ss = '\0';
+
+        switch ( parse_bool(s) )
+        {
+        case 0:
+            opt_pv_l1tf = 0;
+            break;
+
+        case 1:
+            opt_pv_l1tf = OPT_PV_L1TF_DOM0 | OPT_PV_L1TF_DOMU;
+            break;
+
+        default:
+            if ( (val = parse_boolean("dom0", s, ss)) >= 0 )
+                opt_pv_l1tf = ((opt_pv_l1tf & ~OPT_PV_L1TF_DOM0) |
+                               (val ? OPT_PV_L1TF_DOM0 : 0));
+            else if ( (val = parse_boolean("domu", s, ss)) >= 0 )
+                opt_pv_l1tf = ((opt_pv_l1tf & ~OPT_PV_L1TF_DOMU) |
+                               (val ? OPT_PV_L1TF_DOMU : 0));
+            else
+                rc = -EINVAL;
+            break;
+        }
+
+        s = ss + 1;
+    } while ( ss );
+
+    return rc;
+}
+custom_param("pv-l1tf", parse_pv_l1tf);
+
 static void __init print_details(enum ind_thunk thunk, uint64_t caps)
 {
     unsigned int _7d0 = 0, e8b = 0, tmp;
@@ -210,9 +259,16 @@ static void __init print_details(enum in
            (caps & ARCH_CAPS_RSBA)                  ? " RSBA"      : "",
            (caps & ARCH_CAPS_SSB_NO)                ? " SSB_NO"    : "");
 
-    /* Compiled-in support which pertains to BTI mitigations. */
-    if ( IS_ENABLED(CONFIG_INDIRECT_THUNK) )
-        printk("  Compiled-in support: INDIRECT_THUNK\n");
+    /* Compiled-in support which pertains to mitigations. */
+    if ( IS_ENABLED(CONFIG_INDIRECT_THUNK) || IS_ENABLED(CONFIG_SHADOW_PAGING) )
+        printk("  Compiled-in support:"
+#ifdef CONFIG_INDIRECT_THUNK
+               " INDIRECT_THUNK"
+#endif
+#ifdef CONFIG_SHADOW_PAGING
+               " SHADOW_PAGING"
+#endif
+               "\n");
 
     /* Settings for Xen's protection, irrespective of guests. */
     printk("  Xen settings: BTI-Thunk %s, SPEC_CTRL: %s%s, Other:%s\n",
@@ -226,6 +282,13 @@ static void __init print_details(enum in
            (default_xen_spec_ctrl & SPEC_CTRL_SSBD)  ? " SSBD+" : " SSBD-",
            opt_ibpb                                  ? " IBPB"  : "");
 
+    /* L1TF diagnostics, printed if vulnerable or PV shadowing is in use. */
+    if ( cpu_has_bug_l1tf || opt_pv_l1tf )
+        printk("  L1TF: believed%s vulnerable, maxphysaddr L1D %u, CPUID %u"
+               ", Safe address %"PRIx64"\n",
+               cpu_has_bug_l1tf ? "" : " not",
+               l1d_maxphysaddr, paddr_bits, l1tf_safe_maddr);
+
     /*
      * Alternatives blocks for protecting against and/or virtualising
      * mitigation support for guests.
@@ -247,6 +310,10 @@ static void __init print_details(enum in
     printk("  XPTI (64-bit PV only): Dom0 %s, DomU %s\n",
            opt_xpti & OPT_XPTI_DOM0 ? "enabled" : "disabled",
            opt_xpti & OPT_XPTI_DOMU ? "enabled" : "disabled");
+
+    printk("  PV L1TF shadowing: Dom0 %s, DomU %s\n",
+           opt_pv_l1tf & OPT_PV_L1TF_DOM0  ? "enabled"  : "disabled",
+           opt_pv_l1tf & OPT_PV_L1TF_DOMU  ? "enabled"  : "disabled");
 }
 
 /* Calculate whether Retpoline is known-safe on this CPU. */
@@ -770,6 +837,18 @@ void __init init_speculation_mitigations
 
     l1tf_calculations(caps);
 
+    /*
+     * By default, enable PV domU L1TF mitigations on all L1TF-vulnerable
+     * hardware.
+     */
+    if ( opt_pv_l1tf == -1 )
+    {
+        if ( !cpu_has_bug_l1tf )
+            opt_pv_l1tf = 0;
+        else
+            opt_pv_l1tf = OPT_PV_L1TF_DOMU;
+    }
+
     print_details(thunk, caps);
 
     /*
--- a/xen/include/asm-x86/spec_ctrl.h
+++ b/xen/include/asm-x86/spec_ctrl.h
@@ -38,6 +38,10 @@ extern int8_t opt_xpti;
 #define OPT_XPTI_DOM0  0x01
 #define OPT_XPTI_DOMU  0x02
 
+extern int8_t opt_pv_l1tf;
+#define OPT_PV_L1TF_DOM0  0x01
+#define OPT_PV_L1TF_DOMU  0x02
+
 /*
  * The L1D address mask, which might be wider than reported in CPUID, and the
  * system physical address above which there are believed to be no cacheable
